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I have a 1998 Chevy K3500 with a 5.7L engine. I purchased it about 3 weeks ago and have noticed that once it gets warmed up the engine has a tendency to idle at 2000 to 2500 RPM while this is going on the tachometer needle will jump around although the engine is at the same speed. It does not do it all the time, it sometimes idles right at around 800 to 1000 RPM where I expect it to be and the tach needle is right were it should be… The truck has high miles 224,000 and the previous owner was a pool company that used it for service calls and plowing during the winter. It just had work done on the intake manifold. I don’t know if this is a bad control sensor/valve or an intake leak. I have considered replacing the following:
Throttle position sensor
Idle air control valve
Mass air sensor

These are all around $100 parts so I don’t really want to guess. Anyone have any suggestions?

The next time the idle goes high, take the handle of a screwdriver and rap on the IAC valve. If doing this effects the idle speed, replace the IAC valve.


Would that cause the tachometer to jump around as well?

It wouldn’t have a direct effect on the tach. It can send the idle dancing around though - which will register on the tach.

Have you cleaned the MAF? How about the throttle body?