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Discussion Starter #1
Ok...I need the Trailblazer/Envoy gurus!! We have an 04 Trailblazer with 4.2 with a little over 200K on it. Been running perfectly for a couple of years since we got it.
Now about 3-4 times when we first got it if you gave it a lot of gas taking off fast it would go into reduced power mode and was fine on restart.
I never even ran the codes. It never did it to me as I take off easily. My wife got used to not hammering it and it was fine for 2 years.
I knew it was probably the tps on throttle body.
Out of the blue one day it started fine and then after about 20 to 35 seconds died. The reduced engine power light ( REP) came on as it died and stayed on.
It would not restart.
When turning key off and back on all the dash light would come on for the normal test etc and the y would go out but the REP light would stay on. It never did this before.
Tried a bunch of times to restart and no luck.
Was pushing it back to driveway and hit the key and it started and was fine all day.
The next day, same thing. Ran 30 seconds and died.
I put it in N and rolled it about 5 feet and it started up. I did this at least 3 days and it was fine for rest of day.
Then that stopped.

I went to pick and pay lot and got a fairly clean throttle body and replaced it. Same thing the next day.
Then I got another pedal...same thing.
I was getting all the signal a and b mismatch pedal codes etc and the loss of 5v reference and a cam sensor code.

I bought a new cheap amazon throttle body and put it on and did reset calibration procedure and it started right up and ran fine.
Did pop a cam code multiple times and I swapped it out for a used one and no change it was fine for a few days.
Then it would not crank one day. Went out and dash lights were dim but goo battery.
Has son sit in it and AS SOON as it touche the fuses inside the bod under hood the dash lights brightened. AHHH Happy now I'm getting somewhere.
One of the ing or pcm fuses was loose from having someone use one of those stereo taps and bent it out too wide.
I fixed that and it has been fine for about 3-4 MONTHS!

Yesterday I started it and in 30 seconds it died and REP light popped on and no start with light staying on just like originally. I pulled the fused and no change until I got to the yellow 20 amp pcm B I think by the fuel pump micro relay. I pulled the fuse and relay and when I put them back in the REP light was off with key on and it started and ran fine the rest of day.
I cleaned off fuse terms one last time with my shirt tail and hoped for the best.

Today, 20 seconds and died.
Pulled relay and fuse and on the 2nd time pulling them and putting back in it started!!!
I'm at a loss now.
I don't think it's a lose connection at lest not at the fuse and the FP relay wouldn't make the REP light come on.

It only messes up 20 secs after cold multi hour sitting start.

I did smack the top of ecm under hood with palm of my hand but don't think that did it. I pushed on the 3 connectors and wiggled and looked at wires and all look ok.
I think I might pull the 3 big connectors and reseat them but I am at a loss.

HELP!!! Please
 

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2004 gmc envoy_slt
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I would look at the wire harness going to the fan clutch. This harness carries the 5V reference that is also used for the throttle body. Sometimes the wires abrade against the radiator support and short out the 5V intermittently.
 

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Discussion Starter #3
Thank you. I will look. I have had the connector off before as the clutch, as typical, doesn't work like it should. Actually it seems to work ok and never gets hot and only seems to be a little tight sometimes but I have had the code since we bought the truck.
From what I have been told, the 5v reference goes to several places. I saw a list a couple of times the exact ones. Apparently when the throttle body is flaky is shorts out the ref signal too and other things can do the same with losing the signal or not carrying it through that particular sensor so you get a code.

Yesterday I unplugged the throttle body and fan and it still wouldn't start and REP light on.
Once I pulled and re-seated the relay and fuse and light went out it started with or without the throttle body or fan being plugged in.
Demons....
 

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Discussion Starter #4
Today it was fine! Possessed I tell you.

Only thing I did yesterday was pull the yellow 20 amp pcm b fuse out and wipe of terminals with shirt and bend them a little each direction to maybe help connection.
Then I DID swap the 2 electronic black relays for fan and PCM. MAYBE the computer relay it flaky? Maybe something is drawing too much or shorting out and pulling more on relay but not enough to pop a fuse which I assume should happen if a short.

I will see what it does tomorrow.
Fingers crossed....
 

