ECM strategy qs.

[Pablo-UA]

Sometimes idle speed increases up to 750-800 rpm, i tried to unplug IAC, but idle speed sometimes increase. How ECM can do it w/o IAC? What for ECM does it?

[96_4wdr]

1. intermittent intake air leak
2. intermittent MAP or MAF sensor output
3. intermittent false signal from pwr steering switch or AC on switch

what's your O2 sensor V. reading doing during idle rpm increase periods?

[Pablo-UA]

Originally Posted by 96_4wdr

what's your O2 sensor V. reading doing during idle rpm increase periods?

I will try to measure. ECM increases idle speed for 5-20 sec. only. May be canister purge solenoid does it

[xlt4wd90]

Besides the air leaks, any false signal that causes the EEC to advance the timing will cause engine rpm to surge. That's easy enough to check with a timing light. You don't even have to find a real mark, just a reference point, and see if the timing changes when the engine surges.

Also, the EEC uses the IAC to bump up engine speed when running AC. If he's unplugged the IAC, that should no longer happen.

[Aeroman59]

Pretty sure the Cannister purge operates at 1500 RPM's or more, but if it does operate at idle, it could make a fluctation. I would disconnect & plug the purge line with your thumb & see if you can feel a vacuum. Do you have an intermittent sticky PCV ?