Truck tries to drive through the brakes

[PJones96]

I've got a 96 C1500 with a 5.7vortec with 250k on the odometer. It seems like I have a ghost in the machine here. I need some help diagnosing an idle issue. The idle flutters intermittently. I wouldn't call it a surge because it's a very quick blip of 50-100rpm then it goes back to normal. That's a cold start without the ac running.
The real fun happens when I put it in drive. I have to hold the truck back with the brakes. When going down the road it feels like the truck wants to drive itself, meaning that it will gain speed without me using the accelerator. The pucker factor has gotten up there a couple times, not gonna lie. I can stop the truck and once the truck is stopped it eventually will idle down to a normal rpm. I took a short drive from a cold start. Put the truck in park and it revved to 3k. I had to put it in reverse to bring the rpm down low enough to shut it off. There's no CEL or pending codes.
I've replaced the MAP sensor, IAC and EGR valve. I've cleaned the mass air sensor. I realize I've described two different scenarios but I figured I would give as much info as possible. I'm trying to get this truck ready for my 16yo to drive but I can't give it to him like this....

 

[Caman96]

I've got a 96 C1500 with a 5.7vortec with 250k on the odometer. It seems like I have a ghost in the machine here. I need some help diagnosing an idle issue. The idle flutters intermittently. I wouldn't call it a surge because it's a very quick blip of 50-100rpm then it goes back to normal. That's a cold start without the ac running.
The real fun happens when I put it in drive. I have to hold the truck back with the brakes. When going down the road it feels like the truck wants to drive itself, meaning that it will gain speed without me using the accelerator. The pucker factor has gotten up there a couple times, not gonna lie. I can stop the truck and once the truck is stopped it eventually will idle down to a normal rpm. I took a short drive from a cold start. Put the truck in park and it revved to 3k. I had to put it in reverse to bring the rpm down low enough to shut it off. There's no CEL or pending codes.
I've replaced the MAP sensor, IAC and EGR valve. I've cleaned the mass air sensor. I realize I've described two different scenarios but I figured I would give as much info as possible. I'm trying to get this truck ready for my 16yo to drive but I can't give it to him like this....

Throttle plate sticking? Leaking fuel injector spider?

 

[454cid]

Did you replace the sensors trying to fix this issue, or did it start after the sensors were changed?

I'd suggest looking at sensor data with a scanner.

 

[0xDEADBEEF]

Check for vacuum leaks.

 

[PJones96]

Throttle plate sticking?

Did you replace the sensors trying to fix this issue, or did it start after the sensors were changed?

I'd suggest looking at sensor data with a scanner.

The throttle is not sticking. I don't have to give an input to make this happen. I replaced sensors trying to fix the issue...I figured with 250k mile it's probably not going to hurt anyway. I don't like throwing parts at it but here I am. I will check the MAP sensor readings and report back.

 

[PJones96]

Check for vacuum leaks.

I will spray around the intake with brake clean and see if idle speed changes.

 

[Stockdale Dave]

Doesn't the TPS (throttle position sensor) on the side of the throttle body come into play with erratic throttle input?

Sounds like two issues though. One at idle and one while driving.

 

[Schurkey]

Connect scan tool, verify IAC, coolant temp, and TPS. Oh, hell, just verify ALL the sensors and computer outputs--but pay special attention to IAC, coolant temp, and TPS.

Can the computer control idle speed if you use the scan tool to increase or decrease the idle?

How old are the usual "tune-up" parts--cap, rotor, plug wires, plugs? What are the misfire counts for each cylinder?

What is the fuel pressure?

 

[HotWheelsBurban]

Throttle plate sticking? Leaking fuel injector spider?

Or throttle cable or linkage? Check all the way back to the gas pedal. Something is hanging up somewhere.....

 

[PJones96]

Doesn't the TPS (throttle position sensor) on the side of the throttle body come into play with erratic throttle input?

Sounds like two issues though. One at idle and one while driving.

I've replaced the TPS as well. I think I did that first which is why I forgot to include that in the thread. I will use the scan tool and get readings for all the things. Hopefully you guys can help me decipher. Thanks for all the help so far. This is the first thread I've started and I appreciate all the input.