Info: 1991 Z28, stock mechanically.

The issue is that on initial cold startup, the idle speed is 5-600 rpm, after about 15-30 seconds the idle ramps up to 1000-1100 rpms and will idle at that speed all the time. It does seem to search for an idle speed a little, but not much maybe a 50rpm oscillation.

We have checked for vacuum leaks, none. The car runs pretty good otherwise. It does definitely run quite rich at idle, but in driving around does get fairly good gas mileage.

I'm not sure if I have a coolant temp sensor problem, base idle speed problems. Im just at a loss that initially the car will idle so low then ramp up to the higher idle speed. I have cleaned the outside of the IAC motor, but I cannot see it move at all.

Anyway, just looking for a little help.

Scott

 

try checking the o2 sensor mine had the same problem. stock headers use the autozone or napa sensor, on aftermarket headers get a heated sensor.