Rough Idle @ Cold Crank (Worse in Cold Weather) + Fluctuating RPM in Drive

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CruelJung

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You can also test for vacuum leaks using an unlit propane torch with a little propane flow.

The fuel trims indicate a classic vacuum leak that affects both banks. Plug off the vacuum line to the brake booster and retest trims at idle and 2000 rpm. If they are down more at idle, Bob's your uncle. Great trims are less than ±5% (-5 to +5) when adding ST & LT. Anything more than ±10% is a concern, or should be. EDIT: also check the air plenum between the MAF and the throttle body for leaks. Unmetered air is the culprit.

Add each bank's fuel trims together to get the cumulative fuel trim.
For example, in your first pic, B1 = 16.4 + -4.6 = 11.5.
B2 = 16.4 + -5.4 = 11

Compare that to how great they get at the 2000 RPM. B1 total is 0.8 and B2 is -0.8.

It looks to me like the computer is adding fuel at idle, then the LTFTs show it being taken away when it's not needed, which is why they are negative.

I ran a vacuum test on the brake booster hose and attached images of my scan tool data. STFTs seem right with the hose plugged and I'm thinking the LTFTs are likely deep in the negative numbers per the ECM trying to reconfigure with the leak absent. I presume that after the booster is replaced and the engine runs for a couple hundred miles the LTFTs will settle into healthy numbers, but correct me if I'm wrong.

Any tips on the brake booster job? It looks like I don't even need to empty the master cylinder or bleed the brakes, but I'm also not sure if I should replace the master cylinder in tandem. Considering my brake fluid drop last year, should I toss my current MC?

Below is the best video I've located for a visual on what I expect to do. Please advise.


Additionally, how do my MAF sensor numbers look? Should I be thinking about replacing it, as well?
 

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I don't have any advice about the booster change. Some scanners will let you reset the LTFTs; maybe disconnecting the battery and touching the cable terminal ends together would also do that. If not, the LTFTs should adjust over time.

Seems to me the vacuum hose connection at the booster might have some kind of check valve in it, but I'm not quite sure. That might be something to check if it's there.
 
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CruelJung

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With what's been said, how old are your plugs? These engines like the AC Delco iridium.

You also stated you have an oil catch can. I used one not long ago and unfortunately it caused a few issues. Not by design of capturing oil vapor, but my catch can had a screen in the lid that I didn't see and it clogged over time.

This resulted in a fully restricted PCV system on my Envoy and made a mess of the cylinders.

Just something to check since if clogged, you could potentially suck in some gaskets. Make sure your catch can plumbing is completely unobstructed.

The spark plugs and wires are 2 years and just under 30k miles old. I don't have the exact part information on file, but my mechanic will only use OEM parts for repairs--aside from performance upgrades--so I have no doubt they are true AC Delco. As far as the iridium aspect, I'm not sure unless that's standard for the AC Delco spark plugs.

That's unfortunate about your OCC issue. Interesting you mentioned that, though. It made me question my memory, so I checked my notes and realized that cold morning occurred in late December rather than January. I had my sequence of events/repairs correct, thankfully, but I'll edit that detail in my initial post.

At any rate, the cold morning in December and the issues it revealed pre-date my catch can install in early January. I haven't had any issues with my OCC so far, but I'm under the impression that PCV health (or the absence of restriction) can be checked by running the engine at idle with the oil dipstick removed and placing a small piece of paper over the open tube. If suction keeps the paper in place, you have negative crankcase pressure. I checked this before and after the OCC install and, if anything, the suction increased after the OCC and valve cover install. That started with the thread linked below:

https://www.tahoeyukonforum.com/threads/2008-5-3-engine-oil-filter-octane-rating.135750/

If anyone has any dissenting opinion on the 'paper test,' please feel free to weigh in. I'm not an expert, that's just something I saw online and it made sense to me.
 
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CruelJung

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I don't have any advice about the booster change. Some scanners will let you reset the LTFTs; maybe disconnecting the battery and touching the cable terminal ends together would also do that. If not, the LTFTs should adjust over time.

Seems to me the vacuum hose connection at the booster might have some kind of check valve in it, but I'm not quite sure. That might be something to check if it's there.

