Hello. I have a 1992 Mazda Protege with a 1.8L SOHC engine. I wonder if anyone could please help me troubleshoot a problem with erratic idling. The problem is intermittent. It seems to idle properly for a moment then fall to just above 500 rpm before immediately returning to normal idle and repeating the process again indefinitely. Also, the idle drops almost 250 rpm when I have the headlights on and I shift the manual transmission into gear, then the idle stabilizes properly in neutral. I have been told that this is a common problem with this generation of Protege and that it can relate to carbon in the IAC valve. Where is the IAC valve and how would I clean it properly? Are there any other likely culprits? I would appreciate any input you can contribute.

Regards, Richard Fenemore

The symptoms you have are symptoms of IAC valve problems. If you follow you air intake hose it should "T" off to a smaller hose and go into the IAC valve. Should be somewhere around the upper intake manifold, and should also have coolant lines and an electrical connector going to it.

The IAC valve is located near the throttle plate on the engine side.
Clean IAC with an intake cleaner product ,ie: intake medic
from RADIATOR SPECIALTY www.gunk.ca , also clean behind throttle plate
carbon deposits behind throttle plate will cause poor idle and throttle responce.