NEED HELP High Negative LTFT / Poor Gas Mileage

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Hi GMTNation,

My name is Tony and I just got myself a 2007 TB LT 4x4, 4.2L I6 W/ 123,432 miles. During the 2hr drive home I noticed it didn't have the power I remember them having (borrowed one from a friend a few years back) and the gas mileage was not as great as I remember (15mpg Highway/3.42 gears).

Not having all the service records, I cleaned the throttle-body & MAF, replaced the spark plugs with ACDelco 41-103 and ensured gap was at .040, replaced all Ignition Coils and Up-Stream O2 w/ New ACDelco's, installed a Volant CAI and of-course changed all the Fluids except for the Coolant (recent Pump/Thermostat/Flush by previous owner). Finished off by running Royal-Purple Fuel Injector Cleaner in two back-back tanks of gas.

Upon logging the car during my Drives, I noticed that the Long-Term Fuel-Trims were consistently in the -7 to -12 range indicating a rich condition according to these forums. A compression test gave me 200-210 PSI across all 6 cylinders. A vacuum test gave me 20PSI of vacuum at idle and at 2,500 RPM. Blipping the throttle only quickly dropped vacuum before it shot right back up (so I'm assuming cat is not clogged). Fuel pressure is steady at 58 PSI at all RPMs (no fuel pressure regulator present).

I also occasionally get random knock while cruising and accelerating. It seems like I'm down to maybe a faulty injector/s? Below is a screenshot of my log. Does anything stand out?

TB Scan.jpg
 
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Sparky

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You might have a small intake leak causing it to try to richen up your mix constantly (resulting in poor mpg). Check your intake bolts.
 
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You might have a small intake leak causing it to try to richen up your mix constantly (resulting in poor mpg). Check your intake bolts.
Hi Sparky, correct me if I'm wrong but wouldn't an intake leak cause Positive Fuel trims and the car to run lean? I did check the 10mm Intake bolts for good measure and they were all tight...
 

Sparky

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You're right. I got my engine sides mixed up lol.

If your exhaust manifold is cracked (near the O2 sensor) it could be reading a bit of outside air and throw it off. That's happened to others before. Not a job I'd willy nilly do unless confirmed cracked though!
 
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You're right. I got my engine sides mixed up lol.

If your exhaust manifold is cracked (near the O2 sensor) it could be reading a bit of outside air and throw it off. That's happened to others before. Not a job I'd willy nilly do unless confirmed cracked though!
It doesn't smell like raw exhaust gasses when I'm under the hood and it's running, but I will take a closer look to see if I can see anything. Thank you for the suggestion!
 

Mooseman

Moderator
For the knock, can you pull the knock retard data? If it's sensing spark knock, it should retard the timing. Maybe it's related.

Is it throwing any codes? Looking at the pre and post cat O2 voltages, the post-cat is basically the same as the pre-cat, which normally means the cat is dead. Shouldn't be relevant to the rich condition though.
 
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For the knock, can you pull the knock retard data? If it's sensing spark knock, it should retard the timing. Maybe it's related.

Is it throwing any codes? Looking at the pre and post cat O2 voltages, the post-cat is basically the same as the pre-cat, which normally means the cat is dead. Shouldn't be relevant to the rich condition though.

The knock retard data is in the graph I posted. That Red line indicates how much and when it pulled timing. If you look in that row, it shows it pulled 3.2° at just after the peak. Is that what you meant?

Unfortunately, no codes. I've put about 500 miles on it and there isn't even a pending code.

Should the post cat O2 reading be stable? I think this may have something to do with how HPTuners is interpreting the data because the numerical reading seemed pretty steady if I remember right...
 

Sparky

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Post cat usually stable yes if things are all ok.
 

Mooseman

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Ah, missed that. Yes that's it. It shouldn't be knocking. Wonder if you have built up carbon or something. Try some premium fuel and see it it changes. Maybe it has bad fuel?
 
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Post cat usually stable yes if things are all ok.
Okay, so then it would seem that my Converter is okay. Also, I listened for a leak with a hose in and around the exhaust and I couldn't hear/smell anything. I don't think the exhaust manifold is the issue.

