A couple of issues....

[zerovandez]

Hey guys. A few issues I'm starting to have in my 2002 TB 4.2L, 175K miles.

Issues
1. Engine temp reaches 3/4 mark sometimes in 95+ degree SoCal weather. AC is on.

2. Cooling fan clutch fully engages when in issue #1. Turn off AC, and the fan gradually disengages.
- If the fan was engaged before engine off - it will remain engaged until after I turn AC off at engine restart

3. Sometimes I get a pending code of P0172, when I get this, the engine idles at 900rpm
- idle goes back to normal after engine shut off and cool
- when 900rpm idle, while coming to a fast stop, the RPM drops below 500 and is close to stalling but does not. RPM's
- Usually happens when temps are hot
- will hunt for idle and will almost always settle at 900rpm when in this condition.

Changes/Upgrades/Maintenance in the last 40K miles
1. Spark Plugs
2. Valve cover, intake, and spark plug gaskets
3. Clean TB and air filter
4. ACD thermostat w/housing
5. Fan clutch
6. Water pump
7. Coolant flush w/50/50 Dexcool
8. Diff fluids
9. Fuel filter
10. Charcoal canister
11. Purge Valve
12. Vent Valve
13. Oil change done at 173k
14. Fuel cap
15. New ACD Radiator
16. CV shafts
17. New fuel pressure regulator and FPR vacuum hose
18. ACD coil packs
19. New o2 sensor
20. Cabin air filter
*All parts are ACD or GM. For some parts, Delphi was used.

Other
No check engine lights or reduced power. Only pending P0172 when in issue 1 and/or 2.

Need your help because I want to avoid buying/leasing a newer truck. I hate payments. Thanks!

 

[TJBaker57]

If by chance you use the Torque App I would monitor Fan Speed and Desired Fan Speed alongside ECT to see what is actually being reported.

 

[TJBaker57]

Did this high temperatures issue return suddenly or over time since early 2017 when you had a similar issue and did all those cooling system repairs?

 

[mrrsm]

From Delphi on the possible causes for the P0172 Code:

 

[Mooseman]

Was the fan clutch an ACD as well? Some of the aftermarket ones can be troublesome (store brands, Dorman and unknown Chinese). Does it still overheat even when the fan is fully engaged? What's the temps like with the A/C off? Get actual readings using a scanner or an ELM327 and Torque. Have you tried hosing the A/C condenser in case of blockage due to dust, dirt, sand or bugs? @TJBaker57 's suggestion of monitoring the fan speed is a good one. Here's his thread on adding the required PIDs to do that:

More PIDs for Torque App

A short time ago I posted a new pid I discovered for the Fan Speed of a 4.2 in a 2002 Trailblazer. It was properly suggested by MRRSM that I repost that information here so here I am. It is possible that this is my first original post! I have been spending a great deal of time fiddling with...

gmtnation.com

When you replaced the rad and tstat, you did use the correct Dexcool?
[Edit]: Just noticed you did so disregard

For the rich condition, which seems to be only when running hot, maybe you have a leaky injector that doesn't like it when it's hot. What are your fuel trims like overall? If you have a leaky injector(s) and is worse when hot, it might already be running richer than normal without triggering a code.

Using the PIDs in Torque, check the commanded vs. actual exhaust cam angles but it should trigger a code for that if it's off.

How's the cat? Any backpressure in the exhaust? That can cause overheating as well. Might also be something to do with the rich condition.

 

[budwich]

I think the fan operation per item 2 is correct.

You have certainly changes a lot of stuff.... but not any of the sensors associated with fuel air monitoring which relate to the p0172... maybe. Monitor your fuel trims at idle and again while running to see what is currently happening. The MAP is a likely candidate since you have a 2002, it doesn't likely have a MAF also for metering things.

Others queries about the health of your CAT need address as running rich for any extended period is likely push your cat over...

 

[Mooseman]

I think the fan operation per item 2 is correct.

