1995 Wrangler YJ 4.0 Liter High Idle - JeepForum.com
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post #1 of 2 Old 09-05-2010, 07:53 PM Thread Starter
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1995 Wrangler YJ 4.0 Liter High Idle

I've searched this forum for the basics. Checked most of them off one by one, and still have a high idle.

Here are the details.

1995 Wrangler
4.0 Liter
Everything is stock (to my knowledge). I'm the 3rd owner.
No codes are being thrown.

Idle was at about 2200 rpm. I cleaned the IAC and throttle body. This got it down between 1200-1500. Not sure why it varies that much, but at least it's lower. I'm not too concerned about the high idle (should I be?) other than when I first bought it, I was getting 18-20mpg (before the idle went to 2200rpm). Now I get between 12-14mpg no matter what type of driving I do.

I replaced the IAC, TPS, and MAP sensor. Disconnected the battery between each sensor replacement, and took a little drive for it to relearn. Nothing resolved the issue.

IAC: This was weird. I bought a new one from Autozone. The pintle length was different from my original, but I decided to try it. I installed it. I turned the Wrangler on, and it soared to 3500rpm. I immediately shut it off. Instructions said to expect a high idle while it relearns, but it said it should be in the 1800-2200 range while it relearns...not 3500. Instructions said pintle length should not exceed 1-1/8". The old one was 1-5/16". I measured the depth from the mating face to the sealing surface inside the throttle body; it measured approximately 1-1/8". I don't see how the new one from Autozone could seal if it's overall length is 1-1/8". I don't think there's that much stroke in the IAC. The old one at OAL of 1-5/16" puts the tapered sealing surface portion of the pintle head right in the 1-1/8" ballpark. I went back to Autozone to make sure I had the right part number. Best we could tell, I did. However, they didn't have another one to confirm.

Could someone please measure their properly idling IAC for OAL? Also, what's the approximate depth from the IAC mating surface on the throttle body to the sealing surface for the IAC pintle inside the throttle body?

Pending an answer to that, my next step will be to check for vacuum leaks. Now, I've been told 3 ways to do this. Please let me know which is best.
1. With engine running, spray throttle body cleaner on vacuum lines and around intake. If engine revs higher, there's a leak.
2. With engine running, use UNLIT propane torch. With propane on, move the nozzle around vacuum lines and intake. Again, if engine revs higher, there's a leak.
3. With engine running, use a water squirt bottle to spray water on vacuum lines and intake. If engine bogs down, there's a leak.

Perusing the forums has given me some other ideas -- checking bolts for proper torque, etc. Most are related to the vacuum issue. However, with the weird happenings with the IAC, I still think that's my issue. Especially since it did improve some when I cleaned it out.

One other point. My MAP sensor is on the firewall, behind the valve cover. My throttle body does have a vacuum nipple covered with a rubber cap. When I remove the rubber cap, it revs higher. I'm assuming this is where the MAP sensor goes on the newer year models (TJ's). Why did they put that nipple there if they weren't going to mount the MAP sensor there? There are also two threaded holes for mounting the MAP there. The vacuum line running to my current MAP sensor has strong vacuum and no leaks.

Finally, back to the IAC, they are somewhat adjustable. I've seen threads in my searching today with opinions both ways -- adjustable vs. not adjustable. The one from Autozone let's you adjust. Minimum overall length was just under an inch. Again, according to the instructions, max OAL should not exceed 1-1/8", otherwise damage could occur. The pintle shaft is threaded allowing the adjustment when the spring sheath is pulled down. When the sheath is sprung back into place, a bent portion of the end of the spring locks into the head of the pintle. If you spin the head of the pintle with the sheath up, the spring will spin the plastic portion inside the IAC, thus not altering the pintle length. Kinda clever for fine adjustments, but I don't know how well it would hold up with engine vibration, etc. Anyway, just thought I'd share what I learned.

Thanks in advance for your help.


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post #2 of 2 Old 09-06-2010, 12:40 PM
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1996 ZJ 
Join Date: Oct 2006
Location: Vernon, NJ
Posts: 4,758
Check for a vacuum leak. Any vacuum leak will cause a high idle with no codes thrown.

96 ZJ.
05' Grand Cherokee Limited, 2"OME lift
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