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

[CruelJung]

I have seen other posts about similar issues to what I'm experiencing, but I haven't been able to glean any solutions from them as of yet. However, I've been doing what I can and collecting information over the past month, so hopefully the process of elimination has a head-start.



Issue In Question:

I live in a warm climate and back in December the temperature dropped unusually low (mid-30's) overnight and was still hovering around 45 °F when I needed to start my vehicle in the morning. I remote-started it from distance to give it extra time to warm up and noticed it was running rough. While I loaded some gear, with remote-start still going, the engine shut off. I pulled out my Diablo tuner and and checked for codes: I had P0300, P0171, and P0174. I cleared the codes and started the engine again, to the same rough idle effect, and decided to rev the engine a bit a see if there was any improvement. While doing so relieved the engine's struggles, the RPMs were rising well above my pedal pressure and remaining for minutes where they stopped even after I released the pedal before finally dropping. I continued this process, for better or worse, until the engine warmed up to about 160 °F and the engine evened out to normal idle and throttle response. During that process, the dash regained codes P0171 and P0174 (not P0300). Out of sheer necessity, I took my vehicle to the intended destination (regaining only P0300 again on the trip), starting the vehicle again several hours later and having to go through a slightly less intense revving process to warm up the engine without stalling and driving it home.

Since this occurred, I have been continuing the above process to warm up the engine when the weather is on the colder side--the intensity of the rough idle and length of warm up process being mirrored by the degree of cold weather. Also, since the initial day of this flare-up, I have not regained the random misfire CEL. Lean codes for bank 1 and 2 (171 & 174) continued.



Further Background:

Before I get into what I have done in response, it's worth noting that my engine has been exhibiting--to an extremely minor degree--some of this rough idle at cold crank (seemingly regardless of weather) since I acquired it two years ago. At cold crank, the engine typically revs just above 1000 RPMs and settles down to stock idle within 30 seconds. Sometimes the engine vibrates a bit more than other times, but it's all over in a matter of seconds and smooths out to what I understood (with my lack of familiarity) to be normal. Similarly, when in drive while I am waiting at a light, I can feel intermittent vibration; it's not what I would consider to be a rough idle, but it's not perfectly smooth. None of the above trouble codes have been associated with these *features,* but I was getting codes P01153 and P01133 early in my ownership, which led me to replace both upstream O2 sensors. The O2 sensors' replacements did not immediately stop the 1153/1133 code recurrence, but over the months following that work--possibly in conjunction with me adding P.I. (Amsoil) to my fuel tank in regular intervals--these "insufficient switching" codes ceased. My dash had been clean of any CELs for several months to a year (I've had a bit too much going on in my life to remember, precisely) by the time this cold weather rough idle problem presented itself at the end of last year.



What I Have Done Since The Event In December:

1. Cleaned MAF sensor (with CRC MAF sensor cleaner) & replaced air filter (high pleat count, AC Delco).

2. Updated driver side valve cover install.

3. Catch can install & replacement rubber elbow (valve cover to PCV line fitting). The latter was cracked, as I found while installing the OCC. Since replacing the elbow I have not had any more CELs/codes, although a lessened rough idle at cold crank in cold weather persists.

4. Checked for tightness of air intake fittings excluding throttle body to intake manifold--I plan to, but have not yet purchased click-style torque wrenches, and I only have an old beam-style wrench that I don't trust.

5. Cleaned throttle body bore and blade (with CRC MAF sensor cleaner) via weight on accelerator pedal while key in "on" position--I read some horror stories of idle issues following other methods. Throttle body was dirty, but not too bad. I noticed liquid shimmering in the back of the intake manifold while the TB was open (pre-OCC development?).

6. Cleaned MAP sensor (CRC MAF sensor cleaner) and replaced silicone sleeve around nipple. After this was done, a new symptom appeared: my idle, while in drive and stopped at lights, began erratically dipping just beneath 500 RPMs before quickly rising back up to stock RPMs. The engine would not stall, but felt close to doing so when the RPMs dropped.

