Truck accelerating on it's own

btcva19 · July 6, 2004

I haven't been driving truck much lately because I have been riding the motorcycle. I have a new problem, I don't have to even touch the gas pedal and the truck will accelerate to about 35 mph. The rpm's seem to stay at about 1100. Once I come to a complete stop the rpm's will drop down to around 800. Same with or without a/c. Also makes downshifting more abrubt. Anybody got any ideas on this? All ideas will be appreciated.

citydriver · July 6, 2004

Is the A/C on when it does this??

Does it calm down when the engine warms up? coolant temp sensor? just a guess

PFarkas · July 6, 2004

What does the engine rev to if you put it in neutral while you're rolling at about the 35mph that the truck seems to want to?

Does the problem come and go?

When did you first notice it?

99-2door · July 6, 2004

IAC

Idle Air Control

PFarkas · July 7, 2004

99-2door Posted on Jul 6 2004, 10:34 AM
IAC

Idle Air Control

Is this a common problem?

I think I've been observing something very similar to what the original poster describes. For me it's been intermittent and difficult to reproduce.

Is there any way to confirm that the problem is the Idle Air Control?

What is involved in fixing this?

99-2door · July 7, 2004

99-2door Posted on Jul 6 2004, 10:34 AM
IAC

Idle Air Control

Is this a common problem?

I think I've been observing something very similar to what the original poster describes. For me it's been intermittent and difficult to reproduce.

Is there any way to confirm that the problem is the Idle Air Control?

What is involved in fixing this?

Yes, fairly common. The Idle Air Control in located in the side of the thottle body. It is a electric solenoid like valve that controls the idle speed. When it sticks open the engine will idle too high and if it is bad enough it will cause the engine to pick up speed even with the tranny in gear. On a 96-00 Vortec it is on the driver's side and has 2 torx head screws holding it in. Unplug the connecter and pull it out. Sometime you can clean it up with carb cleaner and it will work properly again. Sometimes it has to be replaced though. Around $60-$70 if I remember correctly. Available at any parts store. Go ahead and clean the throttle blade and body while you are at it.

snoman · July 7, 2004

Several years ago I had a simular problem with my 89 TBI engine. It turned out to be a worn gasket between TBI unit and manifold that looked fine until you remove TBI unit to inspect it.

99-2door · July 7, 2004

Several years ago I had a simular problem with my 89 TBI engine. It turned out to be a worn gasket between TBI unit and manifold that looked fine until you remove TBI unit to inspect it.

To check for a torn gasket, while everything is in place spray carb cleaner at the base of the throttle body. If the engine stumbles you have a bad gasket. If not you are ok there.

AZ-Z71 · July 8, 2004

Yeah, mine did this right after I did the three inch body lift.

Had the MAF unplugged and cold air intake off and need to more the truck up a few inches in the driveway. At any rate, when completed and put everything back together the truck started to drive itself the same way. It also had an SES light on. It corrected itself after about thirty miles.

With the MAF off I guess it didn't detect the correct volume of air and changed the idle up.

So maybe you have a small leak in the intake. I would imagine you would get a code for that. Also if the IAC is dirty or bad I also assume it would throw a code as well. I would first check you cold air intake for leaks. Maybe around the throttle body connection.

If you do need to replace the IAC the cost was like $100 from Autozone. I also changed that because I thiught my idle should be running higher when the ac kicked on but there was no change. I might be willing to part with my old one at an extremely discounted price if someone wanted. I kept the old one after I found out there was nothing wrong with it. (Plus the parts store has a little policy about returning electrical goods)

snoman · July 8, 2004

Several years ago I had a simular problem with my 89 TBI engine. It turned out to be a worn gasket between TBI unit and manifold that looked fine until you remove TBI unit to inspect it.

To check for a torn gasket, while everything is in place spray carb cleaner at the base of the throttle body. If the engine stumbles you have a bad gasket. If not you are ok there.

Good trick but mine was fine on outside and missing parts inside. Not sure this test would have effected it.