04 Tahoe.....Sudden acceleration

[jdmidwest]

On 1/21/2023 at 8:34 PM, fishinwrench said:

 

Don't look like mine.

[Daryk Campbell Sr]

Just a tip for if/when it pr something similar happens. Throw it in neutral. Don't fight with it. One of those simple fixes that don't always come to mind. 

[fishinwrench]

On 1/23/2023 at 8:54 AM, Terrierman said:

I always wanted one.

I think they run about the same price as a new Nitro Z21 package, and a tow vehicle.     

Might be worth giving up bass fishing for. 🤔

[MrGiggles]

04 is all computer controlled. No cables anywhere as you know. Cruise should disengage when the brakes are applied, and it also has a minimum speed requirement. 

I'd suspect a throttle body that is sticking or acting funny, perhaps a wire in that harness. It should set a code in the event that the APP and throttle actuator are ever out of sync but you never know, and it may only show up as a pending code if it's a once in a blue moon thing.

I'd try cleaning the throttle body, look over that harness pretty well. 

One other thing that's kind of a longshot, those trucks are known to do some really odd things once the intake gaskets start to leak. Usually when they start to go they will get to running like crap for a few minutes until they warm up, revving up and down, sputtering, etc. The sniff test with a can of carb cleaner works just fine as a diagnostic there.

[fishinwrench]

1 hour ago, MrGiggles said:

04 is all computer controlled. No cables anywhere as you know. Cruise should disengage when the brakes are applied, and it also has a minimum speed requirement. 

I'd suspect a throttle body that is sticking or acting funny, perhaps a wire in that harness. It should set a code in the event that the APP and throttle actuator are ever out of sync but you never know, and it may only show up as a pending code if it's a once in a blue moon thing.

I'd try cleaning the throttle body, look over that harness pretty well. 

One other thing that's kind of a longshot, those trucks are known to do some really odd things once the intake gaskets start to leak. Usually when they start to go they will get to running like crap for a few minutes until they warm up, revving up and down, sputtering, etc. The sniff test with a can of carb cleaner works just fine as a diagnostic there.

Something "sticking" I could believe IF I accelerated and then the throttle didn't return to idle......  But this MF'er goes from idle to 3500RPM all on its own.  Both times it was as soon as I shifted into gear (foot obviously on the brake, because it won't shift until the brake is depressed).   

The first time it happened I had shifted into Reverse (foot on brake) and it revved out of control. threw it into Neutral, and I shut the key off.  Restarted and all was well again. 

The second time it did it, I backed out of a parking spot (at idle, no acceleration at all), shifted into DRIVE and it immediately accelerated like crazy.   I power-braked to a stop, and before I could grab the key it reduced itself to an idle.   Drove fine the rest of the day....and has since.      No unstable idle or any other symptoms.  

I wish you lived closer!   

[Ketchup]

On 1/19/2023 at 5:59 PM, Ketchup said:

Throttle body plates sticking. Have you cleaned the TB?

I still stick to my advice. 

[fishinwrench]

12 minutes ago, Ketchup said:

I still stick to my advice. 

If there's no cable or linkage from the accelerator pedal, then what opens the intake?    Is there a stupid servo motor or something?   

The design of these computer controlled pieces of ---- is annoying as hell.    Why? 🤬

[MrGiggles]

29 minutes ago, fishinwrench said:

If there's no cable or linkage from the accelerator pedal, then what opens the intake?    Is there a stupid servo motor or something?   

The design of these computer controlled pieces of ---- is annoying as hell.    Why? 🤬

Yep, servo motor. Allows the PCM to modulate the throttle for traction control, and whenever the transmission upshifts to reduce wear. Some of these new cars chop the throttle so hard during a shift it feels like there's something wrong with them. 

It also saves a lot of linkage and external components for cruise control, and eliminates the need for an IAC, which is why it's so important for them to stay clean. 

Given the symptoms it seems to me that it's related to that servo motor, whenever the PCM gets the gets the signal that the transmission has been put in gear, it knows to step the idle up slightly for the added load, maybe it's overshooting by a lot. Too bad it didn't set a code, that would likely tell the story. The throttle actuator has it's own sensor (essentially a TPS), that ensures the throttle is in agreement with whatever it's being told to do.

[Ketchup]

Giggles said it perfectly. I have had issues with 2 separate vehicles, both with the symptoms you mentioned. One i was able to clean the intake/TB and never as issue after. The other, i had to replace the servo. It would surge like a jet, but did it at least once a week. 

[fishinwrench]

1 hour ago, MrGiggles said:

Given the symptoms it seems to me that it's related to that servo motor, whenever the PCM gets the gets the signal that the transmission has been put in gear, it knows to step the idle up slightly for the added load, maybe it's overshooting by a lot.

That makes sense to me.     I'd say that you hit the nail on the head there. 👍   

Determining what component needs attention, however......is the lingering mystery.    I mean hell, it could be the transmission sending faulty info to the PCM for all I know. ( Confusing the shift switch with a passing gear....or something like that).

 

All I know for sure is this is a potentially dangerous thing.  Could cause someone to mow over a bunch of kids at the bus stop.....God Forbid!