rough idle

Hello,
I have a 1996 Explorer that has very low (~400 rpm) idle when it's not started correctly. If the starter is disengaged before the engine starts running, then started until engine runs, it idles to low on it's own. I have to give it a little throttle to keep running. Then it continues to idle low until it's driven down the highway 20+ miles or until the engine is turned off and sits for 6+ hours. If not, I have to keep my foot on the accellerator to prevent the engine from stalling.

The only time it has the low idle is when the starter is disenganged before full startup. Any time the engine is started correctly, it runs fine. I have been living with it for a couple years, very careful to fully start the engine before releasing the starter, but i'd like to address the cause of the problem.

I've had a full tune up including fuel filter and injector cleaning.

Any suggestions would be greatly appreciated. Thanks!

Welcome to the forum!

I don't know how the "started correctly" would really have any effect on it. That seems odd. I think I understand what you're stating, just doesn't seem logical to me. Maybe there are others here who've experienced something similar. Could it be that it has more to do with the first start of the day or the first start after sitting for long periods of time, when the engine will tend to crank longer before starting anyway?

Have you had the fuel pressure checked at KOEO and KOER? Maybe the fuel rail is not able to hold pressure due to a leaking injector or pressure regulator.

Also, I'd suggest you check the engine coolant temp (ECT) sensor or just replace it. It could be giving the PCM a reading which implies the engine is warmer than it really is. How's the fuel economy?

Finally, I'd suggest you inspect the throttle plate for carbon build up and consider replacing the Idle Air Control (IAC) servo. One way to test the IAC would be to monitor the engine idle speed when you turn on the A/C. The IAC should compensate for the increased load of the A/C compressor such that you don't notice a drop in engine speed at idle when the compressor clutch engages the compressor.

-Rod

I would take the Idle Air Control Motor off, and give it a clean job. I would also clean the throttle body, and possibly check out the Throttle Position Sensor to make sure it is reading progressively from low volts (.5-.9) to high volts (5v) as you open the throttle lever. Key on, engine off, digital voltmeter hooked up to backprobe the signal and ground wires of the TPS.

The Intake Air Temp and Coolant Temp Sensor (especially the Coolant temp) would be high on my list of things to check, as stated above. Pull codes, and if you can, put the car on a scan tool, and see if all the sensor readings jive with reality.