Ah, missed that. Yes that's it. It shouldn't be knocking. Wonder if you have built up carbon or something. Try some premium fuel and see it it changes. Maybe it has bad fuel?
I'm now on my third fresh tank of gas (last from different station), so I doubt it's bad fuel. I think trying premium is a good idea though, so I will give that a shot on my next fill up. Thank you
 

gmcman

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Do you by chance know how long the TB was sitting before you bought it? I wonder if it had bad fuel.

You could possibly have a faulty fuel pressure regulator.

You said you have a CAI, what does your fresh air ducting look like after the MAF meter? Is that stock?

I'm curious if some of your metered air through the MAF is going somewhere before the cylinders.
 

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(1) If Re-Gapped... or even Touched or Dropped for that matter... the Spark Plugs Center Electrodes can get easily damaged... as the Rare Earth Metals used (Platinum and/or Palladium) are applied thereon as Very Thin Coatings. If the Plugs get Gapped too closely... Ignition and Firing can "Get the Jump" ahead of the correct BTDC Timing and cause the Engine to Knock.

(2) If you cleaned your MAF with anything other than CRC MAF Sensor Spray Solvent... even using Brake-Kleen Spray... it can invite very sketchy MAF Behavior. Re-Check that your MAF-IAT Connector(s) do not have any loose wiring and are making good contacts on the MAF Body. Be very suspicious of the MAF Wiring Harness and examine it very closely, too.

(3) Verify that your Upstream O2 vs. Downstream O2 Sensors are reading correctly after re-setting Fuel Trims. If the Downstream O2 Sensor is reading Rich and upstream one is reading lean, there might be an Upstream O2 Sensor problem.

(4) If you have access to a Tech 2 or High End Scanner (HP-Tuners SHOULD have this as a Feature) Perform a Fuel Injector EFI Balance Test... and note whether or not you can see any excessively HIGH Base Line Fuel Pressure... as a search for a Faulty Fuel Pressure Regulator.
 

Mooseman

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The fuel pressure regulator is built into the fuel pump module on later models. Some testing of the fuel pressure should confirm if it is working correctly. An injector balance test, as indicated by @MRRSM , would also be a good idea.

How's the oil? If it was previously flooded, fuel sometimes gets into the oil and richens up the air/fuel mix via the PCV system.

When you cleaned the throttle, you did disconnect the battery for at least 30 minutes?

The post-cat O2 sensor should be showing an almost flat line, which tells the PCM that it is working. If it's mimicking the pre-cat, then it's likely toast. A pre and post cat exhaust temp check with an infrared temp gauge would confirm this.
 
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Do you by chance know how long the TB was sitting before you bought it? I wonder if it had bad fuel.

You could possibly have a faulty fuel pressure regulator.

You said you have a CAI, what does your fresh air ducting look like after the MAF meter? Is that stock?

I'm curious if some of your metered air through the MAF is going somewhere before the cylinders.
I believe it sat for a year, and there is evidence that there were mice/rats living between the intake manifold and the air-box. All the visible wires appear to be in-tact. But that was 2 years ago, the previous owner was daily driving it for the past year. I believe the 07's don't use a regulator because it is a return-less fuel system, and the Pump controls the pressure via PWM.

Below is picture of what the Volant intake looks like. I don't see how it could escape once it gets past the MAF without it causing a lean condition from the leak though. But you got me thinking. I'm going to install the stock Intake to see what kind of difference it makes.

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(1) If Re-Gapped... or even Touched or Dropped for that matter... the Spark Plugs Center Electrodes can get easily damaged... as the Rare Earth Metals used (Platinum and/or Palladium) are applied thereon as Very Thin Coatings. If the Plugs get Gapped too closely... Ignition and Firing can "Get the Jump" ahead of the correct BTDC Timing and cause the Engine to Knock.

