Comanche high idle

[dlbrenneman]

I have a 1990 comanche,4.0,4 wheel drive,automatic. 64,00 miles on the odometer. Last year heading for home after hunting the idle shot up to about three thousand and stuck. Burnt the brakes off but got it home. Bought another truck but would like to drive this also. Did all the easy stuff. Cleaned T B. ,replaced I. A. C. ,TPS. To no avail. Thing won't do the same thing twice. Idles down now but still revs to 2000 at start-up and sometimes stays there too long. No vac leaks that I can find. Truck has had an exhaust leak at the manifold since i bought it. We all know about the manifolds cracking. Wold loose or missing bolts at the intake do this?

[1989 Eliminator]

Yes, loose or missing bolts that hold the intake AND exhaust manifolds onto the block are essential. Having a loose manifold on either the intake or exhaust will cause a vacuum leak. So make sure you have all the bolts and make sure they are tightened. 21 ft-lbs I think. Sometimes the manifold gasket goes bad and that needs replacing. The intake and exhaust share the same gasket on the block.

 

The MAP sensor will also cause a high idle. So make sure the vacuum line coming out of the throttle body to the MAP sensor is connected properly.

[cruiser54]

Click on the link in my signature and do posts 1,3,4, and 5.

 

My money is on a bad TPS. See post 8 after you've done the others. 

[dlbrenneman]

Replaced map sensor today. Still no good. Starts pretty decent the first time but the second or third time you can bet it will go to two thousand or better. Presently it idles back to normal after about thirty seconds but you can bet one day it won't. Might start on cruiser's renix tips. Sounds like he's been around a 4.0 a few times. Thanks for the link. Lots of good info.

[reubj]

I had a similar problem. When I checked the ground point at the dipstick I found one of the sensor wires was loose from the connector. Since I repaired it, no more problems.

[cruiser54]

These 2 guys ^^^ know what they're talking about. 

[dlbrenneman]

One thing I forgot to mention. This problem only happens at start up. That is while driving the idle doesn't increase. Whatever it does when you start it is what you have to live with. Seems to me when talking about sensors and such there is an open loop vs closed loop. I need help with this.Start this thing ten times and it won't do the same thing twice.

[88ComancheMitch]

Do the tips. Check for vacuum leaks. Intake leaks. I had the same problem and mine ended up being my tps had stuck and wouldn't drop back down to idle.

[GrayWolf1691]

I'm not sure of the differences between the 4.0 and the 2.5 but I believe 90 was still renix controlled... I had a similar problem on my 2.5 though my problem occurred at an idle and always stayed up around a guess of 2500 rpm. I did 2 things to solve the problem, the first was my manifold bolts were loose and I had lost a bolt, and the other was strapping on a 1/2" thick ground wire from the engine to the unibody which smoothed everything out. I would also suggest replacing any old cracking vacuum hoses. A guy at my local O'Reilly's said he had this problem and his turned out to be the manifold gasket itself.

 

Good luck!

[cruiser54]

One thing I forgot to mention. This problem only happens at start up. That is while driving the idle doesn't increase. Whatever it does when you start it is what you have to live with. Seems to me when talking about sensors and such there is an open loop vs closed loop. I need help with this.Start this thing ten times and it won't do the same thing twice.

Normal behavior of a faulty TPS or sensor ground circuit.

 

Get busy on the Tips as suggested first and then test the TPS. 

[M1A1TankerTom]

Exact same thing happened to me when I first bought my rig. Turned out several of the exhaust manifold bolts were loose. Tightened up the bolts and the problem went away.