jeepjerk123

High idle when started, cold or warm 2k rpm. The idle will come down when thoroughly warmed up to about 1k rpm, but seems to lug when put into drive.
I did a pretty thorough search o this subject but didn't find as many threads as I would have thought
1993 jeep grand cherokee LT 4wd, 4.0ltr 6cyl engine
Replaced spark plugs wires and cap with no change.
I'm on the third new oxygen sensor.
Just installed the fifth new IAC motor. When I installed the motor I reset the computer.
The new motor caused a very high idle 3k rpm. Then the idle came down in steps to about 800 rpm like it should. Seems like it isn't adjusting anymore. Sometimes when I engage the air conditioner the IAC will compensate for the additional load.

I can't find a vacuum leak anywhere. I checked the MAP sensor and get the correct voltages for vacuum and normal air pressure. Sometimes when I remove the vacuum tube that goes to the MAP sensor the idle will come back down, but back up when connected.

I thoroughly cleaned the throttle body with carb cleaner. Didn't really change anything.

Like I said above the Idle will come back down when the engine is thoroughly warmed up, but it will go way up again when re-started while warm and takes a while to come back down again.

Well seems like all the old trick don't work anymore. I would be grateful for any tips and ideas to try.

Thanks

jeepjerk123

I can't believe no one here has had this problem.

Radar

check to see if the vacuum lines are all going where they are supposed to

nukinfuts29

Have you tested the TPS?

marmar11993

I am currently having the same problem and my mechanic cant figure out what is up. It would be great if somebody knew what is wrong. If any body has an idea what it is plz let me know.

country_boy454

x2

jeepjerk123

Well how do I check the tps?

It seems to be doing all it's usual functions like shifting the transmission and the throttle.

The idle speed does eventually come down. After about 10min

It's just that the idle goes back to 2000 rpm after a shut down and an immediate re start up. And takes another 10min to come back down.

And when it comes back down it doesn't regulate at 750 rpm or what ever.

It's some where around 1000 rpm and it varies.

It seems that the pcm isn't really regulating the idle. It just comes down eventually.

Radar

i heard that a leaking intake manifold could also be the culprit, and the test is low cost, take some aerosol can starting fluid or wd 40 and spray it where the intake meets the head gasket. if the engine revs or the idle changes then your intake gasket is bad, just make sure that none goes in through the air filter or you will get a false positive result.

country_boy454

TEST

You should have 5 volts going into the TPS. At idle, TPS output voltage must be greater than 200 millivolts. At wide open throttle (WOT), TPS output voltage must be less than 4.8 volts.. The best is to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. One of the other wires should show .26V (or so). The other wire will be the ground and should show no voltage. Move the throttle and look for smooth meter response up to the 4.49 at WOT.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice any flat spots or abrupt changes in the meter readings, replace the TPS.

jeepjerk123

Thanks country boy. I'll probably test that tps today and get back soon.

country_boy454

My TPS went on my 97 4.0 ZJ and it showed a constant 5.8 volts whether it was WOT or not. When you started the Jeep it would idle at about 3000 rpm or so. Replaced it with a new one and was golden!

jeepjerk123

Ok now maybe we're getting somewhere.

I just measured my TPS.

Ok, at the closed throttle position I read 1.2 volts.

At WOT wide open throttle I measure 4.00 volts.

I did not see any open or gaps anywhere.

Now the 1.2 volts sounds a little high for closed throttle.

Now do you suppose this has been the problem all along?

This seems like a pot / potentiometer. Does anyone know what the proper ohm values should be on this pot?

jeepjerk123

I just read my manual. It says that at closed throttle position I should measure about 1v

And at wot, wide open throttle it should read up to 5v. Kind of vague there.

So I don't really think mine is that far off.

What do you think?

I getting pretty tired of running out and buying new parts and finding out that there is no change.

marmar11993

Should all these throttle measurements be the same for every jeep? Because I have a 1991 Jeep Cherokee straight 6. If you guys can figure out the measurements that would be great help.

shlein69

I just replaced both the tps and the iacv and that's when my high rpms started. My 94 Cherokee 4.0 HO was running real rough so I replaced the 2 sensors and it runs awesome now except for the high idle. All the vac lines heem to connected so I guess ill check the manifold for leaks. I also replaced the Valve cover gskt. When I went to loosen the nuts up only 2 of them were tight enough to have to use a wrench! Just got the jeep about a month ago and finding that the PO drove it and that's about it. Any other suggestions would bw great. I love this forum. Thanks everyone!