Ok here goes:
Changed air filter in 1999 Ford Explorer. Noticed that it was idling high after that. Normal idle has always been around 800 RPM.
Problem started getting worse. Idling even higher. Took air intake apart and cleaned the butterfly valve in there. Really gunky. Thought wow that must have been the problem.
Disconnected battery to reset the memory. Started it up and it did fine for about a minute. Idle went up. And by up I mean 1500 RPM and would not come down. Checked all vacuum lines. Seem to be all connected right and in good shape.
Started reading about potential problems and found out about the IAC valve. Went and bought one and installed it. Did the Battery thing again. Started it and thought it helped but it still did not get back down to 800 RPM. Stayed at about a 1000. When I drove it, it got up to 1500 to 2000 RPM's at idle. Read some more. Installed new fuel filter. Disconnected battery and left off for 30 minutes or so. Reconnected battery and started truck. Beautiful idle at 800. Smiled. Left it idling for about two minutes. Put it in gear and idle stayed the same about 800-900. Put it back in Park and engaged the AC. Idle jumped up to 1500 or so AND KEPT CLIMBING. Got to 2000 and I turned off the AC and it jumped to 2500 RPM and would not come down.

Somebody give me a clue as to what I need to do next.

Please

 

Have you pulled the codes yet? There may be a code thrown even if the CEL is out.

 

No codes are showing up???????

 

Intake manifold gaskets. Take a propane torch or carb cleaner or something and spray it around the intake manifold ports and see if the rpm changes. The 4.0 SOHC is notorious for leaking intake gaskets.

 

Hey guys, I have same problem in my explorer 98' 4.0L SOHC.

I did install a new IAC Valve, ALL intake seal, new vacum lines, hoses, clamps etc, etc.

Mine has the idle running 1200 1500RPM sometimes it reduce to normal 650 RPM.

Once, when the idle was about 1200 I did disconnect the IAC valve while engine was running and surprise, the idle was reduced to 450-500 RPM So it indicates the idle is geting hi just because the PCM is comanding the IAC valve to do so...

As far as I remember, the PCM can change the Idle based on engine temperature, maybe it is "tinking" the engine is overheating or so...

Any ideas? no Check Engine light jet.

 

Bump!


Now I am wondering if in fact I may have screwed my electronics or something! Why? Because all of a sudden my water temp gauge is running clear up to hot and the check gage light comes on (not anytime when I am around a checker to check the codes) but every once in a while. Then just as suddenly it will go back down to normal. Pop the hood and there is NO indication that the radiator is over heating or anything.

Then today I get in it and start to leave the house and the red "Theft" light is blinking at me while I am driving it. Only happened for maybe a minute but it has never happened before.

Have I somehow screwed my electronics????? And what will it take to fix it. The computer module???

 

The high temperature readings sounds like a non permanent short circuit at temperature sensor wires. i don't know about the Theft light

 

Check your grounds first of all. Grounds are the culprit of many odd electrical gremlins

 

MisterCMK solved my problem. Idle when engine was cold was like running on 3 cylinders; sometimes would die w/o reving to 2k. As engine warmed up, even just a little idle improved and eventually would idle ok and run ok. Checked IAC, MAF, and ECT and all were ok. Bit the bullet and pulled intake-top and bottom. Bottom 'o-rings' did show some leakage (burnt area toward the outside on some of the cylinders). Replaced lower and upper gaskets and reinstalled. Now runs great and will idle cold. Total time for this shade tree mechanic was 6 hrs, including lunch, a 30 minute nap, one Shiner and lots of kibitzing with son-in-law, who was helping at times. Hardest part was making sure top distribution manifold was lined up right and installing torex screws at back of top manifold.....

 

Glad you got your problem fixed. Sounds like you and I work on vehicles about the same way. A nap here, lunch there, etc.

Welcome to FTE by the way.

 

Guys you aren't going to believe this but the problem is fixed and it turned out to be the weirdest thing ever.

I needed a new thermostat. It was partially sticking open/closed and causing the sensor to think the engine needed to rev up to increase coolant flow. How that all works is beyond me but danged if when I changed out the thermostat it didn't just settle back down to normal and everything has been perfect since then.

Who would have thought it?????

 

Wow, I would have never expected that.

 

ok - maybe that's my problem - a thermostat??? I have a 99 4.0 SOHC & my check engine light came on & my temp gauge is below cold! And my idle is 1000+ - could it be just a thermostat problem?

 

Bad/clogged T-stat or low on coolant. Check coolant level first, then proceed.

EDIT- do these engines have a CTS (coolant temperature sensor)?


CMOS

 

coolent levels are full - that was my first thought too .... prob does have a CTS - seems like it has sensors for everything .... Thanks for your help!!