Fun with MAF's :-(

[budwich]

My 2008 4.2 has been good but fuel mileage has been reducing. I have change out the front o2 sensor (would have done the back but its tough to get at the harness... maybe next time... probably doesn't matter in terms of fueling but it does throw a "weird code" often). I decided to give a new "aftermarket" MAF (ebray... he ha) after cleaning of the existing one didn't do much. It looked ok anyways.
Plug in the replacement, fired up torque, started the vehicle and things (trims, air volume, etc). So OK. But the next drive out, was monitoring things and see the LTFT start to "climb" negative instead of sitting around 0 (old unit and initial run with new). The air intake "volume" is running similar to the old unit at around 6gm/s (I think that the units). Of course, eventually the LTFT for a code of running to rich. I measured the IAT side of the sensor to compare with the old sensor... they both ran about 6M ohm. I then ran a measurement on the ground the signal pin pairs. The old unit is around 2kohms while the new one is around 1kohms. Maybe a design.... potentially doesn't matter since the signal is "pulse based". Further, a diode test of the same pin set give .9 v for new versus 1.9 for the old.

The thing that is unusual is that torque isn't registering air intake flows all over the place like a bad sensor issue..... so then why is the LTFT running away negatively? Any wisdom with a MAF?

 

[mrrsm]

Even though you don't mention getting a P0171 Code... there is some additional information on the O2 Sensors and Scanner Diagnostics that could prove useful here:

P0171 Code

forums.trailvoy.com

2006 Trailblazer 4.2 P0171 - FIXED

2006 Trailblazer 4.2 P0171 - FIXED

www.scannerdanner.com

 

[budwich]

there is no codes normally. the one code that I get on occasion is the code for the second o2 sensor (post cat) which give 013b (strange but true). When the new MAF is in, eventually the rich code is set as the LTFT goes to -25 (and over) for a period and thus the system can no longer compensate, throwing the code.

I guess my question is if the o2 were so out of whack, why with the old MAF in does the system run "normally" (ie. trims are in normal ranges, along with air intake)?

still further, if the intake air "volume" is the basically the same with either sensor, where is the air going that engine isn't getting it?

 

[Mooseman]

Was the replacement MAF an actual branded one or just some sort of CCC? What about the O2 sensor? This engine is sensitive to the parts used on it and ACDelco or Delphi may be required.

And the post cat sensor just reports if the cat is working correctly so no point in replacing it for engine running issues, unless you just want to get rid of that weird code.

 

[budwich]

I cleaned the throttle body... it didn't make much / any difference. Basically, at idle, I am seeing -15-20% lean on the LTFT which if left idling eventually moves to -25 which will code with a rich running code. If I push on the throttle, the LTFT goes "instantly" down (up) to around -2-3... it seems cracking the throttle body causes a significant change of 10-15% which is strange for "long term"... STFT are oscillating as expected. Idle is good at 600 (610-600). Overall running seems ok. O2 front sensor appears to oscillate fine as expected... with slow oscillation at idle and "choppy" (ie. higher frequency at speed). Post cat is relatively flat but does move / drift "plateaus" depending on throttle / speed. The front sensor was a acdelco (at least that was the box). It was replaced last spring.

The replacement is supposed to a "new" but was unmarked so it may be the issue. I am still not quite understand how it functions though since the intake air volume seems the same / similar to the old. So how can it be that the thing is giving the pcm the incoming air volume so relatively stable yet the "mixture" have such drastic changes. To me this has got to be some sort of "table fubar" whereby the "running table" has significant "differences" in some areas that it is indexing (by what) yet the "compute table" for display is saying something different.

I have tried the 3 minutes of idle / 1 minute off type cycling that the GM SM says can be used instead of the scantool reset. The idle does eventually adjust to "normal" (600ish).

Tomorrow, I will probably drop back the old unit and see what happens.

One last question, the "cells" being used by the pcm for various settings are adjusted at idle to get an appropriate idle speed. How are the "running" cells updated or for that matter, erased to start over from "factory" or whatever. I see that the LTFT value is a "average" of the cells in the "range" that is being used.... so to me that "range" has some pretty high values... but how did they get there and where is "there"?

