RPM stutter when braking/low speed in cold weather

nnorton44

Original poster
Member
Aug 5, 2021
16
Indiana
So I am trying to track down a solution to an annoyance I have had with my 05 TB 4.2L SWB for the last 2 winters. I have an issue during winter (30-40F or below) that the RPMs fluctuate/stutter when coming to a stop or stop/go traffic at low speed after letting off the accelerator. For instance when I come to a stop as the vehicle is coming to a complete stop the RPMs jump up and down from roughly 900RPM-400RPM then the idle evens out to the correct speed (~620RPM) when I have completely stopped. Or if I am going through a drive thru or parking garage and give it a little gas the idle has a hard time coming to rest and will fall down like the truck wants to stall but immediately bounce back and idle correctly. This only happens in cold weather. In warmer weather the idle is perfectly fine when stopping and the truck drives great.

It's annoying and I can live with it during the winter but I would like some possible input. The throttle body is clean. AC Delco 41-103 plugs have around 20k on them if I would have to estimate. I do think that last winter I put in a bottle of Techron at 1/8th tank and it idled far better but next fill up the problem came back.

Fuel injectors? Loose intake manifold bolts? Motor mounts?

EDIT: P0420 code but I have a cheapo aftermarket catalytic converter
 

mrrsm

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Oct 22, 2015
7,642
Tampa Bay Area
A Few Ideas...

(1) If your Throttle Body has NOT been completely Removed, Cleaned with Carburetor Spray Cleaner (NOT Brake-Kleen Solvent) and Re-Installed... THIS can cause the lock up of the Butterfly Plate inside if it has become seriously Dirty and Laden with Carbon inside the TB opening. (Here is what to do about this):


(2) If you're feeling any sort of Rapid Vibration through the Brake Pedal at the same time during Low Speed Braking... your ABS my be acting up as a result of THIS Rust Issue (and here is the solution):

 

flyboy2610

Member
Aug 24, 2021
460
Lincoln, Ne.
Is the truck fully warmed up when it does this? It could be the engine coolant temperature sensor acting up. The PCM uses the ECTS to help adjust fuel flow so the engine performs properly at different temperatures. Not talking here about the coolant temperature sending unit for the dashboard.
Is the check engine light on? If so, what are the codes?
 

nnorton44

Original poster
Member
Aug 5, 2021
16
Indiana
Is the truck fully warmed up when it does this? It could be the engine coolant temperature sensor acting up. The PCM uses the ECTS to help adjust fuel flow so the engine performs properly at different temperatures. Not talking here about the coolant temperature sending unit for the dashboard.
Is the check engine light on? If so, what are the codes?
Yes the engine is warmed up when it does this (only during winter). I replaced the coolant temp sensor with an AC Delco replacement when I did the thermostat about 30k miles ago.

I have P0420 code but that should be because of the aftermarket cat.

A Few Ideas...

(1) If your Throttle Body has NOT been completely Removed, Cleaned with Carburetor Spray Cleaner (NOT Brake-Kleen Solvent) and Re-Installed... THIS can cause the lock up of the Butterfly Plate inside if it has become seriously Dirty and Laden with Carbon inside the TB opening. (Here is what to do about this):

I’ll clean the throttle body again with Berryman B12 in a few days when it warms up but last time I did it had no effect. And no pulsating/ABS when braking.
 
Last edited:

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
Remember to disconnect the battery while cleaning it to reset the PCM, at least 30 minutes. Otherwise you will continue having idle issues.
 

nnorton44

Original poster
Member
Aug 5, 2021
16
Indiana
Remember to disconnect the battery while cleaning it to reset the PCM, at least 30 minutes. Otherwise you will continue having idle issues.
Yep I’ll remove the PCM fuses, don’t want to chance wrecking the HVAC actuators

Just not sure why it only does this in the winter and the rest of the year no issue at all
 

JerryIrons

Member
Dec 20, 2011
434
You can buy throttle body cleaner at any auto parts store, it works well and not as caustic as the other stuff. Make sure you remove the throttle body to get the back side, and just spray it on a rag and clean, repeat. And yeah you have to unplug the battery (or fuses) because otherwise the engine is expecting a dirty throttle body and will idle like crap. All this being said yeah it should have this problem during warm weather but you have to rule this out.

cold weather only, kinda weird, things shrink in cold weather though. Vacuum leak? Fuel pressure?
 
