Chevy TBI high idle speed - why?

Question

I have checked all vacuum hoses, made sure the accellerate cable is free etc. - done all the basics and even replaced the idle valve - but still the idle is too high - Cheyenne P/U 1989 - 305 small block with TBI injectors. Any ideas?

Answer 1

Still sounds like a vacuum leak. On these trucks the base gasket under the throttle body is prone to going bad. Squirt some carb clearner like B-12 or some WD-40 around the base of the throttle body with the engine running and see if it speeds up. If it does you have found your problem. You can also do this on the intake gaskets on the plenum and heads joints. etc. Also you mentioned that you had changed the idle speed motor. If so did you clean out the chamber that the plunger goes into with throttle body cleaner or some sort of carb cleaner?? Soot in that hole will cause the plunger to leak air past it and idle to be incorrect, either high or low depending on how bad and where at. Also make sure that the check engine light is out and that none of the sensors for temp, etc. are unplugged on the engine.

Answer 2

It really sounds like the computer is overcompensating somewhere ie o2 sensor or vacum leak. But I think there is a plug that can be drilled out and the idle adjusted on the linkage side also have you checked the engine codes?

Answer 3

Maybe the gasket under throttle body is leaking,simply spray carb cleaner around it to see if engine speed changes.if it cks out ok the intake gasket could be leaking from the inside. pull pcv valve out, block v-cover hole to/ se if theres a vacuum present at other v-cover.Also the timing could be set way to high .Specs call for around 4 degrees.,with EST conn disconnected.

Answer 4

Choke fast idle needs adjustment or replaced.

Answer 5

eng temp senser if it thinks its cold it might rev high

Answer 6

Check your ignition timing. It may be retarded...