2001 wj idle to fast - JeepForum.com
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post #1 of 5 Old 07-28-2009, 07:17 PM Thread Starter
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Location: ohio
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2001 wj idle to fast

I have a 2001 jeep grand cherokee 4.0l automatic
Ok about 3 month ago the idle started acting up was either to fast or slow,
So I replaced the iac and it was ok for about a month then one day it would only start if you gave it a little gas,drove it home and now it idles at 12-1500
So I replaced the iac 2 more times thinkg autozone gave me a bad part,still idle is too fast so a friend of mine has a drb3 scan tool we hooked into the ecm and can change idle up from 1200 to whatever I want but not down from 1200 scan tool shows the ecm has commanded a 0 step which is closed on the iac and if you plug the iac hole it will idle down this eliminated any vaccum leaks,so I ordered a new iac from napa (they have better parts imo)
Put in the iac today and idle came down to 700-800 and I could here a little miss in engine then rpm went up to 1000+ so I pulled the plugs out they had 50-80k on them anyway put in new plugs with the same issue start it idles ok for a sec then misses a little and rpm go up,I tested tps with a vom seemed ok,any body have any clues as to why it would miss at idle the ecm raises rpm to clear miss,I have 140k on the jeep never had any problems other than cooling fan replaced 6 month ago and did water pump just because It was easy to get to after ripping front end off to replace cooling fan.

Thanks for all the help next step I guess will be a dealer

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post #2 of 5 Old 07-28-2009, 07:21 PM
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Try a genuine mopar sensor.
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post #3 of 5 Old 07-28-2009, 08:33 PM
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I would replace the TPS. You said you tested it with a voltage meter and it "seemed ok" I would go ahead and replace it. My sisters bf's Jeep was doing the same thing. It was also throwing 4 different codes including a miss fire. We replaced the TPS ($34.99 at autozone) and it cleared all the codes and fixed the idle issues. Easy and cheap thing to try...

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post #4 of 5 Old 07-29-2009, 03:08 PM
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2000 WJ 
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check all the wiring going to the iac and tps,it gets very brittle from the heat and sometimes causes issue like what you are having.
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post #5 of 5 Old 07-29-2009, 04:59 PM Thread Starter
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I drove jeep home today about 20 miles, and the idle came down to 750 like normal and then the motor developed a little miss for about 3 sec and then the idle went up to 1100 where the miss is gone,so something at idle is causing a miss and the ecm is raising the idle to compensate or clear the miss,like I said I have 140k miles on it(or my wife does) never changed the fuel filter yet,she always runs the cheapest fuel would a plugged fuel filter cause low idle miss or plugged injector maybe?
The new iac from napa is just as good as oem actually thats where our local dealer gets there parts from,after looking at napa's and autozone iac the pintle on autozones was quite a bit bigger around then the napa part but now it will idle down and I don't hear the sucking air sound from the t.b like it did with autozone (ac327) was autozone part number,now when it does idle down it misses a little and idle goes up,put new plugs in yesterday still same problem,I did check all the plugs and harness around engine,and tb all looked good with no breaks anywhere and the wiggle test didn't make any difference on anything so which sensor would affect or cause a miss at idle?
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