(2) If you cleaned your MAF with anything other than CRC MAF Sensor Spray Solvent... even using Brake-Kleen Spray... it can invite very sketchy MAF Behavior. Re-Check that your MAF-IAT Connector(s) do not have any loose wiring and are making good contacts on the MAF Body. Be very suspicious of the MAF Wiring Harness and examine it very closely, too.

(3) Verify that your Upstream O2 vs. Downstream O2 Sensors are reading correctly after re-setting Fuel Trims. If the Downstream O2 Sensor is reading Rich and upstream one is reading lean, there might be an Upstream O2 Sensor problem.

(4) If you have access to a Tech 2 or High End Scanner (HP-Tuners SHOULD have this as a Feature) Perform a Fuel Injector EFI Balance Test... and note whether or not you can see any excessively HIGH Base Line Fuel Pressure... as a search for a Faulty Fuel Pressure Regulator.
1) Visually, the tip of the electrode looks perfectly fine. I did't have to gap them as they came pre-gaped. I verified the gap with a feeler guage, and didn't drop any.

2) CRC Maf cleaner is exactly what I used. I ran the car with the MAF disconnected to see what difference it'd make and I still had the negative fuel trims. Connector is clean and well seated.

3) Up-Stream Sensor is new, and I believe is responsible for reading my current Rich Condition? How much of a variance should I expect on the Post Sensor reading?

4) I don't recall seeing an FI Balance Test on the HPTuners interface but I will double check to make sure. Fuel pressure is solid at 58 PSI at all RPMS.

Thank you for the suggestions!


The fuel pressure regulator is built into the fuel pump module on later models. Some testing of the fuel pressure should confirm if it is working correctly. An injector balance test, as indicated by @MRRSM , would also be a good idea.

How's the oil? If it was previously flooded, fuel sometimes gets into the oil and richens up the air/fuel mix via the PCV system.

When you cleaned the throttle, you did disconnect the battery for at least 30 minutes?

The post-cat O2 sensor should be showing an almost flat line, which tells the PCM that it is working. If it's mimicking the pre-cat, then it's likely toast. A pre and post cat exhaust temp check with an infrared temp gauge would confirm this.
You're right about the return-less fuel system, there is no regulator. Fuel pressure is steady at 58psi. I'm going to try the injector balance test later today to see if it nets any results.

The Oil was changed almost as soon as I got it. I did disconnect the battery and left the switch in the "ON" Position for an hour while I worked on it.

The Post O2 sensor reading hovers from .720-.760 once warm. Pre-Cat temp is ~383° and post is ~482° .
 

gmcman

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You may want to run a compression test, I would strongly suggest this before moving forward. Definitely a base that needs to be covered.
 
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You may want to run a compression test, I would strongly suggest this before moving forward. Definitely a base that needs to be covered.
I did, info is in the OP. 200+ on all 6 cylinders
 

gmcman

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I did, info is in the OP. 200+ on all 6 cylinders
Oh, my mistake.

With it sitting for as long as you say, I can't help to not rule out an Injector issue and/or some type of restriction causing less than ideal spray pattern.

If vacuum is good, needle steady, that will show the intake valves are seating. Now check the exhaust by holding a piece of paper or envelope to the tailpipe at an angle allowing the exhaust to push it away, see if it gets sucked back into the tailpipe. Will either fold over with exhaust flow, or momentarily get sucked back to the tailpipe. If it gets sucked back, then you have en exhaust valve hanging.

Hard to say without seeing it, so just throwing out suggestions.

Would an evap issue show neg trims....what about air injection, does this motor have the air system?
 

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It will not hurt to purchase a LARGE bottle of Chevron Techron after the Next Fuel Fill Up... and dump the whole thing inside ...just to emulsify any residual condensation in the Tank and give the EFIs a decent treatment as well.
 
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Oh, my mistake.

With it sitting for as long as you say, I can't help to not rule out an Injector issue and/or some type of restriction causing less than ideal spray pattern.

If vacuum is good, needle steady, that will show the intake valves are seating. Now check the exhaust by holding a piece of paper or envelope to the tailpipe at an angle allowing the exhaust to push it away, see if it gets sucked back into the tailpipe. Will either fold over with exhaust flow, or momentarily get sucked back to the tailpipe. If it gets sucked back, then you have en exhaust valve hanging.