7. Raised both park and drive idle to +150 RPMs from stock value via Diablo (Predator 2) tuner. I presumed both values in stock form were identical and that raising them equally would keep them equal, but the result of this modification has raised my "park" idle to 750 RPMs while my "drive" idle remains at just above 500 RPMs (perhaps only slightly higher than before). My "drive" idle no longer dips below 500 RPMs or feels as though it will stall, but the needle still fluctuates slightly up and down.

8. Performed Seafoam Top Engine cleaning procedure through throttle body. Minimal smoke, relative to what results I've seen from others, but what I expected with a port injection engine.



Live Data Observations Via Diablo Tuner:

1. MAF sensor shows +/- values per acceleration.

2. MAP sensor shows +/- values per acceleration.

3. Accelerator pedal shows +/- values per pedal pressure

4. Throttle body duty % does not mirror pedal pressure values*



Final Notes:

*I've been contemplating, based on findings in these forums, that my issue is narrowing down to either the accelerator pedal/sensor assembly or throttle body/TPS--leaning towards the entire TB or TPS because the live data APP values seem to reflect actual applied pedal pressure, while even with my pedal at 100% the TB was showing--at max, with key in "on" position--20%.

There is absolutely no trouble in starting the vehicle, so when I mention a rough idle I mean in no way a hard start (my spark plugs and wires are new as of early 2021--if that matters--as is/are the thermostat, water pump, serpentine belt, and upper/lower heater hoses).

Another vexing aspect of all this to me is that every time I see someone solved or came close to solving issues like these, trouble codes more specific to their solutions seem to be present. I probably could have tried replacing the MAF and MAP sensors by now (or the fuel pump, for that matter), but I wanted to explore everything before part swapping--especially since I have experienced some improvement along the way and no CELs are presenting themselves anymore.

I have a family mechanic with an expensive and high end scan tool/computer, another mobile mechanic available with unspecified scanning equipment and expertise, and I'm sure there's more for me to learn about this Diablo scanner/tuner of mine, so please weigh in on what else I can do to troubleshoot and what you think about the current findings.

Thank you!

 

[Foggy]

It sounds to me like a good old fashioned vacuum leak...
The intakes gaskets can get crappy over time (been 15 years)
Check your brake booster lines, etc....
Those are lean codes - find the leak- then check your spark plugs again..
Make sure you use "real" ac delco or similar plugs (fakes are out there)

 

[CruelJung]

It sounds to me like a good old fashioned vacuum leak...
The intakes gaskets can get crappy over time (been 15 years)
Check your brake booster lines, etc....
Those are lean codes - find the leak- then check your spark plugs again..
Make sure you use "real" ac delco or similar plugs (fakes are out there)

I know the spark plugs and wires are AC Delco, and I suspect but have not yet confirmed if the intake gaskets were done when the AFM was deleted--I would expect they were, though, since the valley plate had to be removed.

Is there any superficial observation I could make to determine if the intake gaskets are original or have been replaced?

Once I get a proper torque wrench--that can do inch pounds--I plan to check the throttle body gasket and likely replace it. I wouldn't expect my mechanic would have done this during the AFM delete, so it could be original.

 

[Geotrash]

Thanks for the detailed description of what you're experiencing and the steps you've taken so far. Given that you have a random/misfire code along with lean codes for both banks and idle quality issues, I agree with Foggy that you have a vacuum leak, with the added qualifier that it's somewhere that is impacting both banks of cylinders. With that, I don't expect an intake manifold gasket because they cover individual intake ports. Instead, I would suspect the resonator or something else upstream from the intake manifold itself or something that attaches to the main plenum chamber in the manifold like the brake vacuum booster line. Here's a good video that shows how to find it using either propane enrichment or carb cleaner.

 

[CruelJung]

Thanks for the detailed description of what you're experiencing and the steps you've taken so far. Given that you have a random/misfire code along with lean codes for both banks and idle quality issues, I agree with Foggy that you have a vacuum leak, with the added qualifier that it's somewhere that is impacting both banks of cylinders. With that, I don't expect an intake manifold gasket because they cover individual intake ports. Instead, I would suspect the resonator or something else upstream from the intake manifold itself or something that attaches to the main plenum chamber in the manifold like the brake vacuum booster line. Here's a good video that shows how to find it using either propane enrichment or carb cleaner.

