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Discussion Starter · #1 ·
!990 Jeep Cherokee Limited, 170,050 miles

Today I started the engine and it “idled” at 2000 RPM; I shut it off and restarted and it “idled” at 2500 RPM. I can tap the gas pedal and it revs up but returns to the fast idle when I let off. It was 44 degrees F outside.

It has done this before and eventually idled normally after sitting awhile. Any ideas on what can cause this fast idle?
 

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!990 Jeep Cherokee Limited, 170,050 miles

Today I started the engine and it "idled" at 2000 RPM; I shut it off and restarted and it "idled" at 2500 RPM. I can tap the gas pedal and it revs up but returns to the fast idle when I let off. It was 44 degrees F outside.

It has done this before and eventually idled normally after sitting awhile. Any ideas on what can cause this fast idle?
Usually this sort of thing is caused by a clogged and dirty Idle Air Control valve sticking too far open. Try cleaning that out really well with carb/intake cleaner and resetting the computer so it re-learns to idle with the clean IAC. IF that doesn't solve it it might be time for a new one.

That's engines in general, though. I don't know if the 90 renix 4.0 has any particular quirks that might cause it instead. Still, it costs you nothing but some time and carb cleaner to try so it should be worth a shot.
 

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Discussion Starter · #3 ·
Usually this sort of thing is caused by a clogged and dirty Idle Air Control valve sticking too far open. Try cleaning that out really well with carb/intake cleaner and resetting the computer so it re-learns to idle with the clean IAC. IF that doesn't solve it it might be time for a new one.

That's engines in general, though. I don't know if the 90 renix 4.0 has any particular quirks that might cause it instead. Still, it costs you nothing but some time and carb cleaner to try so it should be worth a shot.
Thanks, I'll look up where the IAC is and check it out. I had the battery disconnected before that too, so maybe it is a slow learner.

BTW, the battery in my 96 Country died and every place I looked on line said I needed a 34 or 34/78 battery, but I had a 34 and it is too long buy an inch for the battery tray. Checking the archives I finally figured out I needed a group 58. I called battery outfitters and they said they had a couple of reconditioned 58s that tested at 610 CCA and 710 CCA for $43.80 out the door so off I went 30 miles to town and picked up a decent looking battery with an eye in it (hadn't seen a battery with an eye in years). I got it home and it did barely fit in the battery tray but it scrunched the radiator hose and the hold down wouldn't fit; the battery was 7 1/2 inches wide and the hold down is 7" or a little less inside. Called them back and the guy went to look at the exchange I took in (one of their own "outlast" batteries actually) and came back and said he had grabbed the wrong battery and I could bring it back. Then I noticed that the sticker on it had 65 written before the hand written 610 CCA. Apparently he gave me a group 65 instead of a group 58, so I'll have to drive back again tomorrow. I think I'll ask them to give me some $$ for gas for the second trip. Bet that will go over well.
 

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Discussion Starter · #5 ·
Usually this sort of thing is caused by a clogged and dirty Idle Air Control valve sticking too far open. Try cleaning that out really well with carb/intake cleaner and resetting the computer so it re-learns to idle with the clean IAC. IF that doesn't solve it it might be time for a new one...
I removed the IAC, cleaned it with carb cleaner, put it back, and the engine still "idles" at 2000 to 2500 RPM so I took it out again and sprayed it again and noticed that the little black collar would turn freely but would not retract like the one in the video and the pintel end is 15/16 from the base with the holes in it. That seems way shorter than the one in the video too.

How does this work? Is there a little motor that turns it in and out to adjust the gap? And when you say reset the computer, does disconnecting the battery do that?

I put the IAC back in a second time and took a drive around the block and it never idled down. If anything it ran even faster and I had to use the brakes to hold it back. Guess it is time for a new IAC?
 

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I removed the IAC, cleaned it with carb cleaner, put it back, and the engine still "idles" at 2000 to 2500 RPM so I took it out again and sprayed it again and noticed that the little black collar would turn freely but would not retract like the one in the video and the pintel end is 15/16 from the base with the holes in it. That seems way shorter than the one in the video too.

How does this work? Is there a little motor that turns it in and out to adjust the gap? And when you say reset the computer, does disconnecting the battery do that?

I put the IAC back in a second time and took a drive around the block and it never idled down. If anything it ran even faster and I had to use the brakes to hold it back. Guess it is time for a new IAC?
It is just a little motor, if yours doesn't have the range of motion it should that could be causing your problem and yes disconnecting the battery resets the computer. The computer has a memory that adjusts how the engine runs. If you change sensors, fuel, air (like the IAC) or spark it's a good idea to reset it so it learns the new characteristics from scratch rather than mixing old data and new.
 

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Discussion Starter · #8 ·
Sounds to me like the normal Renix tPS malfunction. Sometimes it's the ground circuit for it and/or the TPS failing.

http://cruiser54.com/?p=44

http://cruiser54.com/?p=54
Thanks for those links. Is this the flat 3 wire connector you refer to? If so does it matter whether it is connected or disconnected to check the B terminal ground?

I noticed a vacuum line (?) going into the lower hole of an oval rubber block on the side of the throttle body and the upper hole filled with mud dauber mud. Could this have anything to do with my high idle problem?
 

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Discussion Starter · #11 ·
That's the connector. Unplug it and test the ground with key off.
Plug on the side of the throttle body is normal.
OK, Thanks, I measured the resistance between the B pin on the wire loom side of the disconnected plug and the negative battery post. I got 0.7 with my better meter and 0.4 with a Harbor Freight meter on the 200 ohm scale. Wiggling the wiring did not make a difference although my "good" meter did skip between 0.6, 0.7, and 0.8 with nothing wiggled.

