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The cause of high engine idle and surges in the 1989 Corvette is due to..Read More
That usually means it's lean. Check all vacuum lines and gaskets for leaks.
Check throttle positioning sensor.
The inlet manifold is leaking air, weakening the mixture, OR the engine timing/sparkplug electrode gap is outside specified limits.
engine idle set too high or the timing is off and needs reset.
Engine idle too high.
Vacuum leek
low idle and uneven idle
High idle, misfire, check engine light,High idle, misfire, check engine light,
There are several things that can cause high idle. An older carbureted engine may have a vacuum leak or sticking choke linkage. Newer computer controlled engines have an idle control device called an idle motor which could be faulty or receiving bad information from other sensors on the engine ( IE. temperature sensor ).
Added info to question: Vacuum also coincidently went from steady 20psi to 15psi/knock - small backfire/miss on acceleration from idle. Knock is steady at idle.