I have a 1987 z28 with a 305tpi motor and a 700r transmission. Everything is stock however I have a very high idle. It idle at 1500-2000 and goes down a bit when in gear. Also when I put it in drive it makes a loud clunk sound like the transmission is jumping. How can I adjust the idle on this car or is there anything I can do or check to lower the idle.

 

Idle speed is supposed to be controlled solely by the Idle Air Control as it is controlled by the ECM. This can only happen if the intake system is intact with no vacuum leaks, the PCV system is not allowing excessive air flow, the EGR system is operating properly, the throttle plate position is properly adjusted to the 500-550RPM minimum air position, the IAC is clean and functioning, and the throttle position sensor is correctly reporting the throttle angle.

Begin with the easy items. Check for any vacuum leaks, including that PCV valve, EGR valve, brake power booster, and other external vacuum lines.

Once those are verified intact, check the throttle plate position at closed throttle. All too often the throttle stop screw will have been tampered with by someone who mistakenly believes that to be the idle speed adjustment. That stop screw is only provided to set the minimum air position for the TPS baseline and stall prevention in the event of IAC failure.

It is very likely that the 30 year old IAC stepper motor and the idle air passages need to be serviced. Chances are good that the IAC return spring is overly compressed and the stepper rack gear is contaminated with varnish. Removal, cleaning, and lubrication usually solves that.

This might help: https://www.thirdgen.org/forums/tech...ml#post2950871

 

If you need to clean the IAC, this will also help:

https://www.thirdgen.org/forums/tech...ml#post4052702

 

Might also be worthwhile to check the "sport" "appearance" "trim package" - which is NOT an "instrument" unfortunately - in your dash, aka tach, if that's where your RPM numbers are coming from, as those are well and widely known to read WAY WAY high. Out of the pile of em I have, one reads 3 times reality. You might find your idle speed is right where it belongs; in the 600ish range.

 

Do our cars have one of those? I always presumed the Fisher Body people put that dancing display in there to plug the hole the engineers provided in case anyone wanted to install an actual instrument, a little like the blanking plate over the hole for cars ordered without a radio.