 

[mrrsm]

Please do not take offense to this clarifying explanation...

The MAF does not actually alert the PCM as to what the "Volume" of the Air moving through is at any given moment. Rather... it WEIGHS the passing Air using Fractions of Grams to advise the PCM as to How Much Fuel to Deliver based upon its MASS (Hence the MAF Acronym... versus using the Acronym of VAF...for VOLUME AIR FLOW if that were the case for using Air Volume in its F/A Calculus.

The reason for this distinction is that you can have the same VOLUME of any given kind of matter ...and both will be capable of having a greater or lesser amount of MASS. For example... One Cubic Foot of Marsh-Mellows weighs substantially less than One Cubic Foot of Gold. However...their VOLUME within the 12"X12"X12" Cubicle will naturally be... Identical. Thus the MAF's ability to make these precise "scale weight" type of assessments of the Air Mass versus the Engine RPM and other parameters must be quite precise for the PCM to determine whether or not it is achieving a stable, Stoichiometric balance of 14.7 to 1 Air to Fuel Combustion ratios.

The other reason for using the MAF has to do with the fact that not all vehicles have NA engines that breath in available Air...at Sea Level. Consequently, it is the MAF that helps the PCM make variable adjustments of Air Weight for the normal running of the Motor if say... that vehicle traveled from the Pacific Shore all the way up to Pike's Peak where the MASS of the Air would be much "thinner" and so Weigh less. Volumetric Measurements would only work in either one place at one Altitude or another, but not be capable of making these distinct adjustment in the absence of making constant reads of ambient Temperature and Air Pressure for its F/A Calculations.

And so the Quality and Precision capabilities for NON-OEM Delphi or NON-OEM ACDelco Mass Air Flow Sensor will come into question right along with the Quality of the Lead, Upstream O2 Sensor that may not be Kosher. One thing is certain... The only way to conduct a valid scientific solution in any such investigation... is to Only Change One Variable (or Sensor) at a Time. Working with more than One Variable in this case can turn out to be One Too Many.

Take a very close look at the Electrical Harness of the MAF and the IAT Combo for any loose Connector Insert Wires that may be separating when the engine reaches a certain operating temperature or sustained engine vibrations that might be making that MAF act in sketchy ways that cannot be revealed by static probing and testing with the engine OFF.

 

[budwich]

No problem. I agree that the quality of precise of the unit is a question. I guess that I am wrestling with the "precision" at a given "instance". It appears that its "precision" is very bad such that the "value" it sends is 15-20% off... and then slightly cracking the throttle body appears to get it back to perhaps a "somewhat normal" response... maybe.

Of course, as you indicate, it is truly hard to compare things in time as changing / switching out the MAF takes time along with potential that monitor process (torque) has its own "degree of variance" with what can be monitored / displayed at any given instance.

As you indicate / suggest, wiring could be a culprit... I have tried the "wiggle / pull" test at the connector and wire route along the way while the engine is running and monitoring torque.... this is relatively easy to do since the issue (high negative values of LTFT) occur at idle ... thus I don't have climb on the engine while it goes down the road...

I haven't tried a "frequency check" of the output yet to see if there is an indication of poor correlation (I don't actually know what I would be looking for) as I don't like "pin pricking" leads on signal wires especially thin ones as this can lead to further problems later in "life". I am concerned that the meter checks of the MAF side of things gives electrical parameters that are different when compared to the old device.... but again, not sure if they matter at the operational level if the value of interest to the pcm is something like frequency "density" (ie pwm ish) as opposed to voltage level / resistance variance.

The "fun" will continue for a bit before I decide on what to do with the ebray purchase in terms of return. I am concern that the unit has no product stamps codes which leaves one open to the seller's thoroughness in item quality and "cross-reference"... of course, you get what you pay for...

 

[budwich]

fun continues.... So not much has changed overnight in terms of the operation / response of the LTFT.... I was lightly hoping that the "good mechanic witch" would happen by and magically adjust / fix things. Guess I didn't leave enough money under the vehicle.