Maybe, maybe not if it doesn't engage properly when commanded for a lower RPM and only works for higher/full engagement once overheated. Still worth investigating IMHO since everything has been replaced in the cooling system.

He did replace the O2 sensor as indicated in 19. The MAP sensor is a good idea which is just about the only thing that hasn't been replaced.

 

[zerovandez]

Thanks for the replies, guys.

@TJBaker57 - Since those repairs, the truck has not had any issues relating to overheating.

The video was helpful thanks. I checked the new FPR vacuum hose and new FPR - no signs of fuel.

The fan clutch is ACD as well. I don't have an ELM or Torque app. Would BlueDriver provide the same data? How to tell if there's backpressure in the cat? Is there any good tutorials on how to read and understand fuel trims?

Also, after I refuel, the truck is hard to start - it will crank over and eventually start but not immediately as it should. I don't have this problem after driving etc.

Very seldom, it will happen after the car is parked for a few hours and when it does start, there's a more pronounced smell of fuel. Again, this does not happen often at all but it does.

The other question is - When the temp gauge reaches 3/4 hot, is this normal? It never goes beyond that, not like what it did before with a stuck closed thermostat that TJBaker57 was referring to in 2017.

 

[TJBaker57]

Would BlueDriver provide the same data?

No.

 

[zerovandez]

So I purchased OBD Fusion and an OBDLink MX+ Adatper. Should get here tomorrow then I can monitor the STFT and LTFT. The following link gave me really good info on fuel trims - I think I have a better understanding now...

What Is Fuel Trim?

 

[zerovandez]

Results are in.

ECT
179.6F

Vacuum at idle AC on
-9.3

Vacuum at idle AC off
-10.2

STFT at 2k RPM (30 secs)
2.3%

STFT at idle
-5 to 5

LTFT at 2k RPM (30 secs)
3.1

LTFT at idle
5.5 to 8

STFT and LTFT looks about normal based from general fuel trim articles out there. But then again, it's not hot at all today as it was when I started this thread. I'll probably have to wait until next summer.

What does look off to me is Vacuum and ECT. Would you all happen to know what the GM specs are for Vacuum and ECT?

 

[Blckshdw]

ECT should be at least 195. Mine reaches that point with about 5 minutes of driving through my neighborhood (located in FL) How long had you been driving when you captured those readings?

 

[zerovandez]

I've been driving all day - stop and go, about 3 cold starts and AC on. Temp here in SoCal is around 76F. The thermostat is ACD as well installed in 2017 and was brand new.

 

[Mooseman]

Your trims look fine. I can't say if the vacuum is OK or not as I haven't looked at it before but it does look low. It would be normal for it to go down a bit when the A/C is on as the throttle has to open a bit more. You could install a new MAP. It's cheap and easy to replace. Could also check actual vacuum with a gauge.

Yeah, that temp is low. That tstat might have already died or never worked properly. Maybe it's partially stuck open and doesn't open enough at higher temps. Just sucks that you might have to do it again.

 

[TJBaker57]

FWIW, it was about 32F here today and my ECT stays right about 207 at highway speeds.

 

[zerovandez]

@Mooseman - Yeah I know, that job is not the greatest thing to do. Oh well.

@TJBaker57 - Is that 207 based on your dash gauge or from a scan tool?

 

[TJBaker57]

Is that 207 based on your dash gauge or from a scan tool?

Torque Pro app. Pid used has been previously confirmed with a Tech 2.

I spent a ridiculous amount of time this summer discovering additional PIDs as well. Like the cooling fan speed, desired fan speed by PCM, startup ECT, etc. I think I read that ODB Fusion can also setup custom PIDs??

 

[budwich]

my take on the numbers.... your idle LTFT at idle / warm is high. Your vacuum is low. The coolant temp seems low for a warmed engine. This may cause high / rich mixtures which also may be further "aided" by a low vacuum, which potentially might be a small intake leak or exhaust leak at the manifold (maybe).

