hudsonce

92 f-150, 5.0L v8, automatic, E4OD Trans. Very high idle almost immediately when I start up. If i unplug the IAC, the idle DOES comes down (too low even.) I have cleaned the IAC, electrical connections to IAC sensor, and air filter box/ new air filter.

Can I conclude that: 1. the high idle is NOT caused by a vacuum leak; and 2. the IAC needs to be replaced (possibly cleaned again/ better?)

Not sure if this is related, but I can start fairly easy if i step on gas, and NO start if i just turn the key without gas pedal.

Also possibly not related, truck is very crusty and probably sat for some time before I got it yesterday. Don't know too much about previous owner.

Thanks in advance,
Chad

P.S. Good to be back: just got my 6th Ford truck (4 F-150's, 1 Bronco, and drove/ maintained my Dad's F150 in the past.) Will be asking for help with the other problems after I solve high idle.

And yes, I searched and can't find direct answers to the 2 questions I posted above.

88xlt

Actually I think you just proved the IAC is good. I assume the idle stays fast even when the engine is warmed up? You are supposed to have a somewhat fast idle when the engine is cold.

From what I read and from what you did with the IAC, it shows you have a bad sensor. In my experience and from my reading it's most likely the TPS throttle position sensor.

JCH841

Probably the TPS. One of the TPS functions is to signal the ECM to add a fuel multiplier, aka accelerator pump. The ECM responds to a bad TPS by raising idle speed to prevent an off-idle lean bog.

raski

You can test the TPS with an analog ohm meter. Disconnect the plug from it and connect between the wiper(common) and each leg, move the throttle lever slowly from stop to stop. You should see a smooth change in resistance as you move the throttle. Usually, you will get a dead spot somewhere in its travel or the meter will be erratic if the sensor is bad. It's nothing more than the volume control which was present on the old tv's.

hudsonce

Definitely TPS. Took a while to figure out how to accurately "backprobe." I plan to write a detailed description later for others who do not know (like me.)

Manual says TPS should read 0.5 to 1.0V with KOEO. Mine tested at 2.09V. Perfect explanation. The truck basically thought someone was constantly holding the gas pedal down a little bit. Replaced the TPS and the following seem to be fixed:

1. Starts easily without having to step on gas pedal. 2. Idles just right both cold and after warm-up. 3. Previous VERY VERY hard shifting appears totally resolved.

Thanks again, Forum Friends,

Chad