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Discussion Starter #1
Hey guys, I'm starting this thread to document and hopefully get some advice on troubleshooting a minor misfire that's plagued our truck for quite some time. Our gas mileage is now showing around 9mpg, however we have been sitting idle with the a/c on more frequently waiting for kids to come out from their random summer activities. Before the long idle periods, the best we were getting was about 11mpg. When the engine is cold, it will sometimes very clearly misfire for a few seconds, and can hear and feel a very faint misfire while idling while warm. I've noticed the clutch fan engaging more and more when it shouldn't, so I just replaced that, which had little effect on the gas mileage. I will post the latest codes when I read it again tonight.

Here are the items which I've replaced within the last year, based on what I've read from this forum....
Front and rear 02's
champion oem equivalents, however just last night I swapped in AC Delco Oem's as some say other brands sometimes cause problems.
Replaced two ignition coils which I thought were misfiring. Didn't seem to do anything, so I swapped them with the other coils one by one, didn't seem to make any difference.
I also unplugged each coil as the truck was running, and there would be a very clear misfire each time with each coil... however still with them all plugged in, just the slightest misfire that you can barely hear, but it is there.
FPR
Crank position sensor
Cam position actuator solenoid
fuel filter
oem fuel pump
cleaned throttle body thoroughly
reset computer

I just ordered new coils to replace the rest of the old ones, should have them tomorrow. I plan to pressure or leak down test the cylinders and also check the fuel pressure next. Anything else I should check or try? I think it's about time to retire this old truck and get soemthing more fuel efficient, but figured I'd try one last time to get the mpg back up to where they should be.

update
Yesterday my wife said that she went to pull out into traffic and the truck "didn't want to go". I watched the TPS on my scanner, and it won't go past 30%. I checked the potentiometer at the pedal, and also the throttle body circuit. There is continuity where it needs to be and a smooth voltage/resistance when the pedal is pressed or the throttle plate is turned, 0-5 volts as it should. Now today, out of nowhere it started running really rough. A quick inspection shows no. 6 coil is dead. Still waiting on the new set of coils. Any ideas on the 30% throttle?
 

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2004 chevy trailblazer_ls
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Hey guys, I'm starting this thread to document and hopefully get some advice on troubleshooting a minor misfire that's plagued our truck for quite some time. Our gas mileage is now showing around 9mpg, however we have been sitting idle with the a/c on more frequently waiting for kids to come out from their random summer activities. Before the long idle periods, the best we were getting was about 11mpg. When the engine is cold, it will sometimes very clearly misfire for a few seconds, and can hear and feel a very faint misfire while idling while warm. I've noticed the clutch fan engaging more and more when it shouldn't, so I just replaced that, which had little effect on the gas mileage. I will post the latest codes when I read it again tonight.

Here are the items which I've replaced within the last year, based on what I've read from this forum....
Front and rear 02's
champion oem equivalents, however just last night I swapped in AC Delco Oem's as some say other brands sometimes cause problems.
Replaced two ignition coils which I thought were misfiring. Didn't seem to do anything, so I swapped them with the other coils one by one, didn't seem to make any difference.
I also unplugged each coil as the truck was running, and there would be a very clear misfire each time with each coil... however still with them all plugged in, just the slightest misfire that you can barely hear, but it is there.
FPR
Crank position sensor
Cam position actuator solenoid
fuel filter
oem fuel pump
cleaned throttle body thoroughly
reset computer

I just ordered new coils to replace the rest of the old ones, should have them tomorrow. I plan to pressure or leak down test the cylinders and also check the fuel pressure next. Anything else I should check or try? I think it's about time to retire this old truck and get soemthing more fuel efficient, but figured I'd try one last time to get the mpg back up to where they should be.

update
Yesterday my wife said that she went to pull out into traffic and the truck "didn't want to go". I watched the TPS on my scanner, and it won't go past 30%. I checked the potentiometer at the pedal, and also the throttle body circuit. There is continuity where it needs to be and a smooth voltage/resistance when the pedal is pressed or the throttle plate is turned, 0-5 volts as it should. Now today, out of nowhere it started running really rough. A quick inspection shows no. 6 coil is dead. Still waiting on the new set of coils. Any ideas on the 30% throttle?
Your bad MPG is just the misfire, your wasting TONS of gas every time you have a cylinder not firing. Any time I start getting a misfire on these things I always go coils first. Between all my Blazers and s10s I cant tell you how many coils I've had to replace, all the other stuff Id check after doing coils first.

But that being said all your problems listed could be a bad TPS. Cheap fix! Bad coil or bad TPS means way low power, way worse gas mileage, terrible idling, etc.
 

