05 TB 4.2 stumble

[iaminsider]

I am having an issue that I am completely stumped on.

There is a stumble at idle when at operating temperature whether in gear, neutral or park. It does not stall and there are no SES light (no codes stored either).

2005 TB LS 4x4 4.2L 130k miles.
Plugs (41-103 delco plugs) changed when the problem started 3k miles ago
TB cleaned inside and out following the walk through. battery disconnected for the whole process
All maintenance is up to date
PCM was tuned by Lime-Tune roughly 10k miles ago.
My issues started around 3k miles ago

Where should I go next with trouble shooting? All of the threads that I have read talked about the engine stalling which mine is not so I am lost at this point.

Thanks a head of time!

 

[BlazingTrails]

I am having an issue that I am completely stumped on.

There is a stumble at idle when at operating temperature whether in gear, neutral or park. It does not stall and there are no SES light (no codes stored either).

2005 TB LS 4x4 4.2L 130k miles.
Plugs (41-103 delco plugs) changed when the problem started 3k miles ago
TB cleaned inside and out following the walk through. battery disconnected for the whole process
All maintenance is up to date
PCM was tuned by Lime-Tune roughly 10k miles ago.
My issues started around 3k miles ago

Where should I go next with trouble shooting? All of the threads that I have read talked about the engine stalling which mine is not so I am lost at this point.

Thanks a head of time!

I would start by cleaning the grounds in following locations;
•Front inner fender passenger side
•Front inner fender driver side
•Engine block ground driver side next to knock sensor.
•Ground block behind carpet, below radio, passenger floorboard.

Next check the plug on top of the engine at the back on top of the valve cover. This feeds the coils, make sure the connection is clean and tight.

If the problem still persists, replace the ignition switch, it is inexpensive and causes a lot of problems that do not make sense to you. It is the relay for the switch you will still use the same key.

If you are still experiencing this problem it will be time to do the following;
•compression test
•remove valve cover inspect for broken valve springs, and rockers.
•replace lifters
•have the fuel injectors cleaned professionally.
•check for vacuum leaks

Good Luck!

 

[iaminsider]

Thank you for your very detailed response. One thing I forgot to add... The stumble is very intermittent. Sometimes it wont happen for days and other times it will happen at every stop light I reach.

 

[Mooseman]

Mine has always had it. Just an off-idle stumble after idling a while. Never bothered with it. Could be my old O2 sensor. I should have changed it a long time ago.

 

[Sparky]

Does weather seem to play a part in it? Like, does it appear to happen more often when damp, wet, cold, etc?

Basically looking for some common thread when it happens.

 

[iaminsider]

Does weather seem to play a part in it? Like, does it appear to happen more often when damp, wet, cold, etc?

Basically looking for some common thread when it happens.

Honestly, I have not seen a correlation between weather and when it occurs. That is why I have been so darn baffled by it.

It started happening at the beginning of October and I am in Minnesota. We have had a very mild fall up until this past week. It happened at 70 degrees and dry and has happened at 30-40 degrees and damp/wet as well as everywhere in between.

 

[BlazingTrails]

If it is very intermittent I would start with the ignition switch for sure.

 

[iaminsider]

If it is very intermittent I would start with the ignition switch for sure.

Alright. Thanks for the guidance. I have not had the stumble in a couple days but I am certain it hasn't corrected itself.

 

[mrrsm]

If I may recommend an Old School Diagnostic to eliminate or confirm Burned Exhaust Valve(s):

Take a piece of plain White Printer Paper,
Fold it in half- lengthwise and and tear or cut it into two pieces
Start the vehicle and let it idle until the engine warms up.
Hold one half of the white paper completely flat against the exhaust pipe.
The paper should flutter and vibrate in and out quite a bit...but if the paper gets drawn in
Try the same thing again with the other half sheet and if it gets pulled inside again...
Then either one or more Exhaust Valves are burned and leaking enough
To vacuum draw the paper back up inside the pipe on the Intake Stroke
This is a Definitive Test that can be verified by a Compression Test or
A Cylinder Leak Down test...

 

[iaminsider]

That seems like an excessive possibility for something that is very intermittent, when it happens doesn't stall and has full power. Only an idle issue sometimes. Like I said above, it hasn't even done it in days. I drive 60+ miles a day.

