Questions and Answers

Porsche 944 Idle Fluctuations Q. I have a 1988 Porsche 944 with a 2.5 liter 4 cylinder with a 5-speed manual transmission and A/C. When I first start it idles high, after a couple minutes it idles rough between running 300 to 1,400 Rpms. It drives fine but whenever I come to a stop it will either drastically drop in rpms and then shoot back up or stall.

A. You should first check for vacuum leaks and make sure all electrical connectors are clean and tight.

It is probably a sticking or bad Idle Air Control Valve (IACV). There is no simple method of testing the IACV.

Brief Test: Engine Idle Speed (840 rpm).

1. Screw in throttle bypass screw completely when engine is running at steady idle speed. After brief deviation the idle speed must return to about 840 rpm.

2. Connect a new idle positioner on the wire plug to check activation of idle positioner from the control unit. Idle positioner must vibrate noticeably (frequency activation) with the engine running. If engine speed is boosted with the bypass screw, the idle positioner must move in closing direction Idle positioner must turn in opening direction when engine speed drops.

Another possibility is a bad Auxiliary Air Regulator (AAR). This acts like the choke on a carburetor. Under normal circumstances, the AAR will be closed when the engine is warm and open when the engine is cold. To check it:

1. Disconnect auxiliary air regulator wire connector, then using suitable voltmeter, measure voltage between both terminals. Battery voltage should be evident.
2. Using suitable ohmmeter, measure resistance across auxiliary air regulator terminals. Resistance should be 20 to 55 ohms.