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2002 chevy trailblazer_lt
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When you are watching on your scanner do you see both TPS and both accelerator pedal sensors? Do they agree (tps 1 & tps 2 agree and acc pedal 1 & acc pedal 2 agree)

Does the Reduced Engine Power light illuminate on the dash?

I saw no mention of spark plug replacements.
 

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2002 gmc envoy_slt_xl
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Discussion Starter #4
Only one TPS value shown, I will try a different app or see if it will show both sensors, however testing both sensors with the multitester showed they were normal. No reduced engine power light on the dash. Yes, had champion iridiums for quite a while, then just recently switched to the AC delco 41-103, but no change in performance or misfire. Amazon just notified me that my coils are here, I'll replace all 6 and see if that does anything.
 

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If no REP then your throttle and pedal sensors are likely OK. If they don't behave the PCM puts you in REP pretty quickly.
 

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Discussion Starter #6
If no REP then your throttle and pedal sensors are likely OK. If they don't behave the PCM puts you in REP pretty quickly.
That was my thought, but I'm guessing the throttle reading comes from the ECU, which is getting the 0-5V signal from the pedal sensor..... I'm heading to the salvage yard today to grab a pedal sensor and throttle body, that's all that I can do at this point. No CEL, no nothing.

update
Replaced all of the coils last night, reset ECU and checked compression. Compression numbers were all around 190-205. The misfire while idling has gone away, however it still misfires a few times within the first few seconds of starting up. I'll give it a while so the ECU can adjust after the reset, but it runs a lot better.
While I was out I played with the accelerator pedal, and the engine is behaving strangely. So I know the ECU is only reading up to 30% throttle (pedal depressed about 1/4), and it goes no further, no matter how hard the pedal is pressed. If you give it a moderate to a lot of throttle, the engine stumbles for a quick second, accelerates a bit, for maybe a second or two, then the rpm's keep climbing, but the truck barely accelerates. It almost feels like if a torque converter was slipping, as the engine sounds normal, like there is a disconnect between the motor and transmission. This 30% max throttle, slipping feeling, and limited engine power only started a few days ago.
 

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So I know the ECU is only reading up to 30% throttle (pedal depressed about 1/4), and it goes no further, no matter how hard the pedal is pressed.
'Throttle' readings from the ECU refer to throttle plate opening at the throttle body, NOT the accelerator pedal. The ECU determines how much throttle opening is desired based on a number of factors only one of which is the accelerator pedal position. 100% accelerator pedal will not cause 100% throttle opening.
 

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Yeah .... my thoughts too. I just hate beating the drum about transmissions.

Gas mileage as low as he's experiencing... when most things engine-wise are close to normal, usually indicate energy consumption on a grand scale in the running gear or someone is stealing gas from you all the time.

Hanging out at an old buddy's transmission shop (he moved to 59840 ten years before I moved here, rats!) ... I'm seeing a LOT of welded clutchpacks. LOTS OF THEM!

We're thinking that something bad has happened to ATF recently, 'cause we'd see ----- MAYBE ---- onesies or twosies of this occurrence in a year!

I'd be testing for 2nd or 3rd gear startups, dragging Intermediate clutches, maybe crossfeeds or the PWM going lower in range ....
......or perhaps the front pump is in major high output because it stuck there or is incorrectly being told to go 100%.​

That will usually destroy the front pump after pushing it to the limit for too long.
  • The first sign of imminent pump failure--- and this observable window is very small for a timely-enough-to-save-the-unit observation ... is a large ATF leak under the bellhousing.
  • This USUALLY means the annulus is trying mightily to contain scavenged "pump blowby" and it gives up .... sometimes taking the front seal AND the front pump bushing out with it.
  • Bad news either way!
I hope all your troubles are small ones, but sometimes the bear eats us.
Gawd! That sounds like a wedding toast!​
 

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Discussion Starter #10
Wow, hope it's not the transmission. I rebuilt the thing about 10-15k miles ago because I blew out the sun gear shell. New bushings, seals, clutch packs, torque converter and whatever else the message forums recommended be replaced while I was at it.. It had poor gas mileage before that, and the same poor gas mileage after the rebuild. The reason I don't think it's transmission related, is that even in park/neutral, just revving the motor, it is very slow to run up the rpms. It doesn't feel like anything is under load at all, it just feels like exactly as the 30% throttle would feel that I'm witnessing on the logger. I'll swap out the pedal/TB today and see if that does anything at all. Could it be at all possible there is something wrong with the pedal sensor or TPS and no CEL or RES? Also, I've read of mixed information about a TPS relearn, is this something?
 

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I don't think it's transmission related
My thoughts about the transmission are the result of your saying the engine rpms go up but the truck isn't moving (or not moving much) . If not the transmission where are these rpms going then?

