hanging rpm wanting instant idle return

[Frank The Tank]

I've been searching for information on why my 4.0 sits around 1,200 rpms and doesn't want to return to idle...... and I'm turning up a ton of 2.3 and 3.0 fixes, but nill on the 4.0.

I think this is killing my fuel economy, and being rather hard on the engine with cold startups, I'm open to any idea's. and if someone remembers a thread on the topic, I would love to have a simple easy link without being blasted for not searching for something I'm not even sure what it's technical term is.

Thanks
Frank

 

[RotorFX4]

I had the same issue for a while on my '02 SOHC. Mine turned out to be a two part problem. My throttle plate stop was adjusted so that at closed throttle it was open just a hair. This allowed my throttle position sensor to give an open throttle signal when it was supposed to be closed and caused my engine to idle erratically and maintain a very high RPM (3-4000) between shifts (manual trans). I also found that the check valve in my vacuum reservoir was bad, causing a vacuum leak. After fixing the vacuum leak the Idle got a little better but returned to normal after adjusting the throttle plate stop screw. The adjustment for my stop screw was to back it out until the throttle plate sticks when closed and then screw it in until it no longer sticks. I have also noticed that my manual trans will maintain a high Idle for a short time after throttle let off when the Air conditioning or any vent option that uses the A/C (defrost, foot warm, etc) is selected. I am convinced the higher Idle with the A/C on is normal. Hope this helps.

 

[trail B2]

Idle

Man isn't that irritating. I found A place on this site that told me to set A gap between the idle screw and throttle plate of .015 then turn it in I believe it was one turn check the throttle position sender and set it to .6 volts if you don't have any vacuum leaks that really helps but it still kinda floats down so I put A bypass switch on the IAC when i'm not in the mood for that shit.

 

[MountainMike]

I got the same issue in mine, just tagging this thread to see if there is any more info.

 

[Frank The Tank]

Any other information on this?
I guess I'll be looking for that screw to turn when I get off work, it almost never dropped between shifts or at stoplights the whole way home from my gf's.
With aftermarket it exhaust you look like a jack@$$ when your in traffic and your kicking out 2,500rpms waiting for the light to change.

Frank

 

[Earl43P]

You definately have a problem, Frank.

I'd suggest looking for vaccum leaks, which are pretty common. Look at the PCV elbows, etc. The other big problem area can be the intake plenum seals. My Explorer's were sucking air so bad I could hear it. I literally put my body weight on top of the intake and rocked it side to side and could hear the noise change.

Once I got it apart, I could see the dirt/carbon trails on the aluminum ports from where it was sucking the air. I cleaned those surfaces with a scotchbrite pad, replaced the seals and it made a world of difference. I can also understand why this leak point is hard to locate. You'd need "quite a straw" to spray TB cleaner all the way down there looking for a leak.

I wouldn't mess with the throttle plate adjustment screw unless you are going to adjust the TPS to .96v while you are at it. Just asking for trouble if you don't plan on doing it properly.

I highly recommend replacing the coolant thermostat while you have the intake out. You'll see why. I can't imagine doing that thermostat job without removing the intake.