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2006 gmc envoy_denali
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You stated a couple of things that seem to be noteworthy.
1) the REP light and or starting issues has been something the car has done since you have had it.
2) disconnecting power to the PCM then reconnecting seem to allow a normal start and run with throttle housing disconnected.
If I have restated these comments correctly, you may have a faulty PCM but surprised you did not receive a communication link link failure code to PCM. At any rate, I did experience start and almost immediate shut down but did have a comm link failure code on my Envoy. I bought my Envoy used and at first it appeared to act like a dead battery no start on a consistently intermittent basis until finally it begins shutdown while driving. All along it was the PCM being faulty. Possibly, this is a possibility for your vehicle. Replacing the PCM with proper VIN programming fixed the issue. Input on this as a possible solution from truly sharp members like Envoyboy2004 is strongly recommended. My replacement PCM came from PCM OF NC and it’s been perfect since.
 

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Discussion Starter #7
You stated a couple of things that seem to be noteworthy.
1) the REP light and or starting issues has been something the car has done since you have had it.
2) disconnecting power to the PCM then reconnecting seem to allow a normal start and run with throttle housing disconnected.
If I have restated these comments correctly, you may have a faulty PCM but surprised you did not receive a communication link link failure code to PCM. At any rate, I did experience start and almost immediate shut down but did have a comm link failure code on my Envoy. I bought my Envoy used and at first it appeared to act like a dead battery no start on a consistently intermittent basis until finally it begins shutdown while driving. All along it was the PCM being faulty. Possibly, this is a possibility for your vehicle. Replacing the PCM with proper VIN programming fixed the issue. Input on this as a possible solution from truly sharp members like Envoyboy2004 is strongly recommended. My replacement PCM came from PCM OF NC and it’s been perfect since.
It never had any starting issues when we got it for over a year. The REP light did come on 3-4 times on heavy acceleration but it never died or failed to restart and run properly.

I only tried to start it one time with throttle body unplugged.
When the REP light was staying on with key on it still would not start. I had fan unplugged too.
After the fuse was removed and put back in, the REP light went off with key on and then it started with TB unplugged or plugged and ran fine for rest of day with fan an TB plugged in.
It has been fine for 4 months after new TB and used pedal.
The only thing different I can see is the battery was going bad. Getting chilly out and no reserve but it still stated up on its own each morning....just dies 2-3 days and REP light on and then the REP light stays on each time you turn key on and it WILL not start.

I did the fuse pulls ad swapped relays and it was fine for 5 days and a 4 hr trip. Yesterday battery was too weak to crank in morning but it started a few hours later and ran fine.
Then today AFTER new battery it started then died in 3 minutes and has not started since.
My wife is stuck with if 4 hours away. She pulled and swapped the relays and the yellow fuse and FP relay and the REP light went out after key on and I thought we were good as it has always ran fine rest of day after you can get the REP light to go out.
It started and died in like 3 seconds and REP light stays on now!!!

I think something is shorting the 5v ref signal out or wire(s) frayed or rubbing or water bridging or grounding.
I am going to have here pull fan connector and TB and wiggle the wires going to fan and try.
According to GM flowchart, the 5v code is gone after 3 key on self test cycles IF the 5v fault goes away.
I hope it is the fan or fan wiring and she can just get it back with that unhooked.

I wasn't aware of the 3 key on thing so when I first pulled the fan wiring MAYBE I didn't cycle the key and maybe it was locking it out and keeping the REP on due to not 3 key on without the 5v shorted fault no shorted anymore.
 

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2006 gmc envoy_denali
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Lots going on here. I’m no expert like other members but there are certain troubleshooting disciplines that apply in pursuing the cause or base causes.

Unless there were specific codes or observations that clearly demonstrated a failure of the parts you have replaced, changing these may indeed have added to the issues rather than eliminating their potential contribution to the base issue. Hope you still have these original parts.

You stated that perhaps the alignment key on the fan connector was possibly not properly aligned, so you need to make certain that connector is not damaged and is properly mating.

You state the battery may be loosing charge or may be acting like it’s dead or has a dead cell. You must eliminate the battery since it’s always key to lots of issues. I’m not clear at all from what you have stated that the battery is not an issue. This is a priority.

The lack of contact of the fuse holder you discovered and believed was corrected, suggest you revisit this and make certain there is no corrosion and that the two connections are indeed soild and reliable.

If the PCM was indeed struck hand, tool or hood the connecting harness should be checked in detail. This could present a number of problematic issues.

