Offroad Trailblazers and Envoys

RPM drop / stall

Something not working right?

by hilljb » Mon Dec 28, 2015 11:39 am

I've searched around a bunch on here, and I think I may be down to a PCM issue.

What's going on: I drove from Michigan to Colorado for Christmas. I've noticed in Colorado for the past few months that the truck will hesitate a bit when accelerating from idle. The truck idles around 700-800 when warm, and it'll drop to around 400 or even lower before the truck seems to be on the same page as me and we move forward. Hasn't been a big issue, until I'm in Michigan and I start stalling when I hit the gas.

The throttle body was cleaned (removed, cleaned spotless) before leaving Colorado. The air intake bolts were tightened (they were loose). Air filter and spark plugs were changed. All of this had no effect on the near-stalls.

After reading on here, I tested the upstream O2 sensor. The graph seemed slow and there was a sudden drop in voltage when I hit the gas (when the truck nearly stalls). I removed the electrical connector to the upstream O2 sensor and the truck runs like a champ, but the check engine light obviously comes on (it hasn't been coming on until that) and I can't use cruise control.

So, I replaced the upstream sensor. I plug it in, remove the codes in the computer, and we're back to nearly stalling ... with a new O2 sensor.

At least I know I can unplug the O2 sensor and run if I need to. Does this sound like a PCM issue? I do have an original PCM in there and I'm not sure if the previous owner had any maintenance done on it.

Cheers, and Merry Christmas / Happy Holidays everyone.
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by KingBird » Mon Dec 28, 2015 1:07 pm

I had this issue for a while. It ended up being a weak fuel pump. I replaced it with an O'Reilly's pump twice and ultimately ended up putting a Bosch back in and it solved my issues.
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by hilljb » Mon Dec 28, 2015 4:53 pm

That's definitely a possibility. I don't understand why it'd run fine without the O2 sensor connected though, unless that's just a coincidence. I could have the fuel pressure tested.
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by Rickalicka18 » Mon Dec 28, 2015 7:38 pm

My trailblazer use to do that and I got my throttle body cleaned and it went away... I also replaced my alternator and battery at that same time. Do you have any car audio or anything that pulls a lot of power?
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by hilljb » Mon Dec 28, 2015 7:46 pm

I'm reminded that I should clean the battery terminals and make sure the ground is good. I don't have much of anything that would be drawing power, but I have had some weird issues like this caused by a corroded ground terminal and weak alternator in the past.

At least I can check/clean the battery terminals and test the alternator. Good idea.
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by Rickalicka18 » Mon Dec 28, 2015 10:54 pm

Checking the alternator wouldn't be a bad idea, I know mine shit the bed as soon as I put my subwoofer and speakers in. You just said that you have a voltage drop so I feel that may be a tell tale sign of the alternator going. Do your lights flicker at all? I know mine did even before I put my audio stuff in.
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by TBYODA » Mon Dec 28, 2015 11:54 pm

Jason did you reset the PCM by pulling fuses 10 & 28? :scratch: As the throttle body gets dirty the PCM adjusts the fuel to air mixture based on the new position when dirty. Now the TB is clean the fuel to air mixture is very rich thus the suv stalls. Ricky did not have this issue because he replace the battery at the same time. Assuming you did not reset the pcm I would suggest you try this easy test before getting a pressure test. :thumleft: When you pull the upstream O2 you put the TB in open loop.
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by Rickalicka18 » Tue Dec 29, 2015 12:03 am

Makes sense
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by hilljb » Fri Jan 08, 2016 7:29 pm

Good info TBYODA. I pulled the fuses and reset the PCM.

I'm still having the issue, and it's slowly getting worse. I did manage to make it back from Michigan to Colorado and I have the luxury now of letting the truck sit until I can work on it. I'm mostly thinking at this point that it's what KingBird was suggesting, the fuel pump. While it's really curious as to why the fuel pump would appear to work fine in open loop mode, that may just be spurious in the end.

Here's what I know for sure:
1) This happens when the truck is cold or hot with no difference.
2) It happens more when it is cold outside. The colder outside, the worse it gets.
3) It comes and goes as it pleases, mostly. It will hassle me for an hour or so, then go away, then come back.
4) If I ease into the gas pedal, I can mostly make it go away. If I punch the gas from idle, I'm much more likely to stall.