Anyway, go out my "pin probe test lead" as recommended by here a while back.... and tapped into the signal lead. Not sure how responsive my meter is to frequencies especially one that is likely a always positive waveform (no zero crossing) along with not "phase locked" (ie. changing). So the reading at idle was bouncing around 55-65/70 ... could be the meter trying to figure out the waveform. Running the rpms up into the low 1K range or there abouts (helper's foot might be tough to control).... the frequency went into the 110-115/120 range. Not sure what the actual range is for this device. Further, light reading seem to indicate the actual values don't matter just whether is moves up "in step" with the throttle..... BUT comparing with the old, which when put back in, the engine returns to "normal reactions" on the LTFT. The equivalent frequencies are 45-50 at idle and at mid 1k 80-90. It would appear perhaps that frequencies do matter... although my results may be taken with a "grain of salt / accuracy" as the meter isn't very good in this mode of signal.

The other fun now is I lost one of the screws holding the maf in the engine area somewhere... :-( no big deal... except for "its sunday".

Guess it is time to go back to the seller and do that hastle.

Comments?

 

[mrrsm]

As for the Lost Fastener over The Engine Bay... Fortunately... unless you imparted any push one way or another when you dropped it... Gravity is your friend... and a Slender, Telescoping Pocket Magnet will soon reveal that it is probably closer to you than you think. Poking around directly under the MAF/IAT and around on the frame areas and wheel well locations will usually reveal where it fell... and you will very likely find OTHER Lost Fasteners as well.

Avoid using the Telescoping Magnets having the Neodymium inserts the size of a Buffalo Nickel... They will stick to the closest thing made of any Ferro-Metal and are more of a nuisance than an assist.

If you look at the images of the MAP Sensor P0102 and P0102 Codes... even though these have not been thrown yet... their diagnosis will give clues about the expected, nominal performance of the MAF and some possible Causes and Origins of problems similar to what you are dealing with.

That second link to the scannerdanner.com Thread I posted above has what is probably a GOOD WAVE FORM for your comparison... but that fellow is using a much more sophisticated scanner.

You might drop in there and request a Good Wave Form for your analysis, as he keeps a whole library of these available. Read his opening Web Page admonition on "How to Post" first ...as the Members there can be reluctant to jump in and help anyone in the absence of following their "format" for inquiry and problem solving.

I have attached the screen prints of the relevant MAF Code diagnostic break downs that may prove helpful:

 

[budwich]

continue to look around in this area... density versus volume. It would appear that the relationship is somewhat "linear" but in the "inverse" for a given altitude. Basically, as density goes down, volume goes up at the same rate... for an ideal gas. Probably more to it than that, but it would seem to make since that incoming air volume on a vehicle would be hard to ascertain but the density maybe less so and hence why they monitor and use that value. Of course, the "final say" is the O2 sensors that feedback so that adjustment can be made. A 10% error in density will likely cause a 10% air in the volume determination which then would throw the "gross running mixture" out by at least that much. Doesn't mean much other than if the "non-oem" unit has little or poor "certification" to density calibration, the engine / system will be running with one hand tied behind its back. Having said that, is making a MAF sensor really rocket science! I guess so.
:-( Exchanging emails with seller, they indicate their "remanufactured units" are tested but to what, not sure. Further, a visual inspection of the visible components in the air stream does show quite a different in size and placement which may or may not impact accuracy. In addition, there are some small holes in the chrome plate of the main body of the unit... on the replacement unit, they aren't there... not sure if they are important or are just "inspection / test point access"... but a noticeable difference.... which was in the listing picture but on the unit sent.
Still working with seller on next steps.

 

[budwich]

second unit is not much better. It doesn't "code" but results in LTFT running in the negative 14-22 range.... which appears to indicate significant richness that it is trying to compensate for. Replaced the original unit back in and things return to "normal" (ie. in and about 0.0 at idle and slight swings positive or negative with throttling).