Moving the vehicle by pushing it in the driveway or wherever is not an issue normally but the transmission is disconnected from the engine so no engines sensors, start or run are affected or set. So moving the car then discovering it starts normally would suggest a transmission control or body control module is possibly involved. The connection between being able to roll the vehicle slightly followed by a consistent start and run condition should be fully explored with and among the very experienced members because it has been shown to be repeatable as you report.

Suggest you avoid replacement of any further components until there is reasonable traceable proof of defect. Caution is urged using used parts from junk cars and only as a last resort. Again, suggest considering reinstalling the OEM parts removed that were not determined to be defective.
 

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2006 gmc envoy_denali
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Carefully check the relay sockets and electrical harness feed same since these too have been shown to repeatably affect a start run condition.
 

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Discussion Starter #10
Have some consistency now.
Even though it will be fine for days, when it stalls and REP light stays on with key on engine off...if I pull the 20a PCM B fuse and put it back in...it the REP light goes off with key on and it starts.
Today was first time it ever stalled after it was warming up.

Used it this morning at 5:30 am at 38-40 degrees and it was fine. Start, drove 10 blocks, truned off for 10 mins then restart and drive back home 10 blocks.
Started at 11:45 AM and drove 25 mins all fine.
Back home at 2:15pm all good.
Started at 4:50 drove 8 blocks and turned off for 5 minutes then restarted at it ran for 40 seconds and stalled.
Pulled and reinserted fuse and started up and stalled again in 30 seconds. Fuse thing again and it was fine all the way home.
I did pull the wire off of the ac accumulator . I think this is the pressure switch the flow chart mentions is one of the 4-5 things the 5v signal goes to/through...but it could be another at the compressor(high side over pressure kick out) not sure.

Been sitting at 50 Degrees for about 1.5-2 hours. Going to try it again and see what it does.

I have the fan unplugged now which should eliminate the can shorting out the 5v signal if the frayed wires are there like on many of them.
I get the fan code and the Cam sensor code.
I swapped the cam sensor with another used one. I have the original. Both give same code on every start.
I don't see any obvious damaged or chewed wires etc.
Got out with it running and wiggled the fuse and other fuses and it didn't stumble or die.
 

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Discussion Starter #11
Had it die out again today. Pulling fuse did not help.
Wiggled cam sensor harness and connector and pushed on 3 pcm connectors and REP light went out and started up.

I pressed on #3 PCM connector and it stalled out.
Coincidence??? Sometimes it stalls in 20-30 secs and timing could have been right.
I pressed and wiggled again and REP went out and started and couldn't get it to stall again by pressing or wiggling.

I took it home and pulled all 3 PCM connectors and sprayed them with WD-40. (at least twice it has been ok until it got damp out then messed up but can't blame it all on wetness)
Sprayed cam sensor connector terminals too.
Ran it around the block a few times with 3 restarts and several reverse and forward and even some hard full lock u-turns.
Doing fine now. The thing is annoying.

Getting ready to wait until it messed up and look for the 5v at the TPS 1 throttle body connector and if not there which is my guess, start wiggling wires until the voltage shows up.
Got to be a frayed or wires touching somewhere, but haven't seen anything yet.
 

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Discussion Starter #12
Had it die out again today. Pulling fuse did not help.
Wiggled cam sensor harness and connector and pushed on 3 pcm connectors and REP light went out and started up.

I pressed on #3 PCM connector and it stalled out.
Coincidence??? Sometimes it stalls in 20-30 secs and timing could have been right.
I pressed and wiggled again and REP went out and started and couldn't get it to stall again by pressing or wiggling.

I took it home and pulled all 3 PCM connectors and sprayed them with WD-40. (at least twice it has been ok until it got damp out then messed up but can't blame it all on wetness)
Sprayed cam sensor connector terminals too.
Ran it around the block a few times with 3 restarts and several reverse and forward and even some hard full lock u-turns.
Doing fine now. The thing is annoying.

Getting ready to wait until it messed up and look for the 5v at the TPS 1 throttle body connector and if not there which is my guess, start wiggling wires until the voltage shows up.
Got to be a frayed or wires touching somewhere, but haven't seen anything yet.
 

