HO idles fine cold, stalls hot

[jeepcoma]

Hi everyone, I've recently noticed a new problem on my '91 4.0.  When I first start the truck, it idles perfect at the normal RPM (~750 on the tach, or whatever the first line below 1,000 indicates).  I can rev it, I can drive it, it's smooth and returns to the correct idle every time.

 

Not long after, with no apparent change in the way the engine runs, it'll idle really really low (to the point of just stalling out on occasion) when I put the clutch in to come to a stop.  If I veeeeerrrry gradually bring the RPMs down, either in neutral and using the gas pedal, or say being in 5th gear and slowly decelerating until the RPMs are about 750, it'll hold the correct RPM.  It just needs some encouragement.  More abrupt RPM changes, like from normal driving and clutching, result in the low RPM/stall condition.

 

If it DOES stall, and I pop the clutch to restart the engine, it goes back to normal (correct) operation for a while again until it starts all over.

 

I've done some research on this and the conventional wisdom is a problem with the IAC.  I do not believe this is the case here because it doesn't always exhibit this behavior, and it briefly goes away after a stall/restart.  It's independent of both ambient air temp and coolant temp. 

 

Since it takes a few minutes of running for it to occur, I'm thinking it's a computer issue with open loop vs closed loop operation.  I haven't put it to a stopwatch or anything but I bet it takes the same amount of time for the problem to start.

 

Anyone else run into this or have some ideas?  It never fails that if a problem is going to start, it'll be the dead of winter, when it's both freezing cold AND I have to bring the truck in for inspection!  :wall: Maybe I can say I installed one of those new-fangled start/stop systems to save gas... 

[gogmorgo]

Did you check for trouble codes?

http://comancheclub.com/topic/28111-reading-obdi-91-and-92-mj-fault-codes/

 

Seems almost like a throttle position sensor (tps) issue, but check your codes first. It's super easy, just turning the ignition key and counting flashes.

[schardein]

What Gogmorgo said, check your codes.  I had somewhat similar issues on my 91 XJ and the code led me to the Camshaft position sensor in the distributor.  Replaced it and all was well.

[HOrnbrod]

Welcome back jeepcoma, long time no see. :cheers:  Do check your fault codes, especially for a 21 - O2 sensor faulty. Sounds like you're shifting in and out of open/closed loop. If the O2 is old, replace it. Also check it's harness and connector for obvious "eyeball" problems since it's so close the the exhaust.