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Old 01-05-2013, 05:14 PM   #1
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New Member with old problem

Hi all, I've got a problem with my 2002 TJ Sport 4.0 automatic that is driving me crazy. I'm a tech so I'm pretty familiar with what's what but this has me stumped. Random misfire code PO300 that sets only above or at 3100 rpm. Check engine light flashes like it should for a cat damaging code and if I drop below 3000 rpm it stops. Sometimes I get the PO 300, 301,302, 305... Sometimes just the PO300 but only above 3000 rpm. I can duplicate it in park , no load, or in any gear, under load. Here is what has been done. A crank sensor about 4 months ago for a stalling problem. For this problem... A coil rail, champion spark plugs, the Chrysler TSB on decarbonization for the engines, rotating the exhaust valves 90 degrees, today I installed new valve springs, check fuel pressure and engine vacuum, both stable at time of CEL, LTFT looks ok, still does it.. I 'm stumped and frustrated. I'm not sure that this jeep always did this since only recently have we been turnpiking it and need to run at 70. My next step is to check the frequency of the cam and crank sensors to see if anything is weird when it sets codes. Since the vacuum is stable and he fuel pressure is a stable 50 at all rpms, I'm now discounting mechanical and fuel pump issues... The filter is in the tank and not serviceable. This thing has 70,000 miles on it. It runs and idles great otherwise. No compression issues either. Ideas?

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Old 01-05-2013, 06:41 PM   #2
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Can you actually feel the misfire? I know there are a lot of smart people on this forum so I will be interested to see what they say. I'm a German car tech so I understand your frustration. You have done everything that was rolling through my head. I have read on here that the 4.0 is very picky about which plug you use. You might do a search. It wasn't too long ago that this was a topic. Jerry had a lot of advice on which plugs to use. Otherwise I would start to lean toward a bad ECM, even though that is rare. Good luck. I'll keep an eye out to see when you get it fixed...this seems very unusual.

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Old 01-05-2013, 06:46 PM   #3
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I run at up to 80 with the 2001 auto jeep, and never hit 3100. Tire size maybe? I'm not really helping much lol.
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Old 01-05-2013, 06:52 PM   #4
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Is the crank sensor a factory unit or aftermarket? I was working on another chrysler product once in the shop and kept having trouble. It kept coming back to the crank sensor. Replaced it 3x with napa sensors from 2 different lines. Fought the problem over 6months. Finally called napatech and the tech there told me I needed to "use a higher quality sensor". I told him I'd used both lines of napa.. He repeated himself. Carefully avoiding saying not to use the napa ones, but basically saying I needed to go to Mopar. I put a factory sensor in and no more trouble.
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Old 01-05-2013, 07:03 PM   #5
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Originally Posted by chevymad
Is the crank sensor a factory unit or aftermarket? I was working on another chrysler product once in the shop and kept having trouble. It kept coming back to the crank sensor. Replaced it 3x with napa sensors from 2 different lines. Fought the problem over 6months. Finally called napatech and the tech there told me I needed to "use a higher quality sensor". I told him I'd used both lines of napa.. He repeated himself. Carefully avoiding saying not to use the napa ones, but basically saying I needed to go to Mopar. I put a factory sensor in and no more trouble.
I'll say very good advise. I did the same thing with 6 used and perfectly good warranted mass air flow meters...they cant all be bad. Put in a new OEM and it was fixed. Man was I pissed at myself.
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Old 01-05-2013, 07:07 PM   #6
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I worked on an 01 Cherokee that would bring on the C.E.L. but it was also popping on acceleration and would overheat at high speed.... would not leave a code. Made me nuts for a while, Turned out to be a bad plug wire on #3.
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Old 01-05-2013, 10:33 PM   #7
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Misfire

I really don't know what brand the crank sensor is because its the one thing I did not personally install. The jeep broke down on my daughter and a local garage install one for me by my direction. It stopped the cutting off on her. Could be. The plugs are the OE type and number Champions but the coil rail is from AZ. I'm going to put a DVOM on the crank and cam sensors tomorrow and see of there are any frequency glitches. Just doesn't make sense to me. By the way no bad wires near the bell either. Checked that. Really a brain teaser. I wouldn't worry about it but she will be driving the turn pike to college next week and beyond and telling her to stay under 65 mph is like asking her not to drink at college. Yeah right! No problem Dad.
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Old 01-05-2013, 10:36 PM   #8
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Originally Posted by Bgtklbx View Post
Can you actually feel the misfire? I know there are a lot of smart people on this forum so I will be interested to see what they say. I'm a German car tech so I understand your frustration. You have done everything that was rolling through my head. I have read on here that the 4.0 is very picky about which plug you use. You might do a search. It wasn't too long ago that this was a topic. Jerry had a lot of advice on which plugs to use. Otherwise I would start to lean toward a bad ECM, even though that is rare. Good luck. I'll keep an eye out to see when you get it fixed...this seems very unusual.
It will go into limp mode if you continue over 3000 rpm but otherwise it runs great. Cannot feel the miss.
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Old 01-05-2013, 10:39 PM   #9
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Originally Posted by c1skout View Post
I worked on an 01 Cherokee that would bring on the C.E.L. but it was also popping on acceleration and would overheat at high speed.... would not leave a code. Made me nuts for a while, Turned out to be a bad plug wire on #3.
Should of had a PO303 code on that one. My motor has no wires COP system. Coil on plug. And the plugs and coils are new.
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Old 01-06-2013, 12:07 AM   #10
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I'm going to put a DVOM on the crank and cam sensors tomorrow and see of there are any frequency glitches.
Hook your lab scope to each and look at the wave form. Should be a nice, even square wave...
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Old 01-06-2013, 04:55 PM   #11
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Originally Posted by 29ster
Hi all, I've got a problem with my 2002 TJ Sport 4.0 automatic that is driving me crazy. I'm a tech so I'm pretty familiar with what's what but this has me stumped. Random misfire code PO300 that sets only above or at 3100 rpm. Check engine light flashes like it should for a cat damaging code and if I drop below 3000 rpm it stops. Sometimes I get the PO 300, 301,302, 305... Sometimes just the PO300 but only above 3000 rpm. I can duplicate it in park , no load, or in any gear, under load. Here is what has been done. A crank sensor about 4 months ago for a stalling problem. For this problem... A coil rail, champion spark plugs, the Chrysler TSB on decarbonization for the engines, rotating the exhaust valves 90 degrees, today I installed new valve springs, check fuel pressure and engine vacuum, both stable at time of CEL, LTFT looks ok, still does it.. I 'm stumped and frustrated. I'm not sure that this jeep always did this since only recently have we been turnpiking it and need to run at 70. My next step is to check the frequency of the cam and crank sensors to see if anything is weird when it sets codes. Since the vacuum is stable and he fuel pressure is a stable 50 at all rpms, I'm now discounting mechanical and fuel pump issues... The filter is in the tank and not serviceable. This thing has 70,000 miles on it. It runs and idles great otherwise. No compression issues either. Ideas?
Update: installed another new crank sensor. Did not help! Same problem.
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Old 01-06-2013, 06:14 PM   #12
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