Truck accelerating on it's own!

[TONY4X4NC]

16 2500 6.4 280k miles

Just got back from a hunting trip about 1500 miles round trip. Started the truck the next day and put it in drive and hit the gas, RPMs went up slightly but moved slow like a the trans was slipping, then drove fine for the rest of the day. I noticed some weird transmission issues, when coming to a stop from about 60mph or any speed, it held the higher gear and was still trying to accelerate as I was on the brakes trying to coast to a stop. I noticed if im driving at about 20-30 mph, the truck will maintain speed like it's on cruise control and will accelerate if going downhill (while off the accelerator). I was thinking Idle air control valve, im hoping for not a transmission issue, I think the engine revving is giving bad signals to the trans ECM causing weird trans issues. Any help?

 

[Dean2]

Nearly impossible to remotely diagnose this, especially if it isn't throwing any codes. Process of elimination, or put a recorder on the truck for a few days so you have something to work with.

 

[TONY4X4NC]

Don't have a recorder, just the cheapo $100 scanner. Process of elimination what would you go with? Since it seems ot be only an "off throttle" situation is why i'm thinking IACV. Any other suggestions?

 

[Dean2]

Don't have a recorder, just the cheapo $100 scanner. Process of elimination what would you go with? Since it seems ot be only an "off throttle" situation is why i'm thinking IACV. Any other suggestions?

If it was me, I would buy the recorder, rent one or get the dealer to put one on the truck. You can easily spend WAY more than the price of the tool replacing the wrong stuff.

 

[RamRod37]

First thing I would look at if you get a way to read the graph voltage curve of Throttle Position Sensor, Accelerator Pedal Position sensor, and Throttle Body valve position sounds like could be one of them going bad but Diagnosing is the only way to find the issues could be several things sudden intake leak I mean I could go on but I would start first with that they normally would throw codes but I have had two in similar cases that never threw a code so its just a process of Elimination at this point and that is where I would start

 

[1999 White C5 Coupe]

16 2500 6.4 280k miles

Just got back from a hunting trip about 1500 miles round trip. Started the truck the next day and put it in drive and hit the gas, RPMs went up slightly but moved slow like a the trans was slipping, then drove fine for the rest of the day. I noticed some weird transmission issues, when coming to a stop from about 60mph or any speed, it held the higher gear and was still trying to accelerate as I was on the brakes trying to coast to a stop. I noticed if im driving at about 20-30 mph, the truck will maintain speed like it's on cruise control and will accelerate if going downhill (while off the accelerator). I was thinking Idle air control valve, im hoping for not a transmission issue, I think the engine revving is giving bad signals to the trans ECM causing weird trans issues. Any help?

It has been my experience that when a throttle position sensor begins to go bad, it has a “flat” or “bad” spot midway with the accelerator pedal depressed, but doesn’t always set a stored trouble code.

I do not have experience with the 6.4 HD, but with other Jeep & Dodge vehicles (with a defective TPS).

 

[crazykid1994]

You need the live data to see what the computer is trying to do. Actual Throttle position and pedal position as well as commanded throttle position. There is no IAC. That is part of the throttle body now. Could also remove the intake tube to the throttle body and check the throttle body function to see if it’s sticking. Is the engine idling at the normal rpm or is it sitting higher or is it fluctuating more than normal. With that many miles I wouldn’t be surprised if it was a bad throttle body or bad accelerator pedal

 

[TONY4X4NC]

Update: Since the IACV is apart of the throttle body I just ordered a new throttle body from Dallas Dodge $206 and replaced it. Fixed the issue. Just to recap what it was doing for someone in the future:
No codes, when off the throttle coasting to a stop, the truck would maintain speed as if it were still receiving an input from the throttle. If on a downhill, foot off the pedal, truck would accelerate slightly on its own. This caused it to hold a higher gear and not want to downshift while coming to a stop. Truck had that feeling similar to power braking while on the brakes at a full stop. I definitely had some weird transmission shifts that made me think she was on the way out. 280k and back to running like she's supposed to.