Hard to say without seeing it, so just throwing out suggestions.

Would an evap issue show neg trims....what about air injection, does this motor have the air system?
I really appreciate the suggestions!

I'm afraid you might be right about it being an injector issue :cry: . I just want to make sure I exhaust every other possibility before taking on an intake removal.

That exhaust test you mentioned is a great idea too. I will try that tomorrow to see what happens.

It will not hurt to purchase a LARGE bottle of Chevron Techron after the Next Fuel Fill Up... and dump the whole thing inside ...just to emulsify any residual condensation in the Tank and give the EFIs a decent treatment as well.
So far I've run through two tanks with Royal Purple's Fuel injector cleaner and I haven't noticed any difference. I know the Chevron stuff is kinda Legendary, and I usually run that but I wanted to try something different. I appreciate the suggestion 👍
 

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Small h/j... Since you are The Rare User of HP-Tuner Scanner Software... If you have the chance to work up any Articles for the Tech 2 Sub-Forum created by @Mooseman... Every little bit of additional Instructive Information you can provide will improve GMTN and Help other HP-Tuner Users to grow their 'Scanner Skill Set'... The Section for HP-Tuners is way down at the bottom of the Tech 2 Index Page. Thanks In Advance, Brother! :>)
 

gmcman

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The intake isn't that bad, a 1/4" drive ratchet is your friend....the rear bolt is a slight pita.

The front bolt comes off easy with the alternator out of the way, otherwise its tight.

I would hold off on the injectors for now, something else that works well is BG44K, avail at most dealerships.
 

Mooseman

Moderator
Looking at you underhood pic, noticed, and should have clued in, that you have a SAIS. That sucker can be responsible for A/F issues. Try pulling it and block off the port in the head, maybe sandwich some sheet metal or pick up a block off plate from an 02-03. It is a very troublesome system that we usually just delete and get tuned out of the PCM.
 
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Small h/j... Since you are The Rare User of HP-Tuner Scanner Software... If you have the chance to work up any Articles for the Tech 2 Sub-Forum created by @Mooseman... Every little bit of additional Instructive Information you can provide will improve GMTN and Help other HP-Tuner Users to grow their 'Scanner Skill Set'... The Section for HP-Tuners is way down at the bottom of the Tech 2 Index Page. Thanks In Advance, Brother! :>)
Yeah, I noticed nobody uses it for trouble-shooting during my search through this forum. I will look for Moosemans thread and put up some information along with some screenshots of what's available for this platform.

I already had mine for Tuning purposes on a Cadillac CTS-V that I have. I hooked it up to the TB not expecting it to work and it gave me full access to the Tune and Diagnostic functions. So far I've already used it to correct the Speedo, and will be using it further to tune once I solve the issue Im having.

Looking at you underhood pic, noticed, and should have clued in, that you have a SAIS. That sucker can be responsible for A/F issues. Try pulling it and block off the port in the head, maybe sandwich some sheet metal or pick up a block off plate from an 02-03. It is a very troublesome system that we usually just delete and get tuned out of the PCM.
Good catch! Im gonna try to use some sheet metal I have lying around to make a block off plate to see what it does.

The intake isn't that bad, a 1/4" drive ratchet is your friend....the rear bolt is a slight pita.

The front bolt comes off easy with the alternator out of the way, otherwise its tight.

I would hold off on the injectors for now, something else that works well is BG44K, avail at most dealerships.
How many Bolts hold the intake to the Head?
 

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There are (10) 'Trapped' 10MM Fasteners in a staggered pattern holding the Intake Manifold to the Engine Head using ONLY 87 Inch Pounds of Torque and what follows are some of the weird artifacts you should be aware of before you attempt to R&R the IM:

(1) There is a Small Square Port Centered as an Opening between the Upper Engine Crankcase that passes through the Aluminum Engine Head and thence into Intake Manifold. That Cube Shaped Port allows Pressurized Blow-By Gasses escaping from the Lower Crankcase to be routed upwards and 'inhaled' via the partial vacuum present inside of the IM via another Small Plastic Tube which allows those fumes to be ingested right back into the Intake Air Column of all (6) Cylinders and re-burned. (See Attached Images...)

