High/Hanging Idle

[ssmith95]

Hi Folks. Just picked up a 1990 ZR1 #1476 this past weekend and on the test drive the car was hunting for idle, but RPM would settle to an appropriate level. After stopping, idle would fluctuate for a bit, then eventually settle. I figured the car had a vacuum leak, but the hunting has turned into a consistently high idle ~1400 rpm. A datalog with TunerPro shows 0 IAC counts and covering the airhorn fails to stall the engine out. With the airhorn covered, I can hear a "chirping" sound but can not locate it.


I've read through "Finding and Fixing Vacuum Leaks" and repaired the lines coming from the dual vacuum fitting on the front of the plenum. The MAP sensor and PCV valves are free of leaks, and the throttle body was serviced by Marc Haibeck 10k miles ago according to receipts I received with the car. Car also has a tune from Marc.

Anything else I should check before I pull the trigger and pull the plenum?

[rufcar]

I have noticed that even mu 08 zo6 does a similar act. I recently made my 90 zr1 do that on purpose as it solved my stalling issues. I know they all operate similarly though not exactly correctly. I am ok with mine. Everything was replaced and rechecked and nothing helped until I installed an adjustable vacuum line. Others might have more specifics, but I spoke to many, did every test and that line solved mine by increasing the rpms at a stop and holding longer while I pulled away. Even I was at a light it goes back to normal in appox 5 seconds again I am ok with that.

[ssmith95]

Update: Went and bought a Mityvac and cannot pull a vacuum on the cruise/hvac vacuum circuit. Gaskets on the way from Jerrys and a plenum pull within the first week of ownership! Ill keep updating the thread so anyone down the road knows what the solution was.