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Discussion Starter #13
Been a couple of more days since I pulled all three PCM connectors and it has been fine on every cold morning start and every other time.
Hopefully that was it.
I know the PCM or engine has been out because they were marked in white grease pencil as 1:1, 2:2, 3:3
So someone has had it out. Maybe corrosion or something jumped in there as they were installing.

I think I am still gettting a cam senor code on every start though!!

It runs fine but I unplugged cam sensor and it started and seemed to run the exact same.
I don't think the sensor is bad as it did it with last one too.
I think maybe a wiring prob or whatever that no cam signal is getting back to pcm????

Just waiting with fingers crossed now and hoping it keeps running.
 

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Discussion Starter #14
Been a couple of more days since I pulled all three PCM connectors and it has been fine on every cold morning start and every other time.
Hopefully that was it.
I know the PCM or engine has been out because they were marked in white grease pencil as 1:1, 2:2, 3:3
So someone has had it out. Maybe corrosion or something jumped in there as they were installing.

I think I am still gettting a cam senor code on every start though!!

It runs fine but I unplugged cam sensor and it started and seemed to run the exact same.
I don't think the sensor is bad as it did it with last one too.
I think maybe a wiring prob or whatever that no cam signal is getting back to pcm????

Just waiting with fingers crossed now and hoping it keeps running.
Update:
Fine for a few days then on the highway after a good 2 hour drive the REP light popped on and it died.

Pulled over and sat for a minute of two then on turning key on the light went out and it started but ran rough.
Was definitely missing on at least one maybe two cyls.

Only had to drive a few hundred feet to off ramp and into gas station.

Went inside for a few and came out and pulled fuel pump relay and #10 PCM B fuse and #27 and #28 wiped them off and reinstalled unplugged fan clutch connector since I still don't trust it.
Took all three PCM large connectors (that I sprayed with WD40 about 4 days ago) apart and wiped them off.
There WAS some residual WD40 from my over soaking of them....maybe bridging some connections???
Hooked everything back up but left fan unplugged (doesn't seem to make a difference and I get code each time either way), and it started up and was perfect for another 150 miles into town.

Shut it off when arrived and let it sit 10-20 mins and restarted it and ran fine BUT wouldn't come out of gear.
Had this happen for the first time almost a year ago.
Did it 3 times and then never again.
We know how to lift up console and push release button, so no dig deal but???? Related?

Drove to dinner and came out and the shift interlock released properly!! As I have said before, it acts possessed!

Got back to hotel and parked it for the night.
Wed morning still stuck; just dealt with it. Happy it is running fine at this point. Thurs-stuck but decided to check it out....

For future troubleshooting here's some notes: Shifter stuck in Park

You can lift console and push in round button on right side near pass seat.

You can remove two screws T-15 from top slot of shifter and remove interlock soloneoid...or jsut take out front screw and slightly loosen rearward one and let it pivot and it will no longer lock.

You must leave it plugged in or you can't turn key all the way to off or get it out unless you do some resistor hacks to harness end.

FUSE 25 is interlock BUT fuse 27 FEEDS fuse 25! Unless mine is really messed up!!

I had no power at fuse 25. When I jumped power to #25 the interlock worked fine; no shorts, good fuse.

I beat on and wiggled EVERYTHING.
When I got to fuse 27 it released the shifter.
I have had 27 and 28 out a few times but never 25.
I cleaned the, up well with pocket knife ( best I had at time). They do have a bit or oxidation on them but NOTHING that you would ever suspect to be a problem on any NORMAL non POSSESSED car.

I also notice that every fuse instead of having a n area where the terminal contacts it is seems to have a dot like the terminal has a raised dot like on a contact or a pressure switch or many window switch terminals with hardened contacts.
I think these just have a brass dimple or pimple raised area.
This seems to have wore a small dent into EVERY fuse.
I think a slightly large flat area would have been a better design.

Anyway, cleaned and tweaked the fuse term opposite directions and put all back together and it has been fine for 2 days in town and the 350 mile return.

Took some time to rear its ugly head but I think it all boils down to random loose or intermittent connections in the under hood fuse relay block.

I know the top part comes off and is replaceable...wonder why???

What surprises me is I haven't read lots of posts of people having this prob or this part replaced over the years.

Mine started out just like a throttle body needing replacement. I know a lot of people get this replaced and then need more done to get it all straightened out but I rarely read of fuse block replacement.
Mine is old 04 and 230K on it but still....
 
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