Question: My car which is a 1986 trans am 4bbl idles around 13-15 in park when at op temp, but idles normal 7-8 in drive. What could cause this cause im getting pretty tired of my car lurching forward from park to drive and having to turn it off in drive. Ive had this car for 3 years and have rebuilt and replaced just about everything under the hood, so i dont know if this is just one of those things where it is what is is. I would think it would should idle in park and drive around the same range.

 

There is too much idle air for some reason and that needs to be reduced. Some easy things to do are:

Check the setting of the main idle speed screw;




Make sure the high idle cam is completely clear of the high idle screw with the choke open;




Check for any vacuum leaks, including the EGR, PCV, brake booster, intake gaskets, and hoses;


Make sure the secondary throttle plates are closing fully;




Verify none of the throttle plates are sticking in their bores.