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High Idle (Extreme)



Idle on my 84 vette was around 1700 RPM. Replaced the vacuum hose and the PCV valve. Idle went down to around 900 - 1200 RPM after the vette warmed up. Stays at 1200 - 1700 when cold

All of a sudden 2 weeks later the idle jumps to 2300 RPM and will not come down, even when the car warms up.

Does anyone know what the problem can be?

Thinking it may be the Intake Manifold gasket.

Any takers???? What should an 84 vette idle be:
High Idle(when cold) ?????
Idle when warm ?????????


Gone but not forgotten
Jan 30, 2001
Hermosa Beach, CA
1987 Z51 Silver Coupe
Welcome To The Corvette Action Center jlindsey!

IAC first comes to mind, then the TPS. ;)

Ok, ok, I'll explain. :L

The IAC stands for Idle Air Control, and TPS is an acronym for Throttle Position Sensor.

The idle speed on the TPI system is controlled by the electronic control module, through an idle air control bypass channel. When the engine is cold or when accesory loads, such as air conditioning, necessitate a higher idle speed, the ECM, through a stepper motor, opens a bypass valve to provide more air into the sytem. At the same time, the ECM increases the fuel injector open time to maintain the proper fuel ratio.

By monitoring the output voltage from the TPS, the ECM can determine fuel delivery based on throttle valve angle (driver demand). A broken or loose TPS can cause intermittent burst of fuel from the injector and an unstable idle because the ECM thinks the throttle is moving.

Sorry, but my copy of the procedure for testing those sensors is down in the garage, and I'm not going to throw you off further with guesses. ;)

I hope this helps you. :upthumbs



My 82 idles cold start at about 800-900 and 500-600 when warm.

I think Ken hit the nail on the head.

Good Luck


Remove the air cleaner and check to make sure the throttle plates are closing, I have found many different things that can keep the throttle from returning all the way. IAC is a possiblity but most of the time they stick out which give a low or rough idle. TPS againhave not found on to be the cause of high idle a lot of other problems. Vaccumn leak can give you a high idle but find most of the time they just give you a rough idle but you should be able to hear the leak. The spot where the leaks are most apt. to be found is over by the EGR valve the heat from the EGR valve will just bake the gasket and make it brittle and fall apart, just give it a squirt of oil with the engine running and see if the idle changes, you may even see the oil sucked in. You do need to remove the air cleaner assembly plugging the thermac vaccum port and start the car and see what happens when you start the car, watch the spray coming out of the injectors, if it's real heavy you may have to take a look at the coolant sensor, if you hace a round connector going to the coolant sensor you need to replace it with the two pin sensor the original barrel sensor had a real problem with poor connections that will throw off the readings and most of the time fool it into thinking it's much coolder then it is and dump a lot of fuel. But also with it idling if it is still high and you can see the throttle plates are closed then just look for the IAC pintels and see if they are retracted or extended. If extended as if trying to slow down the engine then you will need to look for leaks. If it's retarcted a lot then you have to suspect that for some reason the ECM is comanding the high idle then you will have to look at sensors and or ECM. But I think you will find something holding the throttle open, but check it out and let us know.

David Fulcher


Motor head!!!!
Feb 13, 2001
Valencia, CA,USA
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
And now the stupid thing that happened to me!

When I had my 84.. one of the plug wires got stuck behind the throttle bracket, not letting the linkage go all the way back.

The next thing was the throttle cable broke at the tang which holds the cable into it's mount. It got wedged and held the idle up.

Check for Vac leaks..

Is your check engine light on? Does it turn on when you first start the car?


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