 

[zerovandez]

@TJBaker57 - Thanks. Yes OBD Fusion does have the ability to load custom PID's. There's a few parameters to provide but it should ultimately work after I figure them out.

@budwich - Would you happen to know the acceptable range for LTFT?

I should also add that vacuum was measured in the OBD Fusion app and was added as a calculated PID. Meaning, this might be entirely accurate. I'll have to connect a vacuum gauge to get actual readings.

 

[budwich]

Just like all "gages", in general, the "design" of the system is to have things "easy" to see at a glance and understanding... so "0" would be a "target"... in and around that point would be good... but just like anything, as you move away, what's "good" and when are things are no longer "good". I would say you are on the outside of that. Further, the "fixed" STFT at 2k indicates that the system has dropped out of "closed loop". Not necessarily a problem because it is possible that may be a limit for the closed loop operation in idle.... but I would say this isn't correct.... but I don't know all the parameters that are involved... ie. throttle position, vacuum, speed, "engine load", timing advance, etc...

 

[TJBaker57]

I should also add that vacuum was measured in the OBD Fusion app and was added as a calculated PID

Can you tell me what is the PID and the equation?

I use 221142 which returns a percentage of voltage from the MAP sensor and I translate to PSIA. A/256*5*2.74+1.5. I checked this out this July....

 

[zerovandez]

I'm not exactly sure. I have to try and understand the following before I take a crack at it

OCTech Support

EDIT: I went in the app and noticed there was an "example" of a custom PID. It's equation is (A*256+B) / 4

Are these equations specific to the PID's - do they change or always stay the same regardless of PID?

 

[TJBaker57]

I'm not exactly sure. I have to try and understand the following before I take a crack at it

OCTech Support

EDIT: I went in the app and noticed there was an "example" of a custom PID. It's equation is (A*256+B) / 4

Are these equations specific to the PID's - do they change or always stay the same regardless of PID?

While some PIDs will use the same equation most will be tailored to the PID in question. For some the equation is simply 'A', meaning the value returned needs no further translation. The example you posted I recognize as an RPM equation where the PID returns 2 bytes represented by 'A' and 'B'. This is not the proper forum to go further explaining, but we can take this up in the proper section.

 

[TJBaker57]

should also add that vacuum was measured in the OBD Fusion app and was added as a calculated PID

One other comment regarding your vacuum. I'm fairly certain the numbers you posted are in PSIG, that is to say they are PSI relative to atmospheric pressure. Thus for say folks accustomed to seeing maybe 'in hg' those numbers would be multiplied by 2.03602 putting you around 19 or 20 in hg. Vacuum.

Edit: added a pair of screenshots to demonstrate. One at idle and another under a load. Note the two add to the neighborhood of 14.7, atmospheric pressure.

 

[zerovandez]

Thanks. Im not sure why the custom PID arent working in OBD Fusion. Im sure its something simple.

So this morning - driving as normal ac on. Come to a stop and the rpm dips to 400 then tries to stay steady at 600 then bounces to 1100, then drops to 900, then surges between 600 and 1100 causing the truck to lunge forward at higher rpms. I turn the ac off and made little change. Light turns green and the thing drives as normal. No codes, no pending codes. So strange.

 

[Mooseman]

I doubt that the low temps have anything to do with that but that low vacuum is a likely suspect or a symptom of what's going on. Wish you could get the throttle position data. Can you get the vacuum data while it happens?

 

[zerovandez]

Well the funny thing is that it would seem as though the obd port disables when the truck is in drive. Then, i have to kill the engine and start the bt obd2 connection again. I tried this with a small actron wired obd2 scanner as well - the minute i put in drive the obd port stops communication. Is this normal?

Im not sure how else to get the info we need to properly troubleshoot.

 

[Blckshdw]

That's definitely not normal. Is it only when you're in D? What about reverse or neutral? (I assume the lower gears would also cut the comms)

 

[zerovandez]

Happens when I move it out of Park. I did tap into one of the obd2 pins for my remote start module. I will remove it and see what happens.

