Cold Idle Problem

[Jim C]

I just purchased a 99 Explorer Sport with a 4.0L SOHC Engine. When I start the car in cold weather, the engine will idle fast for a couple of seconds and then quickly drop down and stall the engine. If I give it some gas while the idle starts to drop, it won't stall as long as I rev the engine for a few seconds. I know thier must be something on the engine that prevents it from stalling and keeps the engine at a higher RPM until it begins to warm up. Can anyone help me with this problem?

Thanks in Advance,

Jim C

[shorod]

Welcome to the forum!

There is an Idle Speed Control (ISC) motor, aka: Idle Air Control (IAC) servo, that opens or closes an air passage around the throttle plate to keep the engine idle at the proper speed. As you probably expect, there are various sensors that contribute to the PCM determining the proper engine speed.

If you coolant temperature sensor is out of spec, it could be causing the PCM to think the engine is warmer than it is, so it attempts to drop the idle speed on the cold engine, and the engine stalls.

You could also have an Exhaust Gas Recirculation (EGR) system that is not functioning quite properly. The EGR valve should be closed during idle. If not, it can stall the engine. Sometimes the EGR valve gets contaminated with carbon and fails to close completely, or gets stuck open.

Also, there may be a build-up of carbon on the back side of the throttle plate or in the throttle body itself that snuffs the engine at idle.

You also should verify that the air filter is not plugged, and that the air intake hose is connected properly to the Mass AirFlow (MAF) sensor and throttle body.

-Rod