(2) Concerning the (10) Bolts holding on the IM... There is a Small, Elbow Shapes Rubber Hose that leads from the Small Square Opening via a Right Angle Plastic Tube in the Lower IM Flange leading upwards to another Small Vertical Barbed Plastic Tube. The Rubber Elbow connects these two points together and it acts as the Blow-By Gas Conduit between the Engine Block and the Open Intake Manifold. (See Attached Images...)

(3) One of the (10) 10MM Fasteners is HIDDEN behind that Small Rubber Elbow...and thus it must be carefully removed to gain access to that Hidden Fastener. If the Elbow Hose gets damaged in any way during this R&R... I've included images of the ACDelco OEM Replacement Item below:

gmc-envoy-chevy-trailblazer-im3.jpggmc-envoy-chevy-trailblazer-im2.jpggmc-envoy-chevy-trailblazer-im.jpgACDELCOIMRUBBERELBOW2.jpgACDELCOIMRUBBERELBOW1.jpg
 
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There are (10) 'Trapped' 10MM Fasteners in a staggered pattern holding the Intake Manifold to the Engine Head using ONLY 87 Inch Pounds of Torque and what follows are some of the weird artifacts you should be aware of before you attempt to R&R the IM:
Wow, thank you so much for this information! I would have lost my mind trying to figure out why the IM won't come off after removing the obvious 9 bolts.

I spent part of the afternoon taking screenshots of the HPTuners interface after doing a little more troubleshooting. I'm convinced I must have a leaking injector. I will report back soon :smile:
 

MRRSM

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There is yet another artifact to be concerned with involving that Damned Metal Wire Harness Bundle Support Bracket that is a NIGHTMARE to completely R&R... If there is ANY way that you can remove the connecting (10) 10MM IM Fasteners and then gain enough room to sneak the EFI Rail In and Out WITHOUT having to completely extricate that Awful, Metal Bracket... by all means, do so.

I THINK it might work... as you only need enough room to undo the EFI Rail Hold Down Flange. IIRC... there are just (4) Bolts doing so) and then work the (6) EFIs loose without having to totally remove the IM from the work field. If you can pull this off... it WILL save you HOURS of time trying to get this job done. Start by unfastening the Four Post PCM Hold down Nuts and Studs and laying it off to one side without removing the Three Colored Wire Bundle Connectors. Otherwise, you will have to undo the Negative (-) Battery Cable prior to removing all three connectors and setting the PCM completely out of your work field.

Be advised that I am speculating here about this alternative Metal Bracket R&R possibility after the fact of enduring the harder alternative while performing an Engine Head R&R on a 2002 Chevrolet Trailblazer 4.2L Motor. If it comes down to cases and the FULL IM R&R is required... You WILL need to go through the Driver's Side Upper Wheel Well in order to be able to access those Damned Metal Bracket Fasteners right along with undoing all of the Harness Bundle Restraints attaching them to the Metal Bracket as well... or they will conspire against you and restrain your movements while manipulating the IM ...like the "Snake Hair on The Severed Head of Medusa'". I hope you can explore and succeed with this idea. The Brand New ACDelco OEM Fuel Injectors are available from RockAuto via this Link:

1559207071920.png
 
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There is yet another artifact to be concerned with involving that Damned Metal Wire Harness Bundle Support Bracket that is a NIGHTMARE to completely R&R... If there is ANY way that you can remove the connecting (10) 10MM IM Fasteners and then gain enough room to sneak the EFI Rail In and Out WITHOUT having to completely extricate that Awful, Metal Bracket... by all means, do so.
PARTIALLY SOLVED: My high negative LTFT was being caused by the Volant After-Market intake. Putting the stock Intake back on solved the issue, however, my gas mileage is still terrible.