I was hoping the white plastic swivel piece might be a quick fix (if that's the check valve), but I haven't done any further testing/searching since I vacuum tested last night. In the video, it looks like it can pop right out of the booster, so it's likely a cheap buy if available separately. Thanks for the suggestion.
 

dkad260

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That's unfortunate about your OCC issue. Interesting you mentioned that, though. It made me question my memory, so I checked my notes and realized that cold morning occurred in late December rather than January. I had my sequence of events/repairs correct, thankfully, but I'll edit that detail in my initial post.
Yeah, it was a pain but I managed to get all the carbon cleaned out and return to normal status. Just make sure you have no restrictions through the inlet and outlet line of your OCC.

I would use a vacuum pump and perform a leak down test of your brake booster. Not saying the booster is good or bad, but you said your STFT returned to normal after plugging the hose, but those values are constantly changing and I don't believe that's an accurate judge of your issue. The STFT is a fine adjustment if you will of the "coarse adjustment" of the LTFT. You will need to check and/or bypass the check valve for that test.

Have you checked the throttle butterfly for any "looseness" or "wiggle"? Sometimes the gears wear in the TB mechanism.

For those in the know, I haven't had mine for long enough so I haven't dived in very deep yet, but I assume these throttle bodies have an IAC or Idle Air Control valve or mechanism? Would this be integrated or a seperate part?

When I hear of throttle fluctuations on a MAF engine, I start to think of bad or dirty MAF, worn throttle gears, faulty ignition but you checked that, or even a fuel issue.

Just to rule it out, and I hate to ask, but removing an easy to reach plug and verify the plug type/# is a good start. Cold air is more dense and harder to light off the fuel mixture.

I would also want to know what the vacuum looks like with a gauge. Does the needle fluctuate rapidly or is it steady but with normal, slight varying movements?
 
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CruelJung

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I don't have any advice about the booster change. Some scanners will let you reset the LTFTs; maybe disconnecting the battery and touching the cable terminal ends together would also do that. If not, the LTFTs should adjust over time.

Seems to me the vacuum hose connection at the booster might have some kind of check valve in it, but I'm not quite sure. That might be something to check if it's there.

I replaced the check valve to booster grommet (with one from a universal multi-pack I had on hand) and that corrected my STFTs the same way plugging the booster hose did. Afterwards, I purchased a new (genuine AC Delco) check valve and grommet and installed with no new improvements.

It seems my check valve grommet was the culprit for my wild fuel trims. I'll post an image of LTFTs and STFTs next update, which should give time for the numbers to adjust if that's the end of my vacuum leak.

I still have suspicions of my brake booster, but it passes all the tests and it seems like fuel trims temporarily rising from excessive (3-10 times, let's say) brake pumping may just be normal for vacuum boosted brakes?

Yeah, it was a pain but I managed to get all the carbon cleaned out and return to normal status. Just make sure you have no restrictions through the inlet and outlet line of your OCC.

I would use a vacuum pump and perform a leak down test of your brake booster. Not saying the booster is good or bad, but you said your STFT returned to normal after plugging the hose, but those values are constantly changing and I don't believe that's an accurate judge of your issue. The STFT is a fine adjustment if you will of the "coarse adjustment" of the LTFT. You will need to check and/or bypass the check valve for that test.

Have you checked the throttle butterfly for any "looseness" or "wiggle"? Sometimes the gears wear in the TB mechanism.

For those in the know, I haven't had mine for long enough so I haven't dived in very deep yet, but I assume these throttle bodies have an IAC or Idle Air Control valve or mechanism? Would this be integrated or a seperate part?

When I hear of throttle fluctuations on a MAF engine, I start to think of bad or dirty MAF, worn throttle gears, faulty ignition but you checked that, or even a fuel issue.

Just to rule it out, and I hate to ask, but removing an easy to reach plug and verify the plug type/# is a good start. Cold air is more dense and harder to light off the fuel mixture.

I would also want to know what the vacuum looks like with a gauge. Does the needle fluctuate rapidly or is it steady but with normal, slight varying movements?

Does your Envoy have the 5.3 V8?

Next time I update, I'll upload images of the fuel trims again--post check valve replacement (see above).

