93 idle speed oscillates

[jwclemens]

1993 Ford Taurus 3.0L, not SHO not altnetative fuel.

On COLD startup (its about 60 degF outside temp) with transmission in PARK engine idle speed oscillates with a period of a few seconds from a reasonable speed to a speed so low that it almost stalls. No check engine light or any malfunction indication.

After driving and egine is warm the problem goes away. The problem only happens with a cold engine and only in park.

Any ideas? - Thanks

[shorod]

I'd start by running a bottle of fuel injector cleaner through the car for a hundred miles or so. Probably won't fix anything, but certainly won't hurt either.

Next I would suggest checking the throttle body for carbon build up. You will want to check the back side of the throttle plate. If you see a fair amount of tar-like sludge, get yourself some STP Throttle Body Cleaner, hold the throttle plate open with the car not running, and spray the sludge away. You may need to remove your spark plugs afterwards and crank the engine a few times to get all of the gunk out of the cylinders. Be sure to put rags over all the spark plug holes when doing this or you'll have a real mess.

If that doesn't seem to help the problem either, then check your coolant temperature sensor (not sender, as the sender only controls the temp guage in the instrument cluster, the sensor is what the PCM uses to adjust the air/fuel ratio). If the sensor is out-of-range, it may be causing the PCM to think the engine is warmer or cooler than it actually is and change the air/fuel mixture accordingly.

You'll also want to inspect the Idle Air Control servo for proper operation too. What happens to the engine idle speed if you turn on the A/C or defrost? If the engine speed does not speed up, or the engine completely dies, then it would appear that the IAC servo is faulty.

Of course you'll also want to inspect all of the vacuum lines and air intake hoses. Pay particular attention to the line to the EGR valve. Make sure the Mass Air Flow (MAF) sensor is properly connected and the clamp to the air intake is tight as well. If your Taurus does not have the MAF, then inspect the electrical and vacuum connections to the Manifold Absolute Pressure (MAP) sensor.

You may also want to remove your EGR valve and inspect it for excessive carbon. If you have a MityVac vacuum pump, pull a vacuum on the EGR valve with the engine idling and see if the idle changes. If you don't experience a change, you may have a stuck EGR valve. If the engine stumbles more and dies more quickly, then the EGR would seem to be working.

-Rod