Really High Idle 2300rpm

[barlaam1]

Hey guys I have a High Idle problem on a truck I've had for about 6 months. Here is the history on the truck. Before I bought this truck it had been driven maybe once or twice a year for 3 years. It is a 98 GMC 1500 with the 5.7. No mods that i know about, 140,000 miles. First I had to do a lower intake gasket replacement 1 week into owning the truck. A month later we replaced the rod and main bearings. and the fuel pump. after that it had been running ok and then it started throwing the code for EGR so that was replaced. problem fixed there. Now it has been throwing the code P0430 (bank 2 catalytic converter operating below efficiency). I'm assuming on this one I will probably have to replace the cat. It only throws this code when i granny drive it. i put a bottle of chevron fuel injector cleaner in the tank and took it on a 5 hour/each way road trip. the last three weeks I've been driving it a little harder and the code hasn't come back. But that isn't my main problem right now. (or maybe it is I don't know). This week I was driving around and when i came to a stop the truck was really hard to stop. was idling at around 1000 RPM's in drive. When I put it into park it jumped to around 2300 RPM's. I don't feel like I lost any break pressure/resistance. Had a buddy look at it but he didn't think the brake booster was bad. there was still vacum there once we shut of the truck and popped the booster vacum line off. Break booster vaccum line appears to be in good shape. We are thinking this isn't a vaccum problem because the idle is so high. what do you guys think? what should i check? fuses? IAC valve? TPS? please help me on this one.

[Cliff883]

Yes a vacuum leak can cause a high idle. The tbi gasket was gone on my truck and the idle was really high on my 88. Once I replaced the gasket all was fine.

[barlaam1]

How did you figure out it was the tbi gasket? Do I need to start spraying vacuum lines and gaskets with throttlye body cleaner? I forgot to mention that at one point the problem actually fixed itself but came back 3 days later.

[barlaam1]

So it finally through a code p1509 idle air control valve. $60 later and the truck seems to be running a whole lot better. Hopefully that was the problem.