I'm not as familiar with Envoys. I know the Denali trims of those makes came with 5.3 engines, but I'm not sure they are exactly the same as the 5.3s in the 07-14 full size SUV/truck lineup. If by "integrated" you mean different from the cabled throttle bodies of older vehicles, then I think the answer is yes. I'm pretty sure I can replace my throttle body sensor (which incorporates the IAC or equivalent for electronic TBs), albeit with a potential relearn procedure to follow, but it's attached to the side of the TB with direct internal gear contact to the TB--*I believe*.

I don't know that I've conducted thorough testing of my MAF, TB, or MAP, but I have cleaned all of them. I wouldn't be surprised to find an issue with my throttle body or MAP sensor, so I'll be looking further into those areas next.

When I plugged my booster hose with the vacuum gauge at idle, the needle was steady at/around the 20 mark. I can't say if the needle moved when I revved my engine.
 

dkad260

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If by "integrated" you mean different from the cabled throttle bodies of older vehicles, then I think the answer is yes.

Yes, different from the cabled versions. My guess is the IAC in yours was something you can replace but not sure.
I can't say if the needle moved when I revved my engine.
The needle will definitely move when you rev the engine, it will also move slightly as the PCM controls idle with small throttle adjustments, that would be normal.

What wouldn't be normal is a vibrating needle which would likely indicate some carbon or similar under the Intake valve... but that doesn't seem to be the case.

Forgive me if this was answered, how old are your upstream O2 sensors?
 
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CruelJung

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Yes, different from the cabled versions. My guess is the IAC in yours was something you can replace but not sure.

The needle will definitely move when you rev the engine, it will also move slightly as the PCM controls idle with small throttle adjustments, that would be normal.

What wouldn't be normal is a vibrating needle which would likely indicate some carbon or similar under the Intake valve... but that doesn't seem to be the case.

Forgive me if this was answered, how old are your upstream O2 sensors?

I see. The vacuum gauge was unchanging, unless PCM adjustments would be movements as small as a 64th to 16th of an inch--which I don't so much recall as I can imagine I would have thought unimportant at that time.

Upstream O2 sensors are not old. I had those replaced (with Bosch models, I believe) within the past two years in an attempt to solve the "insufficient switching" codes which remained after the new upstream O2 sensors were installed. Those two codes stopped appearing towards the end of last year--probably due to a variety of things I cleaned and replaced, or perhaps from the use of fuel system cleaners and higher octane fuel.
 

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You can also test for vacuum leaks using an unlit propane torch with a little propane flow.

The fuel trims indicate a classic vacuum leak that affects both banks. Plug off the vacuum line to the brake booster and retest trims at idle and 2000 rpm. If they are down more at idle, Bob's your uncle. Great trims are less than ±5% (-5 to +5) when adding ST & LT. Anything more than ±10% is a concern, or should be. EDIT: also check the air plenum between the MAF and the throttle body for leaks. Unmetered air is the culprit.

Add each bank's fuel trims together to get the cumulative fuel trim.
For example, in your first pic, B1 = 16.4 + -4.6 = 11.5.
B2 = 16.4 + -5.4 = 11

Compare that to how great they get at the 2000 RPM. B1 total is 0.8 and B2 is -0.8.

It looks to me like the computer is adding fuel at idle, then the LTFTs show it being taken away when it's not needed, which is why they are negative.
if it's negative that means it has a restriction in the intake i.e dirty throttle body
 
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CruelJung

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SOLVED - Engine Coolant Temperature Sensor

It's that rare time of year in my state, when the weather dips into cooler temperatures, which gave me the opportunity to test solutions again. Before replacing the ECT sensor, I swapped my MAF sensor for a new one to see if anything changed--nothing of note and the rough idle at cold crank in cold weather continued, so I put my AC Delco MAF sensor back on. Since replacing the ECT sensor, I have had no rough idle from a cold crank in ambient temperatures as cold as 50 degrees (major issues previously started anywhere below 70 degrees).

It's not lost on me that other factors could still be at play--considering I found and fixed a vacuum leak which had little to no *obvious* effect on the original issue--but I waited almost a year to re-focus on this cold crank problem in the prime weather conditions so I'm letting myself enjoy the victory for now.
 
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