1991 Ex strange idle issue

[Thedentedcan]

Howdy, new user and Explorer owner here.

I just picked up a 1991 XL 4X4 5speed, and I'm having a blast with it. I do however have one issue, and despite extensive searching I can't seem to find a solution. I have a high idle (tach shows 1000-1200k) at all times, cold and warm. I have already replaced IAC after not being able to clean the old one to my liking. My MAF is clean and I put new gaskets on the throttle body and IAC. the part that doesn't make any sense to me, is that I can get it to idle down to about 6-800 RPM by pressing the clutch in. For example, if I am slowing down to a stop and I press the clutch and shift to neutral, the engine revs to 1200 rpm. Once stopped I can press the clutch for a couple seconds and the engine will idle down to normal. I am on the verge of replacing the TPS just to see if that helps any, however I can't think of how the clutch would affect the engine idle in neutral. Does anyone have any ideas?

Thanks in advance for any help. I'm looking forward to getting her running smooth so I can hit some dirt soon

 

[Southern_Pride]

Look down on the throttle body and make sure the idle stop screw is adjusted right. Its on the passenger side way down in there and hard to see.

 

[Thedentedcan]

So, I'm going to necro my own thread, in case anyone else is having a similar problem. Its been two years and im still chasing this problem a little bit at a time. I found out when I manually actuate the clutch switch it idles down. Since my post I have replaced the TPS, IAC, EGR, PCV valve, MAF, Coil pack (it failed on me, figured I'd include it,) and verified the vacuum hoses, intake gasket, plugs, wires, fuses/relays, fuel filter, cleaned the throttle body, and checked the throttle cable tension and return spring. I'm thinking at this point its a short on the circuit the clutch switch is on, which include the TPS and the ECT sensor. I'm replacing the ECT this weekend, which may hopefully fix the vanishing coolant mystery as well. I'll report back if this help.

Thank the maker this isnt a daily driver and I can work on it at my leisure, I'd be pulling my hair out after this long of a battle.

 

[FR-425]

Yep, that's a brain teaser.. ... .. wth?

 

[vq5speed]

If your motor seems to increase idle as the engine warms up. And run crappy when restart after warmup. Replacing the ECT could fix that it fixed that issue for mine.
Why do you say you have vanishing coolant.

 

[Thedentedcan]

I have to refill about half a galon every 200mi or so. The oil looks fine thankfully. I did the very scientific test of spraying something flammable around the intake manifold which didnt result in any revving, and when I cover the throttle body it dies right away, so Im hoping its not some kind of gasket issue and just a leak somewhere. You can faintly smell coolant around the engine bay. I'm going to start with both temp sensors, which are probably factory anyway.
Maybe the ECT is shorting something as well, and will help in the RPM issue. After that, I think the only thing in the circuit I havnt covered is the clutch switch and the ECU. All of the wiring I can find looks fine.

It doesn't seem to be revving any differently when cold or hot. If I stop the truck, it stays about 1200rpm or so until I put the clutch in. I can move the clutch pedal switch and get it to idle back down too. As soon as the throttle is pressed, its back up though, and it pulls harder than it should while driving. Not aggressively, but it will almost host speed instead of engine braking.

 

[2stroke]

One thing that it doesn't look like you replaced is a coolant temp sensor. Its vital, and a common cause of high idle. You don't have any engine codes? As for the coolant, a gallon every 400 miles is a lot. It could be a leaking water pump, hose, lower intake manifold gasket, or head gasket.

 

[Thedentedcan]

Yeah, I picked up both coolant sensors, installing this weekend. I need to replace the water pump and hoses anyway, I have no idea how old they are. Hopefully it isnt a headgasket, I'll cross my fingers.

Bought a scanner and it shows the diag info but no trouble codes.

EDIT
Wow, water pumps are cheap. Im definately going to replace it.

 

[Thedentedcan]

Coolant sensor is in; the old one was corroded around the top of the threads and the hex collar, and it wasnt screw in nearly as tight as it could be, so I hope thats the leak.

The old sensor may have been shorted or faulty, it still has the idling issue but its much less severe, and it throws a cel now. Ill pull the codes tomorrow and see what is going on there.

I think the ECU may have been replaced at one point. The code matches up, but the board looks different, and some of the IC's are definitely not 1991 technology.

Should I have any concern with this?

 

[Thedentedcan]

It's fixed! So I guess a bad ECT was masking other problems this whole time. I noticed that after I swapped it the truck ran like crap, wouldn't idle, and smelled rich. I noticed nothing happened after I unplugged the IAC, so I put my old, factory original IAC in and it runs like a champ now. All idle problems are fixed, and the truck can crap along in first gear, engine brake, etc.