I wonder if the "run time values table" has any "initialization" sequence beyond the "let idle for 3 minutes and off, and repeat if necessary"... which seems only for a idle speed / rpm adjustment.

I guess ultimately the MAF provides some "first guess" to the system with the ultimate feedback provided by the O2 sensors for "correction / adjustment" at any given point.

I guess like a lot of "third party stuff", they aren't quite there. :-(

 

[Mooseman]

Have you tried running it with the MAF disconnected? I think it will fall back to speed density mode, like before we had MAFs.

One thing though, with my 5.3, it shifted like it had a racing shift kit. Don't know if it will do the same with the 4.2.

 

[budwich]

I didn't try that "method" yet... as I just swapped back to the original unit and things are back to normal. I will give the "MAF unplugged" a try to see what happens beyond the check engine light which I assume will come on with the MAF disconnected. I assume shifting in some "default modes" goes back toward monitoring / using vacuum levels to help with things.

 

[budwich]

Have you tried running it with the MAF disconnected? I think it will fall back to speed density mode, like before we had MAFs.

One thing though, with my 5.3, it shifted like it had a racing shift kit. Don't know if it will do the same with the 4.2.

I obviously don't know / understand what the heck a MAF does in the overall scheme of things.
So I tried this "test". This resulted in very much like the "third party" MAF response... basically the LTFT "floated" towards a largely negative correction in the -14/-20 range. Didn't notice much difference in shifts although I wasn't pushing the pedal big time.

No check engine light for any codes related to a "mssing MAF"... so it looks like the system doesn't really care if a MAF is there or not as I guess what ever PCM run mode occurs, it has enough "smarts" based on feedback from the O2 sensor to keep things within "reason", emissions wise. One addtional note: an unplugged MAF (IAT) gives a -40F air intake temperature which may have some impact on mixtures that attempt to compensate for the "fake cold" which then the system detects with the O2 seeing rich and resulting in trims being increasing negative to lean out the "fake cold system"... maybe.

So based on this, it would appear that the replacement units either are dead or not functioning / compatible enough to do much for the normal running function.

 

[Mooseman]

The CEL/code would come up after a couple of drives. Maybe the MAF isn't responsible for your trims.

 

[budwich]

The codes did come up on a subsequent "retest" of the "disconnect MAF" so it is possible there is some "time lag / check" that needs to happen. As for the impact of a MAF, the correlation to original MAF to "normal trims" versus replacement MAF to trims heading towards "coding" seems too co-incidental.... but are you saying that any MAF doesn't impact trims or that this vehicle has something strange going on with its trims? As you indicated earlier, a "non MAF operation" falls back to a "rich run" which the system tries to compensate for with negative trims (somewhat extreme). Not sure if that is expected as not sure what the "speed density" theory is for fuel / air.

 

[Mooseman]

It should fall back on presets built in as if it never had a MAF, like the older trucks, using all the other sensors. Maybe there's something else causing the rich mix and with the MAF connected, trying to compensate.

Re-reading a little, maybe it has nothing to do originally with the MAF. Could be the O2 sensor is getting lazy. I have noticed that swapping in a new sensor sometimes improved MPG.

 

[budwich]

could be but I not sure I understand quite what you are alluding to. So with no MAF connected, my vehicle appears to run rich and the system appears / attempts to bring the system into balance by significantly reducing the LTFT to the negative 15/20 ish range. Connecting up the original MAF, after a few minutes, the vehicle runs in what I would consider "normal" with the LTFT sitting in the +-/3 or so depending on throttling and such. Nowhere near the same resulting trims of a "no MAF system".

Trying either "third party" MAF results in the same "rich run" with significant negative trim values and in the one MAF case, throws a "running rich code" (sorry I forget what it was) because the system is unable to compensate because it is past -25%.

To me, this seems like the replacement MAF are non-operational or equivalent to almost being unplugged.... but that just my read. As for the O2 sensor, it was changed well before this MAF "playing around" took place. Further, if the O2 sensor was / is lazy, why is the system acting so different with the two sensors which have nothing in common O2 sensor other than its the same sensor in both cases.

