high idle speed - fixed

[fraxinusalba]

My 97 Explorer (4L SOHC, 248K miles) developed high idle speed rather suddenly. Started just fine and idle speed was normal. Drove it about 10 miles, stopped at store and when I put it in park it was idling at 2000 RPM. Waited couple minutes and it didn't drop to normal so turned it off. Went in store for about 10 minutes came out and it wouldn't start. Waited about an hour and still wouldn't start. Left it until next day, still wouldn't start. Had it hauled home, replaced fuel filter, and it started at first turn of key but was idling at 2000 RPM. Let it run about 15 minutes and idle never dropped to normal. Put it in gear and it immediately died and wouldn't start again. Couldn't get back to it for a week but it started right up again but still idling at 2000 cold. Let it run about half hour and when it got to normal temp idle speed went up to 2500 and stayed there. Put it in gear and it immediately died and wouldn't start again. Checked voltage and resistance on IAC and it's within ranges. Removed it and cleaned it even though wasn't much carbon in it. Checked MAF voltages and resistance and it's within ranges. Removed and checked it and it's really clean - electrodes bright and shiny. Did same with IAT and everything seems fine there. Had upper intake off about 3 months ago and O-rings good. Checked TPS and everything within ranges there too except it only goes to 4.75 volts instead of 5 at WOT. Closed voltage is little less than 1 volt. Can't find any vacuum leaks either. There was a couple times prior to this that idle speed was higher than I thought it should be (around 1500) but always came back to normal idle after it warmed up. Suggestions on where to go next?

 

[koda2000]

Check to see if your throttle body plate is closing. It might be dirty and hanging up or you might have a problem with the throttle cable.

I your check engine light on? I should be.

 

[fraxinusalba]

Throttle plate works fine and TPS gives 1-4.75 volts from closed to WOT, no problem with cables from cruise or pedal. CEL burnt out years ago when original speed sensor went bad and none of the replacements would turn it off. Had the battery disconnected for over 24 hours running tests on all the sensors and trying to reset the PCM so not sure if fault codes will still be there.

 

[koda2000]

You should not have cleared your codes. Clearing trouble codes does nothing except clearing the stored codes. It doesn't solve anything. You can try reading them now but there;s no guaranty they'll still be set. Your're going to need the code(s) to know where to look. I guess it's possible your IAC is bad. Cleaning it is no guaranty that it's now working. You may also have a gross vacuum leak. That will also cause a high idle speed. The ODB II codes would have given you a clue. Without them it becomes a big guessing game.

You need to fix your CEL too. In areas that require emissions testing a non-working CEL is an instant fail. It also let's you know something is not working properly.

 

[fraxinusalba]

Problem fixed - open circuit between IAT and TPS on signal side. Never found breaks but they were somewhere between front of lower manifold where IAT and TPS wires entered larger wiring harness and about 18 inches from PCM connector. Didn't want to remove lower manifold and go through entire wiring harness so ran a couple wires along the fuel rail and spliced on both ends. This fixed the high idle speed but still wouldn't start after engine got hot, turning it off, and trying to start it again. Started right up when cold. Only thing I saw on schematics that would signal PCM for engine temp was ECT. Real pain as upper intake has to come off to get to it. Wasn't totally shot but resistance way out of range. Replaced and runs fine now, starts right up when hot.

Thanks to this site for pinout on PCM as other wires in harness were colored same. Saved going all the way through the wiring harness to find right wires on PCM connector. Pin numbers on my schematics weren't legible.

 

[decipha]

Ect is a critical input, if it reads very low you will be in the extreme cold range of the isc multiplier tables giving you a very high idle, it will also flood out on startup and cranking, holding the throttle to the floor during crank will shut off the injectors and prob would have let it fire up

The full ecu pinout is here
http://efidynotuning.com/pinouts.htm