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00' xj idles higher as it warms up, wtf?

M

Mr. mud

NAXJA Forum User
Location
deridder, Louisiana
Just bought my 00' xj. 4.0, aw4, 242, 8 inch crusty's lift, poweraid tbs. Got a cel a week after I bought it, hooked up the scan tool ( I work at a dodge dealership), tps was bad. I replaced it. Shifts right now, but as the engine warms up the idle steadily increases up to 1200 rpm and thats where it stays. It misses slightly at first startup when its cold too. Ive' also replaced the Iac motor and cleaned the Idle air temp. sensor. Never owned an XJ even close to this new, so I'm kinda lost. It's not throwing any codes right now. Any suggestions? I think Iv'e covered the basics. Oh yeah, I completely disassembled and cleaned the throttle body and checked for intake vaccum leaks as well...
 
What is the idle speed, in DRIVE, both engine and transmission fully warmed up--after about 20 minutes of operation?
 
Yeah, that is wacko.

In DRIVE idle should be 700-750 rpm, there is no spec for P or N.

I would start hunting for intakes leaks at the manifold or in the vacuum system. Other causes could be a lean condition related to sensor input to the PCM, meaning that a sensor is reporting the engine as running rich, and the PCM is leaning out the mixture.
 
Yeah, my 87' and my 92' both did that when the O2 sensors went bad, but only in park or neutral. Usually the things idle at about 800 rpm in park. And i do have a minor manifold leak, but no cel indicating a lean or rich condition.
 
If you can get to the manifold leak try smearing some thick grease on the leaking area and see if that pull the idle down. I will melt away once things get good and hot, but you should be able to see a change for a short time.

The PCM could be leaning out the mixture without it being too far out of spec to drop a code. Also, as you replaced the TPS, you should check its values, even if new.
 
Maybe a leak in the exhaust manifold making the O2 sensor see more oxygen than it's supposed to, causing it to give the engine more gas?
 
kastein said:
Maybe a leak in the exhaust manifold making the O2 sensor see more oxygen than it's supposed to, causing it to give the engine more gas?
Click to expand...

I think that would end up too rich and give a lower idle? :wave1:
 
My 2000 idles at about 600. Its would have to be a massive leak to make the idle go up. A while back when I was playing with a vacuum gage, I pulled off the power brake booster vacuum hose. The idle shot up to about 1500 and then dropped back down to 600 within seconds. The IAC was able to overcome that big vacuum leak!
 
I di check some of the top manifold bolts for tightnees yesterday and they were tight, haven't checked all of them though. I was just fiddling with it here at work while I had a minute. The bottom bolts may be loose. Oh, come to think of it, My manifold gasket looks to be just a thin piece os stainless steel, is that the correct gasket?
 
stainless steel? Doesn't that mean it isn't rusty?

What is it doing on a Jeep? That can't be right.

(just kidding, yeah, that's about what it's supposed to look like)
 
Ok. I'm used to the cardboard type manifold gaskets on these motors, so I just figured I'd ask. Man,this is embarassing to have to ask these questions, I mean I am THE Jeep guy here at our dealership. Guess everyone hits a snag or two once in a while though. I'm going to put my heep on the lift today after work and check it over real good to make sure the manifold bolts aren't loose and see if I can't find a possible major vaccum leak. I need to eliminate vaccum leaks as a possibility before I assume it's electrical and then go from there I suppose.:smoker:
 
You work at a dealer? No need to get in a parts chucking contest then. Put the scanner back on it, what are the long and short fuel trims? 02 voltages, IAC pintle position and so on. Are the TPS percentages where they need to be?
 
wasn't doing that last time I had it on the scanner, but I had just started it up and the motor was still cold when I checked it. Can't believe I missed that. And I call myself a technician, shame on me....:doh:
 
Ok, I have more on this now. Replaced both o2 sensors and no change, put the scanner back on it and went into the datastream and I'm seeing what I think is excess vaccum at the map sensor (17.6 in. at high idle). Replaced with a known good spare, no change. Once it started getting dark I noticed that the cats on the manifold and the runners are glowing dull cherry red. So now I'm thinking maybe the cats are stopped up:flamemad:. Opinions?
 
Mr. mud said:
Ok, I have more on this now. Replaced both o2 sensors and no change, put the scanner back on it and went into the datastream and I'm seeing what I think is excess vaccum at the map sensor (17.6 in. at high idle). Replaced with a known good spare, no change. Once it started getting dark I noticed that the cats on the manifold and the runners are glowing dull cherry red. So now I'm thinking maybe the cats are stopped up:flamemad:. Opinions?
Click to expand...

If the CATs are plugged, then slowly raising the idle will cause the manifold vacuum to drop as things back up. Hook up a gauge and check for that.

I would not expect a high warm idle from a plugged CAT.

As for dull cherry red at night, CATs get hot. Maybe someone else with a late model can chime in here on the manifold CATs.
 
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