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Unread 09-18-2013, 12:51 PM   #1
IKSLM
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1995 ZJ 
 
Join Date: Dec 2011
Location: Maribor, Slovenia
Posts: 16
ZJ 4.0 low idle

This seems to be a "common" problem with these jeeps....

So my idle (when the jeep warms up) falls to about 200rpm, which is really really low...

Just replaced TPS and IAC, cleaned the TB (completely - even the IAC "tunnel"), but low idle still persists (no change).

What could be the problem? There are no error codes, except 12 (me clearing the adaptive memory) and 55.

What's next on the list to check?

I removed the CAT some time ago, but i *think* that the low idle was there before the removal.

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Unread 09-18-2013, 01:03 PM   #2
IKSLM
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1995 ZJ 
 
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One more thing. I also replaced the spark plugs and i noticed a difference in plug gap, could this affect the idle rpms?
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Unread 09-18-2013, 01:51 PM   #3
zjosh93
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1993 ZJ 
 
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Quote:
Originally Posted by IKSLM View Post
One more thing. I also replaced the spark plugs and i noticed a difference in plug gap, could this affect the idle rpms?
Do you mean that the new plugs you bought were not gapped the same as the old plugs? It's normal for the old plugs to wear down somewhat so that the old plugs have a slightly bigger gap than freshly gapped plugs. If you mean that you took the new plugs out of the box and the gap was different so you just put them in you missed a step. Most new plugs don't come with the gap already set: you have to do that yourself with a gapping tool or feeler gauges.

Yes, incorrectly gapped plugs could mess with your idle speed.
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Unread 09-18-2013, 01:58 PM   #4
IKSLM
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Thanks!! Will try this first thing tomorrow.
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Unread 09-18-2013, 01:59 PM   #5
zjosh93
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1993 ZJ 
 
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If you suspect your idle air control valve is dead you can push the pintle back in a little following the instructions here:
http://www.airtexve.com/custom_searc.../990700385.pdf

I do this whenever I clean my valve and the idle will jump up when I restart and slowly drop as the valve adjusts.
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Unread 09-18-2013, 02:30 PM   #6
Oldfrog
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1997 ZJ 
 
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Unplug the upstream o2 sensor and let it warm up to operating temps. It will throw a check engine light...but see if it idles better. If it cures it, replace both 02 sensors with NTK sensors. Rockauto.com has them ( cheap) or you could order them from Kolak on this site. Dont use Bosch sensors, whatever you do.

Check for vacuum leaks too.
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Unread 09-18-2013, 03:51 PM   #7
Kingr98
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1998 ZJ 
 
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Quote:
Originally Posted by Oldfrog View Post
Unplug the upstream o2 sensor and let it warm up to operating temps. It will throw a check engine light...but see if it idles better. If it cures it, replace both 02 sensors with NTK sensors. Rockauto.com has them ( cheap) or you could order them from Kolak on this site. Dont use Bosch sensors, whatever you.
Do the 95 ZJs have two O2 sensors?
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Unread 09-19-2013, 02:22 AM   #8
IKSLM
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don't know - yet
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Unread 09-19-2013, 12:17 PM   #9
IKSLM
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ok, adjusted the plug gap, NO difference!

what's the easiest way to check for vacuum leaks?
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Unread 09-19-2013, 08:24 PM   #10
94grandzj
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1994 ZJ 
 
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When you replaced IAC did you use mopar or aftermarket? I have seen several aftermarket sensors bad out of the box. Just a thought.
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Unread 09-28-2013, 06:22 PM   #11
IKSLM
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Location: Maribor, Slovenia
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IAC is aftermarket, but i don't think that the IAC is the problem, since it works when for example i put it into drive or engage the climate control. The RPMs stay on the same level - low. It's the same with old and new IAC.
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Unread 09-29-2013, 01:09 PM   #12
Oldfrog
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1997 ZJ 
 
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Question. You said the idle is around 200. Is that from the Jeep's tach or from a scanner? My tach says 500 or so, but the scanner says 750.
__________________
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Words of wisdom, Posted by Zeejay:
"..it's not air, it is the opposite of air... suckage so to speak."

Stalling? Work through Zeejay's list
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Unread 09-30-2013, 10:54 AM   #13
IKSLM
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1995 ZJ 
 
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Location: Maribor, Slovenia
Posts: 16
It's from the Jeep's tacho, but i know it's really low, since the engine almost dies when i come to a complete stop.
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