I would be happy to find the intake gaskets are not the source. Mine do have a metal framework around them, which seems to indicate they are not original? I don't recall the color of the actual seals within the framework (I want to say I could see a bit of teal), but I can post an image if that if it comes up again.

There's some backstory that would explain my lack of knowledge on some of my mechanic's AFM delete process, but I'll save that for an official intro or build thread.

Thanks for the video. When you say "resonator," are you referring to those many chambers built into my stock intake tube? Otherwise I think I understand what you're pointing out about the cause not being cylinder-specific.

 

[Fless]

Read and document your short- and long-term fuel trims at idle, then after a minute or so at 1500 or 2000 rpm. If it's adding fuel at idle (high positive fuel trim numbers) and they go down at the higher rpm, that's typically caused by a vacuum leak. If on both banks, look for an air leak between the MAF and the throttle body, or something common to the intake for both banks as @Geotrash noted.

 

[Geotrash]

I would be happy to find the intake gaskets are not the source. Mine do have a metal framework around them, which seems to indicate they are not original? I don't recall the color of the actual seals within the framework (I want to say I could see a bit of teal), but I can post an image if that if it comes up again.

There's some backstory that would explain my lack of knowledge on some of my mechanic's AFM delete process, but I'll save that for an official intro or build thread.

Thanks for the video. When you say "resonator," are you referring to those many chambers built into my stock intake tube? Otherwise I think I understand what you're pointing out about the cause not being cylinder-specific.

The factory intake gaskets have a plastic frame around them - at least they do on the 6.2.

Yes, that's what I'm referring to. There's a lot of plastic there to crack or otherwise leak.

 

[CruelJung]

Read and document your short- and long-term fuel trims at idle, then after a minute or so at 1500 or 2000 rpm. If it's adding fuel at idle (high positive fuel trim numbers) and they go down at the higher rpm, that's typically caused by a vacuum leak. If on both banks, look for an air leak between the MAF and the throttle body, or something common to the intake for both banks as @Geotrash noted.

Thanks! I've included some screenshots of my video recording below; the results line up with your description. More on that to follow.

The factory intake gaskets have a plastic frame around them - at least they do on the 6.2.

Yes, that's what I'm referring to. There's a lot of plastic there to crack or otherwise leak.

Okay, that's what I've seen on all the displacements in videos explaining the replacement process--with the implication being the ones removed are 'factory.' I won't entirely rule it out, but hopefully those are not the problem for a number of reasons.

It's probably just my lack of experience, but the liquid spraying technique is less attractive to me at the moment and I don't mind nosing around my engine bay to educate myself on what's what first.

Since my last post I have replaced the 'clean air' PCV rubber elbow--just because it's cheap and I recently replaced my 'dirty air' PCV elbow due to cracking--and swapped my intake tube with another stock intake tube that I cleaned and inspected. No change. I also cleaned and inspected my original intake tube in the process and reinstalled it. Not the most scientific, I know, but I moved on and think the brake booster diaphragm may be half or all of the problem. The below video was enlightening and I found some of what is described to hold true while reassessing things with my scan tool: my B1 and B2 fuel trims rise in tandem with RPM drops whenever my brakes are applied and, frankly, I think my brake pedal is a bit stiff and has been since I acquired the vehicle 2 years ago--I just didn't know any better. I need to get a pressure gauge to check the booster vacuum line and also revisit my MAF numbers (like in the video), but I think the jigsaw is falling into place.

I'll have more updates after I cross-examine a few more concerns. Any further input during the interim is of course ideal, though--I am just learning all of this as I go.

Some additional information: sometime last year, I had "Service ABS" and "Service Traction Control" on my dash and responded by checking my master cylinder--it was low. I added DOT 3 and the dash displays vanished. Following that, I had my brake lines inspected and no leaks were found. My brake pads were said to have +/- 60% wear remaining and I was told my fluid drop could be due to brake pad wear. I didn't have any further issues after refilling the master cylinder, but the fluid level hasn't stayed where I filled it.

 

[Fless]

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.

 

[dkad260]

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.

 

[CruelJung]

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?

 

[Fless]

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.

 

[CruelJung]

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.

 

[CruelJung]

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]

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?

 

[CruelJung]

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]

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?

 

[CruelJung]

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.

 

[BG1988]

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

 

[CruelJung]

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.