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-   -   ZJ 4.0 low idle (http://www.jeepforum.com/forum/f13/zj-4-0-low-idle-1599265/)

IKSLM 09-18-2013 12:51 PM

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.

IKSLM 09-18-2013 01:03 PM

One more thing. I also replaced the spark plugs and i noticed a difference in plug gap, could this affect the idle rpms?

zjosh93 09-18-2013 01:51 PM

Quote:

Originally Posted by IKSLM (Post 16115017)
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.

IKSLM 09-18-2013 01:58 PM

Thanks!! Will try this first thing tomorrow.

zjosh93 09-18-2013 01:59 PM

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.

Oldfrog 09-18-2013 02:30 PM

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.

Kingr98 09-18-2013 03:51 PM

Quote:

Originally Posted by Oldfrog (Post 16117353)
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?

IKSLM 09-19-2013 02:22 AM

don't know - yet :)

IKSLM 09-19-2013 12:17 PM

ok, adjusted the plug gap, NO difference!

what's the easiest way to check for vacuum leaks?

94grandzj 09-19-2013 08:24 PM

When you replaced IAC did you use mopar or aftermarket? I have seen several aftermarket sensors bad out of the box. Just a thought.

IKSLM 09-28-2013 06:22 PM

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.

Oldfrog 09-29-2013 01:09 PM

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.

IKSLM 09-30-2013 10:54 AM

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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