I had already purchased a set of refurb injectors so I decided to take on the task of replacing them, and Mr. RSM, you were NOT kidding about that bracket bolt! 😩 Luckily, I was able to get it off somehow with the help of a small ratcheting 10mm wrench.

The injectors all looked just fine, and replacing them made no difference but at least I was able to clean out all the trash the rats had left behind.












89523
 
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So after placing the stock intake back on and replacing the injectors, my gas mileage is still ~10mpg in the city and ~16 mpg on the highway, and the truck just feels sluggish and weak.

After going out for a cruise so I can log some data, I noticed that my Calculated Engine Load is pretty high. In park w/ AC off and idling with the engine up to temp, it sits at about 37%. Putting it into drive and at a stop, it goes up to 43%.

Cruising with the cruise control at 63 mph, load is at 88% and the torque converter takes a good while to lock up despite the road being flat (60+ seconds/no incline). I commanded the Coverter to lock up and the engine load went to and stayed at 100%. Coasting to a stop the engine load hovers at 33% and the Wideband O2 shows a steady 14.7:1 AFR even on De-cel.

Do these values seem normal? I found this thread about high engine load and the problem ended up being a bad torque converter...

High Engine Load

My brake temperatures are pretty even after a long drive, so I don't suspect a stuck brake caliper. Does the front driveshaft spin even when in 2WD on these vehicles?

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MRRSM

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If you can do this without inviting the Invention of a Whole New Set of North American English Swear Words... Remove the Upstream O2 Sensor... and while trying to ignore the horrendous racket it will make while you take the SUV on a Short Test Drive ... take it out on The Highway just long enough to evaluate your Transmission Shifts and to see if there is ANY Bump in Performance. If the difference is real and significant... Your Culprit will probably be a Clogged CAT.

By the way... Now that you've Conquered the IM R&R (Congratulations on that work, by the way)... You'll never have to Prove your Manhood in any other way ...for as Long as You Live... :>)

 
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If you can do this without inviting the Invention of a Whole New Set of North American English Swear Words... Remove the Upstream O2 Sensor... and while trying to ignore the horrendous racket it will make while you take the SUV on a Short Test Drive ... take it out on The Highway just long enough to evaluate your Transmission Shifts and to see if there is ANY Bump in Performance. If the difference is real and significant... Your Culprit will probably be a Clogged CAT.
I removed the Up-Stream O2 and plugged the hole with a rubber cork that attaches to a vacuum gauge, and there is almost no back pressure at all. Even holding the RPMs at 3k showed no measurable back pressure. I get a slight blip to about 4 psi when I stab the throttle at idle but it immediately comes back down. Pre-Cat temp is ~383° and post is ~482° once warm.

By the way... Now that you've Conquered the IM R&R (Congratulations on that work, by the way)... You'll never have to Prove your Manhood in any other way ...for as Long as You Live... :>)
The experience definitely made me feel more like a man once I finished :2thumbsup:

And thanks again for the info you supplied me with. I couldn't have done it without it!
 

gmcman

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The calculated load will read near 100% at the slightest throttle input, nothing abnormal.

Do you have 4WD? If so, are you driving around in A4WD or 2WD? A4WD will consume more fuel as your front wheels are partially engaged....for lack of better terms.
 
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The calculated load will read near 100% at the slightest throttle input, nothing abnormal.

Do you have 4WD? If so, are you driving around in A4WD or 2WD? A4WD will consume more fuel as your front wheels are partially engaged....for lack of better terms.
I do have 4WD, but always drive around in 2WD. What doesn't make sense to me is why the converter takes so long to lock up, if at all. Also, when logging AFR in other GM cars, I've noticed that during deceleration, AFR goes sky high as I'm guessing the PCM pulls fuel to reduce consumption when not necessary. Here, it stays at right at 14.7 as if it was under load.
 

gmcman

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If your trans is stock, then it will still have the Pulse width modulated lockup. The lockup will be almost inperceivable until you step on the gas slightly and the tach doesn't jump up. The converter will unlock on a coast.
 