Reading a bit more on speed density systems, ultimately my question is: is the table in the PCM "corruptable / changeable" and is so what is the technique to do either.... and further, what table does the system use with "MAF enablement" with the same question... corruptable / changeable?

 

[Mooseman]

Quite possible the PCM falls back to an over rich parameter when the MAF isn't working. Would explain that you got the same result with the bad replacement MAF and having it disconnected. Maybe @limequat could explain a bit more how this system works.

I wouldn't use any data taken when the MAF is reconnected with the engine running. It should be cycled or do a full PCM reset to get a good read as it relearns the MAF.

Engine temps good? Could explain the richer mix.

 

[budwich]

Engine temps are good as I watch them closely especially when towing heavy (4.5K lbs).

Your suggestion of the "disconnected MAF" was a good one as then it gave me one more "data point"... system with old MAF, system with "non oem" MAF and system with no MAF. The last two seem similar in results and system response, hence my "suspicions" about the "non-oem". Of course, as you indicate, how the system is supposed to set up in a "speed density" mode (in a system that is supposed to actually have a MAF) isn't obvious to me... and in general, I don't know much beyond a quick read of some stuff on "speed density" / MAP advantages and disadvantages versus MAF. I guess I could "ping" limequat although I don't want to take up his time as he does this stuff as a "money maker" in some form.

I guess one other point / thought.... with no MAF, if the system is running in a "speed density mode" AND the result is rich, that would appear to indicate that the tables are quite right for the configuration (ie. how much air is supposed to be moving thru the intake). It could point to a constriction of some sort in the intake path. In my case, everything is stock. The air filter was changed last august... the old wasn't especially dirty. I am not sure I appreciate the incoming air port which seems to be almost blocked by the rad / front end.... but surely GM designed it appropriately.

 

[limequat]

Quite possible the PCM falls back to an over rich parameter when the MAF isn't working. Would explain that you got the same result with the bad replacement MAF and having it disconnected. Maybe @limequat could explain a bit more how this system works.

I wouldn't use any data taken when the MAF is reconnected with the engine running. It should be cycled or do a full PCM reset to get a good read as it relearns the MAF.

Engine temps good? Could explain the richer mix.

In theory the system is supposed to work normally with the MAF unplugged, doesn't always happen in my experience. 2nd'd that this issue is not MAF related. At idle, the MAF is mostly not used, fueling is done based on the MAP. I would suspect a dribbly injector or vacuum leak.

The reason the LTFT changed so quickly is because there are a number of different cells separated by engine rpm and MAP. Blipping the throttle puts the engine into a different cell which has learned a different value.

MAF codes set after 2 consecutive ignition cycles.

Hope some of the above is of use

 

[budwich]

thanks... our ships crossed path as I sent you a "conversation starter"... sorry.

Thanks for the further information. The effect of the "non-oem" MAF is both at idle and as well at speed. Less so at speed but still significantly negative. If the original MAF is placed back, I do not see this type of "negativity" degree as the system runs near / around 0 LTFT.

It is possible that a dribbly injector exists as they have never been service in the 200kkm. I haven't done any vacuum testing but have replaced the MAP in the past... about a 1.5 years ago.

Lastly, from your explanation, about the "non MAF" operation, how does the system determine a "faulty MAF" beyond a "coded fault"... meaning if electrical readings are OK, then what would make the system with the MAF plugged in operate like a "speed density" system which is what appears to happen with the non-oem MAF?

 

[Mooseman]

I do believe there are correlation error codes that pop up if the expected flow is really out of whack compared to the throttle and MAP.

 

[budwich]

hmmm... so that doesn't explain the MAF differences (non-oem vs oem) as there is no codes when either are attached. I am going to pull / clean the MAP sensor just in case something has happened there along with trying the "spray for leaks" test to see if anything happens... not so much about the MAF but general system operation / maintenance check.

I guess the question is: can cells be corrupted and then reset is some form by an enduser?

