With the car in drive it idles AROUND 600 when up to operating temp. But the annoying part is that it doesn't operate at 600 steady. It's anywhere from 740-690 so there is some noticeable rumble and I can watch the tach fluctuate slightly and I can get a video if you'd like. I'm not sure what would cause this. It's a 90 Formula with an lb9. IAC valve is new and adjusted according to the manual.

Thanks to anyone with ideas.

 

throttle position sensor maybe.

 

Its an old car, with an engine thats got some miles on it, so it wont idle as smooth as it did stock. Also, the older ECMs didnt have very sophisticated idle controls like the newer models, so they cant control the idle speed as precisely. Newer computers also use the fuel and spark advance to control the idle speed in addition to the IAC (or e-throttle), so they idle much more smoothly.

Also, as for the tach speed, its likely to be off. Youd need to use a scanner or an external tach to get the true idle speed.

 

Yeah the scanner is what I used to see the actual idle speed (740-690 RPM). And yeah, I can understand how the older technology could be the cause.