 

[mrrsm]

It might be worth using a 10 MM socket and 3/8" ratchet (and keep a 6" or a shorty 5/8" Spark-Plug extensions) and pulling the Coil On Plugs one at a time and look with a small mirror and a flashlight for the presence of either AF, water or oil collecting in the Spark Plug Wells. I would refrain from pulling the plugs before making sure that a lot of fluid is not sitting down there because if you pull the plugs without knowing and then install new ones...if any fluid leaked down inside the cylinder... you could wind up hydro-locking the engine and perhaps even bend a connecting rod.

Its wise to use compressed air and a rag to collect anything that might have accumulated over time from leaking Valve Cover Gasket(s). Then pull each plug and see if they all look "Fried Chicken Brown" indicating excellent and consistent combustion over time. If any are either black and oily or have the electrodes badly eroded... replacing them with the exact factory COP replacements and new OEM plugs would give you the chance to find out if one or many COP units have gotten "sketchy" and in need of replacements.

Without Spark-Plug Wires to be able to observe any "lighting flashes" from bad wires...the only way to really see what has been happening inside the cylinders is to pull the COPs and the spark-plugs to do a visual diagnostic. MAY03LT has an excellent Youtube video on how to do the COPs and plugs with great efficiency:

 

[iaminsider]

That isn't the case. They were bone dry when I replaced the plugs in October (the stumbling was already there when I replaced the plugs).

 

[iaminsider]

I ordered a new delco ignition switch this AM. cheap enough to just replace and see.

 

[mrrsm]

Since the damned Throttle Bodies on these things are almost solid state and you can't remove TPS or an IAC components...is there any chance that a complete TB replacement might solve the problem? When you re-installed it after cleaning it...is there any chance the Blue TB Gasket got turned sideways a bit? All the connectors nice and tight?

 

[iaminsider]

I cannot say 100% that the TB isn't the problem. If it is the problem... I will not be replacing it until the stumble turns more constant or causes stalling. I am fairly confident that the gasket is secure and correctly fitting and that the plugs are seated properly.

 

[mrrsm]

I don't know why I didn't think of this earlier...because I have been staring these components in the face almost on a daily basis for the last few weeks. There are two sensors/solenoids than can either individually or in concert... actually cause the GM Atlas 4.2l Engine to stumble like an electrical misfiring problem, but not necessarily throw any codes while doing it because it has to do with very fine changes in variable oil pressure. Both are fairly inexpensive components to replace, but while one is fairly easy to R&R... the other is a PITA because you have to remove the three bolts holding the Power Steering Pump to the front of the engine in order to gain enough access to slide it out. Lets start with the easy one first:

CPS = Camshaft Position Solenoid (EASY)

This is a solenoid that picks up the physical position of the Exhaust Camshaft in relation to the position of the Reluctor inside the V-V-T (Variable Valve Timing) Sprocket and signals the PCM to make timing adjustments depending upon how far advanced or retarded the camshaft is changed depending upon the actions of the second solenoid actuator:

CPAS = Camshaft Position Actuator Solenoid (PITA)

This unit is an oil pressure sensitive device that changes the internal positions of a helical gear inside the V-V-T based upon oil pressure either increasing or decreasing pressure depending upon Engine RPM. If the device internals and filter screen(s) are particularly dirty and or have metal particles in the oil stream blocking the entrance and exit of the oil under pressure, then the V-V-T helical gear will not perform correctly and the PCM can become misinformed causing a timing mistake when firing possibly throwing P0013 and or P0014 codes and making the engine stumble. It is possible to clean this device as shown in one of the below listed videos. However...considering how sensitive it is to very slight changes in oil pressure, it is recommended to be replaced with an OEM unit along with an oil change and oil filter replacement. This is yet another problem that can be reduced by doing more frequent oil changes using Mobil1 5W-30 Fully Synthetic Motor Oil and Filters or K&N Oil Filters to clean the oil of micro-fine offending metal particles in the oil stream.

https://www.youtube.com/watch?v=GsgItRsAWZQ

https://www.youtube.com/watch?v=N9d1_6PLXuQ

 

[BlazingTrails]

I ordered a new delco ignition switch this AM. cheap enough to just replace and see.

 

[iaminsider]

I think my truck is fixed... I ran a bottle of Royal Purple Max Clean and in the last week, I have not had any stumbling.

I previously ran 2 bottles back to back of Techron and that did nothing to help.... Maybe Max Clean will replace that as my go to.

I have the ignition switch sitting on the shelf, but I am not going to change it unless the issues re-appear.

 

[AzTruckGuy]

I am having the same issues, so you think it was just by running a bottle of Purple max?

 

[iaminsider]

It helped me but remember I changed plugs, cleaned the TB VERY thoroughly, and changed O2 sensor. I never had any stalling either which is different from you.