I've got a 1985 that varies its idle RPM from 550 to 750, in park and in drive. Car runs great above 750 rpm. New plugs, cap, rotor, and MAF sensor. Can't detect any vacuum leaks. I was told that the 1985 did not have a power or burn-off relay for the MAF sensor. Any ideas. Thanks.

 

Vacuum leak. 100% probability.

https://www.corvetteforums.com/m_37283/tm.htm

print it out, put it in the bathroom.

 

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My C4 '87 runs at 500 in park or drive(foot on the brake). Not sure why your's varies...

Ask Mech259 [8D]he might know...

 

The answer to your burn-off relay question is also in the article. Fifth post (p. 23). It won't matter for yours however; we already know whatcha' got there...

 

Throttle plates probably need cleaning. When they get carboned up, it is difficult for the plates to seal in throttle bore and the ECM has trouble controlling idle.

 

Thanks for all of the input. I guess I'll try cleaning the throttle plates next. I read the article and tried the propane with no luck of locating a vacuum leak. The car has 30300 miles on it and is like new except for my idle problem. I've set my idle at 700 rpm in park and the car purrs, but at 500 it surges to 700. What do you guys think of an EGRissue. I currently do not have any codes stored or a service light. By the way I also have a 2000 and a 1979 vette. Thanks.

 

One more thing to note, this is a 1985 car that has had about 2000 miles put on it in the last ten years. Thanks.

 

30K doesn't seem like enough distancefor avacuum leak to develop. I can't imagine carbon build-up either.

...or anything else failing, mechanical-wise. Rubber and other organic fiber compounds - vacuum hoses and gaskets,COULD fail, given the time period.

So I gotta' stick with an intake leak somewhere. Get back on this one tho'... I'd really like to know on this one.

 

I agree with cleaning the throttle body and the IAC also. You can usually hear vac leaks....like a hissing sound.

Does it make a difference if the engine is cold or hot? Usually a vac leak will disappear once the ECM goes closed loop.A easy test to do if you have an ohm meter is to check the resistance of the injectors hot and cold.

 

the burn off relay is behind the breadbox check the fuel filter and like said above clean the plates