How much resistance is too much?
 

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OK, Thanks, I measured the resistance between the B pin on the wire loom side of the disconnected plug and the negative battery post. I got 0.7 with my better meter and 0.4 with a Harbor Freight meter on the 200 ohm scale. Wiggling the wiring did not make a difference although my "good" meter did skip between 0.6, 0.7, and 0.8 with nothing wiggled.

How much resistance is too much?
2 or 3 ohms. My wife's 88 had 54 ohms!!

Test the tPS next. Have you cleaned the throttle body?
 

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Discussion Starter · #13 ·
2 or 3 ohms. My wife's 88 had 54 ohms!!

Test the TPS next. Have you cleaned the throttle body?
I haven't cleaned the throttle body yet. I waited for the new IAC to arrive today, but there was no change with it installed; it still "idles" at 2200 to 2500 RPM. The pintel length on the new IAC was between 15/16" and 31/32" same as the old one. The throttle linkage is free and returns to the idle position against a little grub screw.

So...Clean the throttle body next?

And then do the TPS checks in http://cruiser54.com/?p=54 ?

Is there any chance of damaging the TPS with carb cleaner when cleaning the throttle body? I assume that I should not remove the TPS before doing the checks.

And thanks again for the advice.
 

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I haven't cleaned the throttle body yet. I waited for the new IAC to arrive today, but there was no change with it installed; it still "idles" at 2200 to 2500 RPM. The pintel length on the new IAC was between 15/16" and 31/32" same as the old one. The throttle linkage is free and returns to the idle position against a little grub screw.

So...Clean the throttle body next?

And then do the TPS checks in http://cruiser54.com/?p=54 ?

Is there any chance of damaging the TPS with carb cleaner when cleaning the throttle body? I assume that I should not remove the TPS before doing the checks.

And thanks again for the advice.
good plan. Carb cleaner shouldn't damage TPS. Have you done tip 5?
 

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Discussion Starter · #16 · (Edited)
good plan. Carb cleaner shouldn't damage TPS. Have you done tip 5?
Yes to tip 5; got 0.7 ohms with my better auto-ranging meter and 0.4 ohms with a Harbor Freight meter on the 200 ohm scale. Wiggling the wiring did not make a difference although my "good" meter did skip around between 0.6, 0.7, and 0.8 with nothing wiggled.

Yesterday I decided to check per "TPS ADJUSTMENT FOR ENGINE ISSUES" before cleaning the throttle body (I prefer to do one thing at a time). I wasn't getting a good reading by back probing the 3 wire flat connector so I measured the voltage between pins pushed through the wires. A to B measured 4.88 volts so voltage between B and C should be 0.83 but it was 1.079. I removed the intake tract and the screws holding the TPS because the little oblong shaped washers were turned 90 degrees and jammed against the body so it would not move.

The bottom screw came out a little hard but OK. The top screw turned hard, stripped the end, and wouldn't back out so I had to lever the TPS body out while turning it to remove it; These appear to be 10-32 thread and a 10-32 nut threaded onto the good screw but they seem to be a slightly smaller diameter (although not as small as an 8-32). I tried to tap the top hole with a 10-32 tap and got it a little way in but there was not much room for the tap wrench to swing so I didn't get it very deep I reinstalled the TPS without the washers on the top screw so it would grab a good thread or two and turned the TPS as far clockwise as it would go: the output voltage was 8.63 Volts at as far as the TPS would turn.

I put the intake tract back on temporally to start the engine and It started at 1000 RPM, idled down to 800 RPM, and then as it warmed up some more it went down to 500 RPM (half way between 0 and 1000 on the tach). It might have to be turned back up a little if it idles too low when fully warm. I am still wondering what changed to raise the idle because it was apparent the TPS could not have moved and it had happened intermittently a few times and had settled back down before it became the permanent condition. Or might this be an intermittent ground issue? Tip 6?

Finally, is there a specified clearance for the throttle butterfly? It is .002" on my older Jaguars. Oops, never mind. Tip 14.
 

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I think you found it.

Seems to be the same for the throttle body adjustment, but it shouldn't be used for that. http://cruiser54.com/?p=94

A bad oxygen sensor can cause a low idle. Unplug yours and probe the plug on the sensor itself. Should be 6 to 8 ohms resistance on it's heater circuit. Red and black wires.....
 

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Discussion Starter · #18 ·
I think you found it.

Seems to be the same for the throttle body adjustment, but it shouldn't be used for that. http://cruiser54.com/?p=94

A bad oxygen sensor can cause a low idle. Unplug yours and probe the plug on the sensor itself. Should be 6 to 8 ohms resistance on it's heater circuit. Red and black wires.....
Thanks. I still need to clean the throttle body but it is wet and cold today so it will have to wait. This is a good day to read the rest of your great tips.

I drove it a short distance yesterday and it seemed OK, but I will need to get it out on the road to see if the TPS change affects the auto trans shifting.

The ABS pump was making a rattling noise and the brakes seemed a little weak so that will have to be addressed too. Got to find out more about that Bendix 10 lifetime recall on the ABS pump and if I can get a Jeep dealer to do it.
 

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Discussion Starter · #19 ·
Update: Cleaned the throttle body yesterday and set the butterfly clearance at .003"; It took about two turns in on the grub screw stop. Thanks to cruiser54 for the tip that it takes a 3/32 hex wrench on the screw (from the back, BTW, and after a soaking with liquid wrench in my case). Idle seems good now; a little over 1000 cold and about 800 as it warms up.

Now to deal with the noisy, failing ABS pump making the brakes a bit weak.
 

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