I ordered a fuel pressure gauge from Amazon, after I realized that you can get a well-rated one for about $35. The irony here is that my girlfriend's truck just blew its fuel pump a few weeks ago and she wound up having the dealership ($$$) look at it while I was away visiting family. I could have diagnosed her truck before I left as I was pretty sure it was the fuel pump as well in that case ... and it was.

I'll check the fuel pressure next week once my pressure gauge arrives and post here on my findings.

This all reminds me of a time in December of 2004 when I was driving a back road in Vermont and the fuel pump on my Cavalier instantly died without warning. It was -40F at the time. Talk about a bad day.
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by KingBird » Sat Jan 09, 2016 1:29 am

Then I doubt very seriously that it's your fuel pump. Mine would only act up once the fuel pump "warmed up".

I mean, sure, it could be a problem with the electrical connection to your fuel pump but I doubt it. Once you verify good pressure, I'd look at upstream O2 and catalytic converter.

I gutted my cat after I had some issues with it.
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by DirtyBacon04 » Sat Jan 09, 2016 9:55 am

How many miles are you at? If you're anywhere close to 175-200k, and still on original fuel pump, you might want to look into replacing it anyways. I'm sure someone wants to chime in that they've got over 350k on their original pump and they haul trailers of unicorns every day and park on rainbows. But that's one of those thing where you don't want to wait for it to fail before you replace.
My parents learned that the hard way. In the middle of GA, with a trailer full of dirt bikes, and a full tank of gas. Literally, we only made it about 3 miles from the gas station.
You replace it on your terms, you get to run the tank down to empty when you have to drop it. I'm pushing 145k (indicated) and I'll be looking into doing mine soon.
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by KingBird » Sat Jan 09, 2016 3:57 pm

Good point. I was lucky the first time, as mine had come apart inside the tank, moving the pick up to about half tank. So I wouldn't have issues until I tried to run past half tank. Then I replaced it with a junkyard unit. Two years later, I'm limping home because it's cutting out and then catching up with the engine's demand. Then came the cheap (cheaply made not inexpensive) O'Reilly's pumps, just save $100 and buy a Bosch off Amazon.
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by Mrandersonxd » Sat Jan 09, 2016 11:02 pm

FWIW I just had a similar issue that was solved with a new VVT Solenoid, Crankshaft Pos. sensor(need crank relearn), camshaft position sensor and manifold pressure sensor(about $140 for all 4 on Amazon.) Haven't had an issue in 3 weeks. Also seem to have recovered some low end power and response. Hope this helps!
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by M_C_U_02 » Sun Jan 10, 2016 12:53 am

Had a similar issue but not as drastic.... mainly was a hiccup in the idle and felt like it was going to stall, but only actually did twice.
I too replaced the crankshaft position sensor and it has been smooth sailing.
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by jrivs4x4tb » Sun Jan 10, 2016 12:54 pm

Last time I cleaned my entire air system I had this same issue, turns out I didn't connect the vacuum hose that goes from the vortec box to the intake in the center, plugged it back in and its good to go. Might try checking that, seems almost too simple ha-ha.

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by jrivs4x4tb » Sun Jan 10, 2016 12:57 pm

Image

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by jrivs4x4tb » Sun Jan 10, 2016 12:58 pm

Sits under the air box as shown in this picture.


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by hilljb » Thu Feb 11, 2016 2:37 am

As a few had mentioned, I checked the crankshaft position sensor. The bolt holding the sensor was loose and it was about ready to fall out. I had ordered a new sensor from Amazon before I discovered this, so I just did the 2-minute install of the new sensor anyway. Seems to have resolved the issue. Cheers!
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by fishcreek » Tue Feb 16, 2016 12:01 pm

I have same issue. Rough idle and have pending P0300 when parked. P0300 goes away when driving. Replaced potential bad coil, replaced upstream ox, cleaned and relearned tb. Wondering if its a bad gas or plugs. Plugs were replaced two years ago with original ACs.
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by hilljb » Fri Feb 19, 2016 4:40 am

The camshaft position sensor may be the culprit. It's a cheap and easy replacement.
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