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Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
Looks like the valve cover gasket. Not related to this issue.
 
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Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
Also just noticed that you have an 05 and your SAIS has been deleted as evidenced by the new block off plate. Has the PCM been reprogrammed to delete the codes in there too? Anything else tuned that would alter air and fuel?
 

nnorton44

Original poster
Member
Aug 5, 2021
16
Indiana
Also just noticed that you have an 05 and your SAIS has been deleted as evidenced by the new block off plate. Has the PCM been reprogrammed to delete the codes in there too? Anything else tuned that would alter air and fuel?
Yes this has been happening prior to SAIS delete, I just did that the other week and should have my PCM back from limeswap tomorrow. I’ll clean the throttle body tomorrow and give an update.
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
Since you will have a reset PCM, be sure to clean your throttle body. By clean I mean it's been taken off and thoroughly cleaned.
 
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Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
Certainly. If it has a small leak, it can affect the air/fuel ratio, especially when cold.
 

nnorton44

Original poster
Member
Aug 5, 2021
16
Indiana
Certainly. If it has a small leak, it can affect the air/fuel ratio, especially when cold.
Okay, intake manifold bolts were not loose.

- Removed and clean throttle body
- Replaced throttle body gasket
- Replaced air filter

Problem still persists.

I did notice on the throttle body that the plate is not completely closed and there is a small gap you can see through on the top of the plate. Is this normal? Should it be 100% closed? Also is the TPS built into the throttle body? When your throttle body is bad does it throw a code?
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
I did notice on the throttle body that the plate is not completely closed and there is a small gap you can see through on the top of the plate. Is this normal? Should it be 100% closed? Also is the TPS built into the throttle body? When your throttle body is bad does it throw a code?
That's normal. If it was bad, it would certainly throw a code and like go into REP mode.

I'd recheck the plugs. Depending on where you got them, there are fake ACDelco plugs floating around, mostly on Amazon and eBay.

Beyond that, I'm out of ideas.
 

nnorton44

Original poster
Member
Aug 5, 2021
16
Indiana
That's normal. If it was bad, it would certainly throw a code and like go into REP mode.

I'd recheck the plugs. Depending on where you got them, there are fake ACDelco plugs floating around, mostly on Amazon and eBay.

Beyond that, I'm out of ideas.
Okay I will check them, if I do decide to replace the spark plugs should I also replace the coil boots/springs? A kit of 6 new isn't expensive at all from RockAuto.
 

budwich

Member
Jun 16, 2013
2,027
kanata
I suggest that you get some operational data when your vehicle is in "cold mode" and you see the problem and again once it warms up... compare the information and go from there. Look at fuel trims at idle, brake on / brake off... etc along with vacuum. My guess is that you are running rich which has problems with cold as then you are almost in "stumble" mode when air intake is removed with throttle shut down.
 

nnorton44

Original poster
Member
Aug 5, 2021
16
Indiana
I suggest that you get some operational data when your vehicle is in "cold mode" and you see the problem and again once it warms up... compare the information and go from there. Look at fuel trims at idle, brake on / brake off... etc along with vacuum. My guess is that you are running rich which has problems with cold as then you are almost in "stumble" mode when air intake is removed with throttle shut down.
Okay I have a Bluetooth OBD will torque pro be sufficient?
 

budwich

Member
Jun 16, 2013
2,027
kanata
that will work (other apps as well). This will at least provide some "baseline" to determine whether there is a "operational fuel air issue" or mechanical problem or otherwise.
 

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