What causes rough idle and vacuum drop during warm up?

EUWINN

MEMBER
1988 MAZDA 929
84,000 MILES

Car runs well except for idle during warmup. I come to a stop, rpm about 850, then after a few seconds, drops to 500. The vacuum drops from 18" to 12" and runs rough and may stall. There are no error codes and the timing and hot idle is set correctly with green 1 pin connector grounded. Slight throttle feed will smooth out idle with rpm at 850. I have tested egr, pcv, purge control 1, 2. It acts like a vacuum leak, but. The Throttle sensor idle to ground is 60 ohms, the gap is set correctly, the prc and tics solenoids work. Any clues? Isc valve? Water thermo sensor or switch? The car does warm up slowly. Can ICS valve be cleaned?

1 Reply

KHLOW2008

MECHANIC
41,814 POSTS

When idle speeds drops, the vacuum would drop. Any vacuum leaks would normally increase the idling speed. Erratic idling speed are usually due to air passage restriction reducing the amount of air getting to the manifold. The most common cause would be dirty ISC and throttle body. Get them cleaned and retest. There are many commercially available carb/throttle body cleaners and they do work wonders.

This vehicle uses an air flow meter therefore any vacuum leakages between this and throttle body would cause erratic idling.