 

[limequat]

Mooseman has it. The PCM compares the MAF to the MAP, RPM, and APPS readings and makes sure that it's within an expected range. There's also an maximum expected airflow value.

There is no possibility of corruption in the PCM. Everything is protected by checksum. If by chance something wonky did happen, it would set a P060x code and probably go reduced power.

 

[budwich]

As perhaps a happy ending to this fun, finally after 4 different ebay "efforts", I finally got a MAF that appears to work or at least isn't any worse than my original unit. It looks that same as the original in terms of physical properties in terms of the casing (chrome part) and stamped labeling whereas the other "tries" were very "generic" in look and weren't what was pictured in the listing (which is what I wanted / needed). The chrome cover is a bit scuffed up which perhaps means that it isn't "new".... maybe. Of course, time will tell but at least the LTFT is not all over the place like the others.

 

[mrrsm]

This is a Pretty Good "Explanation Video" in which the VOP goes into some finer grain details already nailed down by @Mooseman and @limequat concerning the Real Differences between Speed Density vs. MAF Tuning (...or when BOTH systems can be used) with some additional explanatory stuff:

And even if you never intend to use HP Tuners HW & SW to "Fine Tune" anything you own... The VOP over at the "Goat Rope Garage" has some VERY contemporary and ON TOPIC Instructional Videos in several Complete, Multi-Part Series.

This stuff is the "Bread and Butter" of everyday PCM-ECM Controlled Vehicles as well as High Performance Turbo-Charged GM Trucks and SUVs. The Vidz have a wealth on information regarding "What is Going on Inside of the PCM" in the way of MAF, MAP & VE Tuning and so much more that is excellent stuff to know... even for Strictly Stock, Never-to-be-Tuned Vehicles:

Goat Rope Garage

Almost two decades of owning and working on performance cars that include a custom turbo GC8, STi, FD RX-7, GTO, Corvette and plenty others. Stop by to learn...

www.youtube.com

 

[budwich]

I had a chance sit down and watch the linked video. As you said, its targeted towards tuning but provides some basics on the overall system which is good. One thing that they brought up was when the system moves out of "closed loop" into "open loop" because of throttle "demands" (ie. wide open throttle). To me, this is interesting because I have monitored the trims while pulling a trailer and have found that the STFT stop moving at about 40 % throttle and I never understand how that could be because I could see that the O2 sensor was still oscillating.... and yet no codes were being thrown for this "condition". This brings up questions like: is there a throttle point that reflects that change of state (closed to open) and if so what is the designed parameter for the system? ie. is my system going into open system because of a "misread" of "open throttle" or whatever the trigger is? Another question: is there a PID that monitors the "open/closed" running state and can it be displayed by torque?

And another point: I can see that if the system is going out of closed loop, then it is relying on tables for mixtures (which I assume is either the speed / mass). I can see that perhaps if those are not tuned well, that you maybe very fuel inefficient which I can readily see once you get about above 60 mph which is about where the throttle point is.

 

[mrrsm]

This is why @limequat and @Kelly@PCMofNC deserve "The Big Bucks"... 8>)

Glad you found that particular segment interesting... The same goes for the one he has over at his "Goat Rope Garage" YT Channel on strictly "Tuning using the MAF Sensor" as well. He must have around (20) or so "Targeted Topic" Training Videos that can help lift the veil from the eyes of us who want to know more about "What Makes Tuning ...Tick?". Ill be covering these topics in the HPT section off-shoot of the "Hoping for Loping" Thread as well.

 

[Blckshdw]

Another question: is there a PID that monitors the "open/closed" running state and can it be displayed by torque?

Actually yes there is, I have it on my display. It's listed as Fuel Status, when it changes it says "Closed Loop, using 02" Here's a screen shot, engine off obviously.




When you go to add a display item, scroll down further, below all the different display types, the Fuel Status one is by itself, towards the bottom, as is SAIS for anyone that has it

 

[budwich]

OK... but does it change with "wide open throttling" or otherwise as opposed to temperature?
I notice that your ACH is not registering. I wonder if you need to "rescan" your available PID. I know that sometimes my torque display loses some of the display PIDs such as gear, rpm and a few others including ACH.... not sure why. I usually do a "restart" / shutdown of the tablet to get things working again although I also have done a rescan in the past.

