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Once engine warms up idle becomes low/rough and hard to start

Tiny
HVACTECH7
  • MEMBER
  • 2000 GMC SIERRA
  • 7.4L
  • V8
  • 4WD
  • AUTOMATIC
  • 160,000 MILES
Just bought the truck listed above 3500 and everything seems great until it warms up. Then the idle becomes tough and almost shuts off at times when I’m waiting at a stop light. No error codes so far (been 2 months) and it passed smog easily several weeks back. Is it likely a sensor problem or something else fuel related? When it’s driving it runs great with no issues. It’s only at idle after warming up for several minutes.
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Wednesday, May 6th, 2020 AT 2:20 PM

15 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Hi,

It could be a few different things. First, have you checked to make sure there are no engine vacuum leaks? Also, an engine coolant temperature sensor can be causing this as well. It provided info for the power-train control module (PCM) which in turn determines what the air fuel mixture should be. If the sensor's reading is wrong, so is the fuel mixture. Interestingly, there not be a code set because the PCM doesn't realize the signal is wrong.

Here is what I suggest. First confirm there are no engine vacuum leaks. Here is a link showing how that is done:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

If that checks good, I need to know if you have or can get a live data scanner. If you have one, let me know what the short term fuel trims are when it is running rough. Also, check to see what temp signal the engine coolant temp is sending to the PCM.

Let me know what you find or if you have other questions.

Joe
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Wednesday, May 6th, 2020 AT 9:15 PM
Tiny
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I really appreciate the fast response Joe. Other than the PCV hose I’m not sure what to check. I have a hydroboost and I did spray a little carburetor cleaner around to see if I got any change. As well as the throttle body area. It got dark on me and I didn’t get any further. I grabbed a PCV valve just in case but won’t put it in until tomorrow. Not sure if that’ll do anything.
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Thursday, May 7th, 2020 AT 9:07 PM
Tiny
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  • MECHANIC
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Hi,

When you replace the PCV, make sure to inspect the hose to it for cracks, dry rot, or if something is disconnected. Also, do you have access to a live data scanner?

Let me know.
Joe
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Thursday, May 7th, 2020 AT 9:25 PM
Tiny
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I will double check the hose for sure. Live scanner is ordered but won’t likely be here until Tuesday. I’ll update you as soon as I do.
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Thursday, May 7th, 2020 AT 11:24 PM
Tiny
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  • MECHANIC
  • 109,747 POSTS
Let me know what you find. Also, once you get the scanner, let me know. I need you to check a few things.

Joe
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Saturday, May 9th, 2020 AT 7:50 PM
Tiny
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Updated shipping says it arrives tomorrow. Let me know what data you need.
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Saturday, May 9th, 2020 AT 8:44 PM
Tiny
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Okay, with the engine running and at operating temp, go to the live data setting and look for STFT and LTFT. That stands for short term fuel trims and long. I can usually tell by that reading of there is an engine vacuum leak.

Good luck with the new scanner. Honestly, it's something you appreciate for a long time.

Take care,
Joe
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Saturday, May 9th, 2020 AT 9:01 PM
Tiny
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  • 21 POSTS
Here’s what it read while I was warmed up, in gear, brake pressed, at idle.
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Sunday, May 10th, 2020 AT 7:40 PM
Tiny
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I did change the PCV valve and I don’t know if I’m just being positive but it seemed to help some. Still a bit rough but not as bad. It’s not always consistent though.
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Sunday, May 10th, 2020 AT 7:51 PM
Tiny
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Hopefully that is part of the problem. As far as the fuel trims, I can see the computer is trying to lean the fuel mixture. In other words, the engine is getting too much fuel.

Have you checked fuel pressure and the regulator? Also, can you go back into live data and bring up the oxygen sensors. These sensors should fluctuate between.1v and.9v back and forth. Those are the extremes of the acceptable range. If they are not changing, either the sensor is bad or we have an issue with wiring. Note that if you find the sensor is reading a stagnant reading (not changing or far off from what is considered normal, pull a vacuum hose off the intake and see if it changes the reading. And yes, that can cause the STFT to be off and a rough idle.

Let me know.

Take care,
Joe
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Sunday, May 10th, 2020 AT 9:11 PM
Tiny
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Sorry, I’ve had all kinds of other issues. But I did check the O2 sensors and the are fluctuating normal between those voltages. I even replaced them recently. The truck is getting worse the last couple days. It’s hard to start almost every time. Sounds like it’s “bogging out” and it’s got a noticeable amount of white smoke in the beginning as well as a significant amount of water coming out of the tailpipe. I’m getting a bit worried. I’m also smelling a fair amount of fuel in the exhaust as well as in the engine compartment area.
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Saturday, May 23rd, 2020 AT 10:59 PM
Tiny
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Is the engine coolant level dropping? Take a look through this link and let me know what you find.

https://www.2carpros.com/articles/head-gasket-blown-test

Joe

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Saturday, May 23rd, 2020 AT 11:01 PM
Tiny
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I finally just ended up taking it in. I do AC work and we are swamped! It was the fuel pressure regulator causing the issue and it was even leaking, hence the fuel smell. Just glad it’s fixed. Engine is running absolutely smooth as butter. Thank you guys so much for your help! Really appreciate it!
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Friday, May 29th, 2020 AT 6:00 PM
Tiny
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That is why it showed the computer trying to lean the fuel mixture. There was too much pressure.

Regardless, I'm glad to hear it's fixed. Let us know if you have questions in the future.

Joe
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Friday, May 29th, 2020 AT 6:21 PM

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