idle speed

[302Z28]

On a 2.2L 97 Cav, how does the computer maintain the idle speed? Is it done by retarding the ignition timing or a metered vacuum leak? My car will sometimes stall when turning the steering wheel at slow speed. I've had two different codes for rich fuel mixture. The light always comes on at an idle, but then goes off when driving. At times, the throttle seems very "mushy" when coming off idle, up to about 1000 RPM, then it runs great. I could change the 02 sensor, but first wanted to get some responses on an idea I have. The stalling, combined with the mushy throttle and the fact that the engine seem,s to come alive at 1000 RPM and the rich fuel light lead me to believe there is a shortage of air available at low engine speed when the butterfly valve is closed. Would it help to open the butterfly valve on the throttle body a turn or so of the adjustment screw, or would that create other problems? Any input would be greatly appreciated. Thanks

[J-Ri]

The idle speed is controlled with the Idle Air Control valve, which is controlled by the PCM. The IAC does the same thing as the throttle valve, except it is a pintle style valve, it allows more air for higher RPM and less air for lower RPMs by moving the valve into and out if it's seat. If the IAC is sticking the engine may be dying because the IAC can't open enough to maintain idle with the extra load of the P/S pump. Remove the IAC valve and clean it and the passages in the throttle body,

Because of the codes you have for running rich only at idle, and the way the throttle feels "mushy", my first thought is that your fuel pressure regulator is leaking into the vacuum line. It is probably leaking and providing more fuel than the engine can burn at idle, but less than it needs at higher RPMs. That means that the PCM cannot control the fuel mixture at idle because the leak is giving more fuel than the engine needs, so the PCM may not be commanding the fuel injectors on at all (since it's rich already), and then once you get to an RPM that uses more fuel than the leak is providing the PCM starts commanding the fuel injectors on and it seems to run like normal. This problem could also be caused by one or more leaking fuel injectors, but that is less likely than the fuel pressure regulator.

There is no idle adjustment screw. The idle is controlled 100% by the PCM. If there is what looks like an adjustment screw, it's only there as a stop so the throttle valve stops at closed, rather than opening the opposite way. Don't mess with it, doing so may make the symptoms go away (if so, probably temporarily), but the problem will still be there.