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Unread 01-02-2014, 09:59 AM   #1
Ctpetersen
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1988 MJ Comanche 
 
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High idle on my 4.0

I have a 88 jeep comanche with an idle problem. 4.0l 5spd 4x4. At start up it idles to about 2000 rpm but goes down to normal right away about 700 to1000rpm. When you rev it it Goes up and back down ok. But when i start driving and take it out of gear it will idle at between 1500 and 2100rpm. Shut it off and start it again and it usually goes back to normal. Could it be a vaccum leak? I did a quick clean of the IAC and that help for a day or two. How do i clean and or set the TPS or IAC. Is carb cleaner ok to use to clean my throttle body?

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Unread 01-02-2014, 12:49 PM   #2
tjwalker
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The Idle Air Control (IAC) is mounted on the back of the throttle body. (front for 87-90) The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

“Gently” wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality
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Unread 01-02-2014, 02:14 PM   #3
Ctpetersen
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ThAnk you. I am going to clean the iac tonight and possibly the whole throttlebody. I hope it works, i dont feel like trying to chace down a vaccume leak
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Unread 01-03-2014, 01:03 AM   #4
demonoid369
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Quote:
Originally Posted by Ctpetersen
ThAnk you. I am going to clean the iac tonight and possibly the whole throttlebody. I hope it works, i dont feel like trying to chace down a vaccume leak
Be on stand by to get a new one though........ For the most part those things don't like being cleaned and working afterwards properly(they're sensitive) but your iac could also be on its last leg already and needs to be replaced
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Unread 01-03-2014, 06:44 AM   #5
cruiser54
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Quote:
Originally Posted by demonoid369 View Post
Be on stand by to get a new one though........ For the most part those things don't like being cleaned and working afterwards properly(they're sensitive) but your iac could also be on its last leg already and needs to be replaced
They're fine with cleaning as long as you don't push or pull on the pintle.

There's always the chance that the TPS is taking a dive also.
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If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

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Unread 01-03-2014, 11:03 AM   #6
Ctpetersen
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Well i cleaned the iac and throttlebody and it did not help at al(was very dirty tho so it needed it)l. Going to try a new IAC and maybe TPS and hope that works.
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Unread 01-03-2014, 04:47 PM   #7
cruiser54
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Quote:
Originally Posted by Ctpetersen View Post
Well i cleaned the iac and throttlebody and it did not help at al(was very dirty tho so it needed it)l. Going to try a new IAC and maybe TPS and hope that works.
Rather than throw parts at it, click on my link below and do the procedures outlined in posts 1 through 5 first. Don't cut any corners or skip any steps.
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Quote:
Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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Unread 01-11-2014, 09:32 AM   #8
Ctpetersen
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Well i cleaned everything and nothing helped. Got a nrew IAC that helped a little but not much. Then i replaced the vacuum harness between the valve cover and airbox and now its finally idleing ok. Still a bit high tho (1100-1200). I think after i set the TPS it might be back to normal. I hope.
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Unread 01-11-2014, 03:44 PM   #9
SKELITOR117
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Could also be a bad fuel pump. Stuck injector maybe?
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Unread 01-11-2014, 05:39 PM   #10
cruiser54
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Originally Posted by SKELITOR117 View Post
Could also be a bad fuel pump. Stuck injector maybe?
Not causing a high idle.......
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Quote:
Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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Unread 01-12-2014, 03:12 PM   #11
Kalali
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After all the other items, you may also want to check your throttle cable assembly (at the intake manifold) to make sure it moves freely and is not binding.
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Unread 01-17-2014, 08:30 PM   #12
Ctpetersen
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Ok so i tried adjusting the tps. The repair manual i got said for a renix MPI engine(which i would imagine it is, or is it a TBI) i should check input for 5.0 volts, which i have. And to adjust output volts to 0.80. And i did that. But it idle at 2500 rpm like that so i put it back to about what it was(0.24 volts) and it idles decent that way but still a little high and bounces around a little. Could the TPS be bad? Or does this sound more like something else like a intake gasket or a diffrent sensor.
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Unread 01-17-2014, 08:34 PM   #13
Ctpetersen
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I should have said earlier that it does idle a little rough and backfires but its only every now and again and only at idle. Never when im moving, runs great then.
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Unread 01-17-2014, 09:06 PM   #14
cruiser54
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Quote:
Originally Posted by Ctpetersen View Post
I should have said earlier that it does idle a little rough and backfires but its only every now and again and only at idle. Never when im moving, runs great then.
Use my tips as in my signature to adjust your TPS.
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Quote:
Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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Unread 01-17-2014, 09:56 PM   #15
Ctpetersen
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I will give that a try. So if all else fails, i can use the TPS for an automatic and just not plug in the 4 prong conector?
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