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'89 4.0L VERY high idle problem

I have recently acquired an 89 Jeep Cherokee with an inline 6 cyl which is fuel injected. It quite suddenly developed a dangerous problem with the idle speed. At idle it sits at anywhere between 2300 to 3300 rpm so you almost have to have 2 feet on the brakes at a stop sign. When I pull the vacuum hose off the map sensor it drops down to about 1100 with some white smoke out the tailpipe. I pulled the tps out to see if it was maybe stuck but it seemed okay and moved freely. I am not sure where to go from here to fix this thing. Anyone else with a similar problem?

Reply to
scmadhatter
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Reply to
L.W.( ßill ) Hughes III

(sigh) Is there anything that beer can't do?

the throttle stop screw.

Reply to
brianc

First off. pul the connector on the tps with contact cleaner (or WD40 in a pinch). after you plug it back in, take a volt meter and check the voltages. I can't remember which pins are which, but with the key on (don't start it) probe the plug from the wire side. One pin should read 5 volts, one about .9 volts, the third zero - that's ground. probe the one that reads about .9 (or somewhere between about .5 and

4.9 some-odd) as you move the throttle cable - it should vary smoothly with no breaks. If you can't find a pin that does vary from just under 1 volt to something close to 5 volts, you have a shot tps. If the tps checks out then what you describe could well be the MAP sensor. Probe it the same way, but you really need a vacuum pump (hand style) or good lungs to see if the reading changes as pressure changes. AIR, the MAP sensor is relatively cheap - $20 range.

The dirty TPS connector is a common problem on the old Renix systems.

Reply to
Will Honea

THANKS! I spent the evening reading that fuel systems manual. I liked the fact that it explained how each component worked rather than just stating what it did and assuming that the reader was savvy enough to figure it out! Sort of like fuel systems for dummies! I will start testing tommorow. I was wondering though, could the stepper motor plunger get carboned up and stick enough to cause a high idle condition? Thanks again for the info.

Reply to
scmadhatter

Reply to
L.W.( ßill ) Hughes III

If all the other fixes mentioned don't work, then I would be looking hard for the vacuum line that fell off.....

If a vacuum line has fallen off, you will get your symptoms. One check is to see if it will go into 4x4. If it does, then you likely don't have a leak and it is a sensor or sensor connection like on the TPS issue.

If it doesn't go into 4x4 with the dash light on, then it is likely a vacuum leak.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

scmadhatter wrote:

Reply to
Mike Romain

I think that might have done the trick. It was really caked with grime so I took the degreaser to both the stepper motor and the hole it mounted in and cleaned it all up. When I re-installed it I started it up with the intake elbow off and nothing had changed. Then I put my finger over the odd shaped hole which ports down to the stepper which is on top of the intake flange just to see what would happen and it immediately dropped down to a normal idle!?! Remounted the intake hose and took it for a test drive and it ran just fine. I dunno! Hope it stays that way. The stepper motor was really caked and I don't think it could have moved the plunger at all in the condition it was in. Thanks for the info, but don't go too far away as I am posting another problem after I eat dinner. Steve in OKC

Reply to
scmadhatter

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