1991 Explorer idle revving mystery

[bigdavet]

I have a 1991 Explorer 4.0 4x4 automatic that's been a great car except for one thing...at random times the idle revs to 2000 rpms for no apparent reason. If I shut it off and turn it back on, it will return to normail, but then will rev back up again, randomly. My mechanics couldn't figure it out, and I trust them with all my cars. This has been happening for a long time. It doesn't get driven much anymore, and may go up for sale, so I'd sure like to get an idea what's wrong.

 

[Roadrunner777]

Possibly an issue with the IAC valve on the upper intake, or something with the powertrain control computer.

 

[poppyseed]

faulty tps (throttle position sensor)

 

[Roadrunner777]

This can happen if the diaphragm of the fuel pressure regulator springs a leak. Fuel gets sucked down the vacuum line to the intake manifold.

 

[Corik]

My 91' Explorer has been doing the same thing lately. It will rev to 1000 when stopped in any forward gear. and 1500 in park and stay that way for a few minutes or until I turn the truck off and turn it back on.
I've tried unplugging the FPR while it was happening but no fuel in line, and it seemed to react but only revving higher.
I unplugged the IAC and the idle came down to what I get normally. plug back in usually comes back but today it didn't.

I cleaned my IAC but it didn't help. It happens usually if I hit the gas hard like passing someone on freeway or going up a steep hill. Saturday while down hill braking...

 

[Mettler]

Mine used to get stuck at 2000 rpm if I'm doing a road trip. Come to a rest stop and it would just stay at 2k. Unplug the iac and it drops back to normal. I just recently replaced my iac with a completely different one. Totally fixed my problem. Also, doesn't idle surge anymore. Used to get a 50-100 rpm idle surge when I start it up. And it would do it while driving aswell. No more.
I'm telling you, replace the iac. You'll be happy with it again.

 

[eddie91explorer]

i have 91 exploder that had this problem only when idling in nuetral (i got 5 speed) and i found it was vaccum leak. Usually if pop ur hood u can hear air whistling in my case i could not. But i took starting fluid and sprayed vaccum lines till it reved up usually they get little pinholes in them.. if this help it was vaccum line for my smog system right near radiator

 

[RustySJohnson]

Alternator

Hear me out......

I chased this problem for months only to find out the bearings were bad in the alternator and it wasn't charging properly because the rotor inside was misaligned from the stator coils. The engine would idle up to attempt to charge the battery (i think). I'd have to stand on the brakes while stopped or sometimes it would chase it's idle, surging. The bearing on the rear of the alternator was dry and had almost fell apart. I could grab the front pulley and make it pivot. Check it.

The alternator can be repaired easily with new bearings. The one on the front is a 6203 and the rear I can't remember, but they are both available from your local parts stores or industrial supply distributors for $5-12. Brushes and the diode packs can be replaced separately as well as a cheaper alternative to a rebuilt one.

It's odd, I know. And why would someone check the alternator if it's surging, it doesn't even seem relevant? It can cause this exact problem, I've had it.

 

[$$exploder$$]

I've had the same problem for a while now. Iac.?