Perhaps some
clarification of the former statement is in order. Are we talking like 3000 engine rpm while little or no acceleration or are we talking 1200 or 2500? 4000? Details matter.
 

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My thoughts about the transmission are the result of your saying the engine rpms go up but the truck isn't moving (or not moving much) . If not the transmission where are these rpms going then?

Perhaps some
clarification of the former statement is in order. Are we talking like 3000 engine rpm while little or no acceleration or are we talking 1200 or 2500? 4000? Details matter.
His having to break something loose to get it to move in Reverse is typical of welded clutches too.... just sayin'..........
 

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Discussion Starter #13
My thoughts about the transmission are the result of your saying the engine rpms go up but the truck isn't moving (or not moving much) . If not the transmission where are these rpms going then?

Perhaps some
clarification of the former statement is in order. Are we talking like 3000 engine rpm while little or no acceleration or are we talking 1200 or 2500? 4000? Details matter.
I get what you are saying, it makes the most logical sense, but off throttle everything feels pretty good. I've had a torque converter slip before and it doesn't feel like that (although it can come with many different symptoms), it's hard to explain. I only hit the gas like that a couple times down my road, my eyes were fixed on the road for the most part, and my perception could be off. It may be that it was slowly going up to speed, and while still in 1st or 2nd gear, felt like it was not accelerating much. The sound of the engine struggling may have made it feel as though it should have picked up more speed. One thing I've read that matches a lot of what people describe is a clogged catalytic converter. I don't see how one day it's fine and the next there is a clog, but just to eliminate airflow problems I'm going to pull the front 02 sensor and see if it breathes better before I swap TB & pedal unit. I'd guess the rpms were up to around 4000 rpm when I let off the gas. It did accelerate, just very slowly it felt. Again, maybe my perception is just off. I will try again, this time record speed/rpm & throttle position tonight and see what you guys think.
His having to break something loose to get it to move in Reverse is typical of welded clutches too.... just sayin'..........
That's how the sun gear broke. I put it in reverse while on a hill and POW!. It shifts and drives perfectly normal now, just when you push beyond 30% on the accelerator, things get weird quick.
 

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OK - now you've got me going down another rabbit hole.... so here goes.....
  • Thermostat stuck partially open - makes engine too cold and ECM will compensate the best way it knows how and turns UP the fuel - bad mileage and crappy performance.
  • Bad CTS - that falsely reports lower/other-than-normal temperature to the ECM.
  • Plugged CAT.
  • Low fuel delivery.... pressure-wise and/or flow-wise.
Now my rabbit is going out on a limb here - but if you've already tested and dismissed these thoughts, then iggy them.....

Does your vehicle have Traction Control?
If so, is the Traction light ON?​
Have you tested the FPR?
Pulling the pre-cat oxygen sensor should provide a LOT of exhaust - if the cat element(s) have all congealed into a massive lump that won't pass exhaust.
Cats are known to plug up in a New York Minute too.......
Have you scanned the post-cat O2 sensor?​
Is it talking normally somewhere in mid-range or stuck HIGH or LOW?​
Is the voltage of the pre-cat and the post-cat the same value?
Ya know - I'm spitballin' this here and I'm not even sure you HAVE a post-cat sensor ---- <ahem>​

the rabbit's gonna take a nap now........ZZZZZZZZZZZZZZZZZZZZZZzzzzzzzzzzzzzzzzzzzzzz,,,,,,,,,,,,,,,,,,
 
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Discussion Starter #15
OK - now you've got me going down another rabbit hole.... so here goes.....
  • Thermostat stuck partially open - makes engine too cold and ECM will compensate the best way it knows how and turns UP the fuel - bad mileage and crappy performance.
  • Bad CTS - that falsely reports lower/other-than-normal temperature to the ECM.
  • Plugged CAT.
  • Low fuel delivery.... pressure-wise and/or flow-wise.
Now my rabbit is going out on a limb here - but if you've already tested and dismissed these thoughts, then iggy them.....

Does your vehicle have Traction Control?
If so, is the Traction light ON?​
Have you tested the FPR?
Pulling the pre-cat oxygen sensor should provide a LOT of exhaust - if the cat element(s) have all congealed into a massive lump that won't pass exhaust.
Cats are known to plug up in a New York Minute too.......
Have you scanned the post-cat O2 sensor?​
Is it talking normally somewhere in mid-range or stuck HIGH or LOW?​
Is the voltage of the pre-cat and the post-cat the same value?
Ya know - I'm spitballin' this here and I'm not even sure you HAVE a post-cat sensor ---- <ahem>​

the rabbit's gonna take a nap now........ZZZZZZZZZZZZZZZZZZZZZZzzzzzzzzzzzzzzzzzzzzzz,,,,,,,,,,,,,,,,,,
That's just what I wanted to hear! Leaving work now, will elaborate later...but I have had strange things with the downstream 02 recently!
 

