1996 K1500 Manual Transmission

[crsgmc]

This is going to sound strange.....but starting truck and idle are fine. Put it in gear and accelerate normally and in any gear then remove food from gas petal the truck is still going strong. Push in the clutch and it is running at 2000 rpm some times it will drop back down but most of the time you need to stop the truck and it will return to 700 rpm idle. A few times I got it to do it in the garage in neutral reving it up to 1500 or 2000 and it started going crazy. Sometimes when cruzing with foot off the gas petal (when it is running down the road at 1500 to 2000) it speeds up and slows down. I replaced the throttle position sensor and the idle air control valve and it did nothing. Cleaned the MAF did not help so unplugged MAF and it ran a little worse but the problem remained as before. Cleaned the intake this did nothing. It is not throwing any codes. I also inspected the throttle cable and it works smoothly no binding or sticking. About a year ago I installed a new gm 350 crate engine and at that time I replaced a lot of parts dist, wires, plugs, fuel filter,new injectors, most sensors etc. I ran fine until this problem came up. What am I missing? Someone must have had this problem before.

Thank You For Any Help You Can Provide.

[Palf70Step]

My only guess is the throttle return or the cable. I can't think of anything else that might give you that issues. I don't know enough about TBI adjustments to know what to look at there.

Did it just start, or has it been doing it since the engine swap?

[crsgmc]

No it just started the truck ran fine for a year after engine replacement. It is crazy, start truck fine, put in gear and start to accelerate and it seems to hold a 2000 rpm on its own. Foot off the gas and I am doing 27 mph in second gear. The cable is fine works smoothly. Truck has never been to the dealer since I purchased it but I am running out of ideas. Thanks for the reply.

[crsgmc]

Just so people know the problem was the iac valve. The new one purchased and installed was bad. If you are having these symptoms it is probably the iac valve. I have purchased a lot of aftermarket parts with no problem, and in some cases they have been better in quality. In this case it did not work out, I am going to stay with name brands on anything electrical in the future.

[Palf70Step]

Glad you got it worked out.