Problems w/ new rebuilt 4.0 liter 1999 WJ -
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post #1 of 5 Old 01-20-2014, 03:39 PM Thread Starter
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Location: Elizabethton
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Problems w/ new rebuilt 4.0 liter 1999 WJ

I had a rebuilt motor put in it with a new WATER PUMP, RADIATOR, TIMING CHAIN, 2 O2 SENSORS, TPS, IAC, PLUGS and BATTERY. Starts fine, highways speeds fine, just bogs out when cold or warm at low speeds (0-15 mph) for about 5 seconds to 1 minute. If I let off of the accelerator the rpms are fluctuating between 300 and about 700 and will occasionally just die if I don't put it in neutral OR let off gas for a split second then go WOT. When I push the accelerator in normally the rpm's are at about 1K when they should be at 2K. But then it just picks back up again, and it doesn't do it again until I turn it off and restart it. Again, it still does it even if it's warm (I shut it off and get gas quick) and usually happens at about 0-15 mph but never at highway speeds. The mechanic that did all the work on it is at a loss right now, and has had it for about 5 months. By turning key on/off 3 times I get P0141 and P0161 codes (both downstream o2 sensor heater element malfunction.) Looks like they pertain to the O2 heaters, but not sure if that would cause this. There are 2 mini Cats. at the manifold both of which are about 700 degrees F. The last cat. w/ no o2 sensors is about 400 degrees F. Mechanic said fuel pressure tests fine at the rail.

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post #2 of 5 Old 01-31-2014, 10:38 PM
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1997 ZJ 
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Location: willimantic
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my wifes 97 grand with the 4.0 would start fine when cold but when it warmed up the rpm would drop to about 300 to 500 and barely idle. i unplugged the first o2 sensor while it was running like that and it started idling like normal. replaced it with a new sensor and it never did it again. i never had any codes in the pcm though. it was a guess by a mechanic buddy of mine, but it worked.
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post #3 of 5 Old 01-31-2014, 11:16 PM
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1992 XJ Cherokee 
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Location: umatilla
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Sounds like maybe Tps. Do you have a cel? How's your battery and alt?
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post #4 of 5 Old 02-01-2014, 10:52 AM Thread Starter
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Ok, a quick update. It turns out the mechanic only replace one downstream O2 sensor. So I unplugged both downstreams, and that had a very minimal effect. Then unplugged both upstreams and that fixed the problem. I dont have the $400 to replace them right now, but it runs pretty good without them. Maybe a little rich but nothing serious.
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post #5 of 5 Old 02-07-2014, 09:27 AM
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1995 YJ Wrangler 
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You want to watch running it to long with the o2's unplugged because the over fueling can burn your cats out.

White jeep club member 157
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1999 , 4.0 , help needed , rebuild , stall

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