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Discussion Starter #16
Wife just scanned after a CEL, P0171 , system too lean, bank 1. This is a new CEL which came on before the throttle issue, and I thought It was due to a vacuum leak, leak has been plugged, and now the P0171 is back, a separate issue or is it tied somehow?
 

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Discussion Starter #17 (Edited)
OK - now you've got me going down another rabbit hole.... so here goes.....
This has helps a lot, I really appreciate the info. I'm very mechanically inclined but a terrible troubleshooter. So I'm putting all of the pieces together, and I'm finally getting somewhere with this truck. 1st, checked the actual temp reading, it's 84*C/183*F. Just picked up a new T'stat, will install that over the weekend. I will look into the CTS sensor if the temp doesn't deviate after the new T'stat.
No traction control, and a new FPR was installed last year, although I plan on checking the actual pressure with a gauge at some point in the near future.
As for the cat, I did pull the pre cat 02 and ran down the road and did some aggressive acceleration. So it did provide more pep, and was able to accelerate a bit quicker, but not a definitive amount. I'd say I probably do have at the very least a partially clogged cat. Your downstram 02 comment solidifies this as well. Last year I was getting a CEL. I forgot the code, but it had something to do with the downstram sensor was reading constant. I replaced with a generic 02 and same CEL. This time I watched the graph live. front 02 was cycling as normal, rear was stuck (don't remember which way). If the throttle was blipped or pressed, it would begin to cycle sometimes, very erratic though. I then installed a more reputable brand. I don't remember if it was bosch or AC Delco. That removed the CEL, but the rear was never cycling as it should. My theory is that a partially plugged cat is causing low exhaust flow, the rear 02 to read abnormal, and the ECU throws the P0171 code, system too lean? There is also a very minor exhaust leak which could further provide a place for exhaust to escape before the rear 02. I also noticed that the long term fuel trim has climbed to 14%, the ECU was just reset a couple days ago, so it may go up even more, so you are correct, it is trying to compensate with more fuel, possibly due to the inaccurate rear 02 reading?
I did notice when accelerating hard this time with the 02 out that once the pedal was depressed 1/4, this is when the strange hesitation begins for a second or two, then the engine revs up, and it does in fact accelerate in line with the rpms, somewhat slowly, if I press the pedal to the floor, it takes a second, but it then downshifts, but it takes an extended period of time, the timing and intensity of the shift feels harsh and off. The shifting I've noticed is out of place as well vs. where the pedal is and how aggressively I push. I pulled the TPS and pedal plugs and check resistance on the pins again. This time I've found the the pedal sensor 1 AND 2 both lost continuity when the pedal was pressed about 1/4 of the way. It read normal the other day when I did the exact same tests. edit - I also had my daughter press the pedal to the floor while I observed the TB plate, and it would only crack open 1/8 of the way. This should explain the strange shifting, stumble, and revving that is going on, as the throttle plate, fuel, and transmission are getting the wrong reading, everything goes out of whack, and the ECU has no clue that there is a problem coming from the pedal sensor, as evidence by no CEL. I went to install the used pedal switch I picked up from the salvage yard, and the plugs were different, so back to the yard I go today to get the correct one. Once the throttle is straightened out, I'm going to log 02's, then remove the cat, and log 02's and see how the truck feels before I jump to buying a new cat. I have a feeling it's partially clogged. I will update this thread with results.
 

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Downstream O2 sensor should be fairly steady once at operating temperature (catalytic temp, not ect). Upstream O2 sensor should oscillate. Both will go high with hard acceleration. Both will oscillate before the catalytic heats up and 'lights up'.

Normal cruising:

Screenshot_20200710-145645.png


Jumps high at hard acceleration:

Screenshot_20200710-145714.png
 

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Discussion Starter #19
Downstream O2 sensor should be fairly steady once at operating temperature (catalytic temp, not ect). Upstream O2 sensor should oscillate. Both will go high with hard acceleration. Both will oscillate before the catalytic heats up and 'lights up'.

Normal cruising:

View attachment 55456

Jumps high at hard acceleration:

View attachment 55457
Okay, thanks. My only other experience with 02's is with a V6 that had the 02's right after the turbos on each bank, so they would behave the same. Obviously the truck is different with one pre & post cat, would have thought that both should oscillate the same in closed loop though, but it clearly doesn't.
 

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The main function of the O2 sensors in these trucks is to monitor efficiency of the catalytic converter.
 
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