MRRSM

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The next time you Datalog... include making some observations of the ACC pedal High and Low Voltages in response to the different positions its in when it feels like the engine is struggling. I'm just curious if they are within acceptable PWM Signal levels, starting from an Idle Pedal all the way up to "Flooring it for WOT". Also... observing the percentage that the Throttle Plate finds itself precisely positioned at when the Engine struggle is present may also be revealing whether or not the Throttle Body is also NOT behaving itself.
 

gmcman

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I took a few screen shots today of the Torque app. Ambient temp was 92 deg, my LTFT range from 0 to -10ish depending on conditions..generally average around -5.

This was after sitting in traffic then was able to get up to speed. Note the high intake temp which can change things. If I'm not mistaken, at or around 110 deg intake temp, either the timing and/or fuel trims change to compensate for the less dense air charge. I believe the fuel trims lean out a little...don't hold me to that.

Cruise at hwy speed, level ground, after sitting in traffic.

Screenshot_20190625-175201_Gallery.jpg

This is accelerating after the previous screenshot.

Screenshot_20190625-175225_Gallery.jpg

After about 5 miles, the Intake temp dropped to the average 5-7 deg above ambient at cruise.



Screenshot_20190625-175427_Gallery.jpg


My MPG's are pretty good, and the engine runs great, some negative LTFT's of -5 to -7 have been quite normal for me.
 

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My MPG's are pretty good, and the engine runs great, some negative LTFT's of -5 to -7 have been quite normal for me.
Thank you SO MUCH for showing me these logs. I had already started looking for Trailblazers that I could rent on TURO just so that I can compare my logs to one that is working well. When you say you get good gas mileage, what are you averaging?

Do you have the stock intake on your vehicle? If it's not too much to ask, can you show me what engine load is when cruising at HWY Speed?
 

gmcman

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No problem.

Yes, intake is stock, except I added some thin vinyl from a gutter guard and fabricated a more direct path for cold air to enter the air filter from next to the radiator. Other than that, from the air filter to the TB it's stock.

I tried to find a level section of road, here's a steady cruise.

Ambient temp 91 deg.

Coolant temp 210 deg


Screenshot_20190627-015114_Gallery.jpg

Very slight downgrade

Screenshot_20190627-015247_Gallery.jpg


Very slight acceleration

Screenshot_20190627-015202_Gallery.jpg


Hope this helps.
 
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Thank you @gmcman , this most definitely helps! :2thumbsup: What year TB do you have? Is is 4WD? Your cruising Load is lower than mine, but not by much. I wonder if my tire size is what's making up the difference.
:squint:
 

gmcman

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2002, 4WD.

Tires play a huge part. Pretty much low rolling resistance (LRR) tires are almost a necessity.

Do you have aggressive tread? Is the vehicle lifted?

I have had the best mileage with the now discontinued Michelin Cross Terrain (OE tire), Michelin Latitude Tour (Non -HP), Michelin Defender LTX, and not far behind was the Continental Cross Contact LX-20.

For all the above listed, 35-36 PSI cold seemed the best. At around 30-32 PSI cold I could see a slight drop in fuel economy.

Check for dragging brakes, you would be surprised at how little the calipers need to stick to drag you down. Sometimes it could be an old brake hose.

Summer heat will lean the fuel mix a little, as well as elevation.

If you drive at say 75 MPH then don't expect the best fuel economy.
 

whodwho

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... Also, when logging AFR in other GM cars, I've noticed that during deceleration, AFR goes sky high as I'm guessing the PCM pulls fuel to reduce consumption when not necessary. Here, it stays at right at 14.7 as if it was under load.
I would like to bring this up again as no one had commented on this. I have also witnessed this and it doesn't seam correct. On warm up I will see it vary downwards of 8.5 but once in CL it stays exactly @ 14.7 no matter the load.

This was observed on a trip I just took with a ScanGuage, I need to check with a scanner app and see if it shows the same. I also noticed it doesn't seem to go into fuel cut on decel often and I believe still showed at 14.7?? but again that could be the ScanGauge or I might of missed it, I need to dig out my adapter.

Does anyone see it vary from 14.7 once warmed up?
 

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