 

[Blckshdw]

I never have a WOT scenario, so that's not something I'd be looking for. The AC pressure isn't registering in this case because the key was just turned to ACC to get the radio to come on. So far, the only display that gives me intermittent issues is the tranny temp. It doesn't lose the display, but gives me a false reading, and always the same temp (74 degrees)

 

[budwich]

right about the "ach display"... sorry I forgot about your car state for the posted display.... but it is good that you too are seeing the AC high side pressure as lots have reported on here that they don't see it,.

My tranny temp some times goes "-" but a restart of the app usually brings it back.

I put up the "fuel status" and hope to follow it in an up coming "tow". As I indicated, I don't believe my truck goes into to "WOT" mode in terms of the throttle position but it would appear from watching the trims that it leaves "closed loop". I am hoping that the PID you have pointed me to will give a better understand of where the system is running. I am thinking that my throttle body might have some issue both physically and electrically... maybe cause there is some play in the butterfly and not sure that the sensor is always picking up the correct position at least versus the feel or pushing on the gas with my foot.... maybe a delay in the torque app talking with the pcm.

 

[budwich]

saw an interesting "event" on the shopping trip today. Now that I have the "fuel status PID" being displayed, I was keeping an eye on things for the short trip.

So within seconds of a cold start, the "fuel status" goes from "open loop" to "closed loop using O2". Seems strange but maybe with the warmth and heated sensor, it doesn't take much...maybe. Then about 5 minutes into the trip, I happen to notice that the STFT was no longer "oscillating"... hmmm ... AND the fuel status flashed back to "open loop low temp" and then about 2 seconds later went back to "closed loop" again which then saw the STFT start to "oscillate" again. The rest of the trip and few starts happened with no further repeat of the "event". No sure why but I have seen some strangeness in the sensor (actually two different sensors) at certain periods in the past whereby it appears to go into a sort of "square wave" period for a long time 1-2 seconds high and then 1-2 seconds low... almost like it was initializing or something like that. Not sure if it is an idiosyncrasy of torque, pcm or the o2 sensor electronics.
Comments?

 

[mrrsm]

It is a given that more than most of us… YOU clearly understand these concepts and principles concerning the “Strange Balancing Act between STFT and LTFT”. However… I thought that Duane from the “RealFixesRealFast” YouTube Channel has put on a Really Good Two Part Clinic involving a different platform than ours as his exemplar repair subject vehicle… but with a much more in-depth approach to the topic. I hope there are a Few More Clues as to what you are dealing with nested within in these Videos:

 

[budwich]

thanks again... interesting problem in the video. Sure glad our intake doesn't work like that and needs totally replacing because of a small non-serviceable part.

In general, I monitor my trims regularly just cause they usually are easy to spot things when they suddenly differ, as the videos indicate and thus begin the investigation deeper.

At this point, because of the state change in the "fuel status" which I haven't been directly watching before the forum suggestion, I am somewhat concerned of the unusual reaction. Driveability was not noticeably different and no coding has shown up which is good so far in that the p013b code that was popping in the past potentially has been addressed with the downstream o2 sensor replacement (and monitor of the unit appears to show a better response). Further, related to this and readings in the area, there are some that suggest that certain systems use the downstream sensor in some "fall back mode" to do some adjustments when the results from the trims and upstream do not appear to begin the expected results in the downstream readings. Not sure if the trailblazer has this type of mode in the PCM (even in the readings in general, most "3rd party people" are unaware of any manufacturer's who are willing to specific state this capability on vehicles).

Anyway, those that have and can monitor things, maybe they have experiences in this area to share. I will keep actively monitor this aspect to try to better understand the relationship between state changes and why they occur and if the change is expected or an error condition.

 

[Blckshdw]

So within seconds of a cold start, the "fuel status" goes from "open loop" to "closed loop using O2".