 

[TJBaker57]

Im not sure why the custom PID arent working in OBD Fusion. Im sure its something simple

Here is a sample PID I set up in OBD Fusion....

The entry for 'Module/header can be either 10 or ECM. You must add '01' to the pid or it fails, or at least it does for me.

 

[zerovandez]

Thanks again, @TJBaker57. I changed the priority to med-high and the PID tested successful.

The next step to this drama is to get the ODB to communicate while driving. Thanks for hangin in there with me, guys. I really appreciate all of your replies.

 

[zerovandez]

So quick update on the high RPM. After driving for 40 minutes keeping steady at 75MPH, I get to the exit off ramp and come to a stop. The idle stays at 900 RPM - turn off AC no change. I drove to the destination and put in Park - idle stays at 900 RPM. I get in OBD Fusion and take the following readings....

Outside temps around 85F

ECT - 189
STFT - -9 to +10 (the ranges vary but does not exceed -20 or 20)
LTFT - -4 to +15 (the ranges vary but does not exceed -20 or 20)
Absolute throttle position - 9% (not really sure what this means)
Vacuum - -10.4 PSI (-21inHg)

Is there anything that seems off? Anything else I should look at? I'm still working to get that single lead from the remote start module off of the OBD port so I can monitor while driving.

EDIT: I came across a related thread. Looks like the ECT sensor resolved this issue for another user. I'll make sure to replace it as well when I work on the thermostat.

High Idle(Not talking about startup)

forums.trailvoy.com

 

[budwich]

the low temperature output of the ect continues to be a concern. After that speed and time, the engine should be at or above normal temp. Although the sensor can readily be checked with a meter (comparing resistance to chart), its probably a good idea to replace if there is any question in terms of its reliability. A faulting sensor will cause the engine to runs rich (ie. cold running). This may change some of your running parameters thereafter.

 

[Mooseman]

And since replacing that sensor is about as difficult as the thermostat itself, might as well replace both while you're there.

 

[TJBaker57]

Absolute throttle position - 9% (not really sure what this means)

OBD Fusion calls for the standard pid #11 for service 01 to get what OBD Fusion calls "Absolute Throttle" . If interested in this stuff this link is a good place to start.

OBD-II PIDs - Wikipedia

en.m.wikipedia.org

It is a representation of throttle opening in percent. It is not the accelerator position. If monitored one will notice it pretty much never goes to zero or 100%. The PCM directs where this is desired to be based on a host of inputs.

that single lead from the remote start module off of the OBD port

Do you remember what pin that is? Seems very odd to be losing the power when shifted from park. Equally odd to lose the communications, if it is not a loss of power.

 

[budwich]

The throttle position number seems relatively low for an idle number, iirc.

 

[TJBaker57]

The throttle position number seems relatively low for an idle number, iirc.

Agreed, while it isn't a number I look at much I seem to remember that being somewhat higher, like mid-teens maybe? I would be curious what the desired idle is.

 

[zerovandez]

Do you remember what pin that is? Seems very odd to be losing the power when shifted from park. Equally odd to lose the communications, if it is not a loss of power.

Hey TJ, Im using the iDatalink ADSTBSLPL module for OEM key fob remote start. Its orange lead (data) requires to be tapped to obd2 pin #2 - looking at connector - it is on top row, 2nd furthest to left.

 

[TJBaker57]

Hey TJ, Im using the iDatalink ADSTBSLPL module for OEM key fob remote start. Its orange lead (data) requires to be tapped to obd2 pin #2 - looking at connector - it is on top row, 2nd furthest to left.

So it is connected to the class 2 data bus. I see from online information that is a security bypass device. Would seem somehow to be interfering with data transmission to/from the scanner(s).

 

[TJBaker57]

I am of course assuming you verified power to the OBD Link MX after you move the shifter out of park.