Mine does this too, but I've never seen it go back to the open loop indication (while driving like yours does) until shutting off the engine.

Edit: I wonder how much of a difference it would make if this was winter time in a colder part of the country, and things were actually COLD on start up.

 

[budwich]

the "fun" continues....
so I had to do a run to pick up the trailer from storage. With my "new found fuel status pid"... I was ready and watching. So, it initially went from "open" to "closed loop" very quickly... probably with 2-3 seconds.... as before. Then about 4-5 minutes into the journey (going - no trailer), the prime O2 sensor did a "square wave thing" (high for about 3-4 seconds of graph and then low for another 3-4 seconds) and then went back to normal oscillating activity. Before this is was acting normal. During the "square wave", the fuel status changed to "open loop low temperature".... some what strange by this time engine and coolant temperature are well into "normal warm" at around 194. I don't understand why this "event" occurs.

The next "fun" was when towing. As mentioned, when I get my throttle position in and around 40% (higher a bit) and stable (ie. constant pedal), the STFT stops cycling. Now I found with my "new PID" that the fuel status goes into "open loop - engine load / decelaration". The O2 sensor is some what normal although not as regular in it oscillation but not a "square wave" or pegged at some level. I didn't have an engine load PID displayed (not even sure there is one as opposed to an internal calculation by the PCM). I assume the engine load was at some value threshold (as opposed to deceleration which I know wasn't happening). For sure, I am not any where near WOT. The engine seemed to be pulling find so I don't think I was at 100% load and certainly not in terms of rpm (probably around 2800, in 3rd gear, speed 60mph ish).

I guess the new question is: is there some engine limit that causes a "drop out" of the closed loop and if so, is that kind of the normal operating limit or is there an issue such that the engine load is higher than expected because of some underlying issue? Not sure that I have ever seen this in a "non towing" activity although I can't say that I can get there legally (ie. the speed might have to be a bit higher than allowed).

 

[budwich]

OK... now the "quicksand" has started.

So I added the PID for "engine load" (not the absolute one but the other). But now the question is: what the hell is it or how is it calculated? At idle (cold), the thing was at 35% already... so my engine is "1/3 loaded" at idle.... doesn't sound good. I haven't run down the road yet but I would expected the thing to be more related to speed / rpm and pedal position / timing and at idle maybe around 20% or less. Looks like I need to do some more read.

 

[mrrsm]

This is a DBOT (Dead Balls On Topic) Reply to a Related Thread over at TV:

OBD II Scanner (Engine load at idle)

forums.trailvoy.com

Quote:

Originally Posted by Chickenhawk Ignore that. Look at the oxygen sensor data for sensor 1, bank 1. It should fluctuate rapidly above 0; from about 200 mV to 900 mV. What are your readings, and how fast are they changing? Now check oxygen sensor 2. This is your downstream sensor, and the readings should be pretty solid, with few fluctuations. It should read close to 0. What is that reading and how fast is it changing? Now look up your readings for short term fuel trim (STFT) and long term fuel trim (LTFT.) These readings will be expressed asa percentage and should be +/- less than 8%. The STFT should bounce a lot between +/-5%. The LTFT should be pretty steady and ideally will be near 0. What are your readings for STFT and LTFT? Next, read the many threads here on mileage. Nobody can say exactly what is wrong or whether anything is wrong because driving conditions and driving styles vary. But there are certain common causes of much poorer than normal mileage figures. The funny thing is that we tell people to look for these things time after time, and for some reason, very few people actually come back to report their findings. I guess some people would rather just throw parts at the problem or hope maybe some mechanic will stumble across the answer (or combination of answers.) The common causes of decreased mileage are: - city versus highway driving - driving style - plugged cat - bad thermostat - lazy oxygen sensor - worn or wrong type spark plugs - winter gas - bad fan clutch - oil sludge buildup - dirty throttle body - changing tires to ones with a higher rolling resistance (These are in rough order, from the most effect to the least.) So let us know what your scan tool reads, what your mileage is and what you have already checked. Good luck and stick with it!