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Old 10-27-2012, 09:22 AM
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'04 WJ, bizarre misfire of sorts

Hey guys!

Our 2004 GC Limited has a strange misfire issue of sorts... When I say misfire, it sounds and feels like a misfire, however, there's "no" check engine light, no codes. Exhaust appears louder as I'm driving along and there's a distinct one cylinder that stands out as different. You can feel it as well, especially as RPMs hit above 1500 or so, all gears. With my head under the hood it doesn't sound loud at all. It happens regularly when the ambient is below 67 degrees F, only as the engine gets up around operating temp, it's like someone hits a switch and it happens. Cold engine runs great! If the ambient is "above" 68 degrees or so, it runs excellent cold or hot engine.

I've seen many posts about head gaskets and fuel injectors and such, but those all seem like more permanent issues. I took it to a friend's shop and we put a scope on it. No misfires recorded, but it was obviously missing and the engine stumbling and jumping around. The only thing noted out of spec was the IAT sensor was reading 108 degrees "F". I replaced the sensor and it reads ok, but no change in operation. I also changed the TPS. I've also changed plugs. I thought also it may be coils or fuels injectors as they heat up, so I used freeze spry on each with no difference. Besides, that wouldn't explain why it runs good in warmer air... Anyone else seen this?

Thanks!
Lorne
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Old 10-28-2012, 05:46 AM
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Re: '04 WJ, bizarre misfire of sorts

Temperature related issues most of the times have to do with a sensor problem.
Maybe a crank or camshaft sensor, which is messing with the injection or ignition timing. Just an idea of me. Probably 2005JGC can help you further without changing all kind of components in the process of troubleshooting.
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Old 10-28-2012, 08:06 AM
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Re: '04 WJ, bizarre misfire of sorts

What TPS did you use? I have read that only OEM Mopar or you will get all kinds of strange behaviors. Which engine? I assume it is a 4.7, not the HO. Is that right?
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Old 10-28-2012, 12:57 PM
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Re: '04 WJ, bizarre misfire of sorts

Yes, it is the 4.7 VIN "N". I did use an Auto Zone TPS to correct a little stumble at idle and that helped that issue. The problem I'm having started prior to changing that sensor.

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Lorne
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Old 11-01-2012, 06:04 PM
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Re: '04 WJ, bizarre misfire of sorts

Update: OK, we hooked the WJ up to a diagnostic scope and near as we can tell, the crank and cam sensors are working. Notably, however, we realized that the IAT sensor is still out to lunch (we think). With the sensor connected, the PCM shows the ambient is 108 degrees F steady state, with it disconnected, the PCM reads 84 degrees F (as did the old IAT). Is 84 degrees a default if the IAT shows an open? Anyone else seen this?

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Lorne
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Old 11-01-2012, 07:30 PM
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Re: '04 WJ, bizarre misfire of sorts

Yep, the 84 F is a default setting when the temp sensor info is lost.
Did you already measure the IAT wiring to ground? From PCM connector C1 point 15 to ground and from the same connector point 4 to ground. The resistance should be infinite, otherwise you could have wire shaving somewhere.
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Old 11-04-2012, 11:13 AM
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Re: '04 WJ, bizarre misfire of sorts

The resistance of the IAT sensor at 45 degrees F was around 21k ohms. As I blew warmer air on it, the resistance went down. I have not disconnected the main trunk into the PCM (will later today). I did measure the resistance at the IAT connector with the PCM hooked up. The BLK/RED wire (IAT return) is not infinite (but in the m ohm range) and the BLK/BLU wire is dead short to ground. Question.... Is the sensor ground out of the PCM (pin 4 BLK/BLU) and chassis ground supposed to be the same potential?

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Old 11-04-2012, 03:16 PM
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Re: '04 WJ, bizarre misfire of sorts

Not completely sure about this, but they talk about a low noise sensor return, so it should not be directly shorted to ground. It can be that inside the PCM it will switch to ground. For that reason you need to remove the C1 connector, so that you can measure the wiring only.
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Old 11-12-2012, 12:36 PM
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Re: '04 WJ, bizarre misfire of sorts

Well, at this point I'm thinking either harness at the PCM or the PCM itself. It seems I've heard mention of problems with both. When I disconnect the harness from the PCM all is well, everything looks OK in a resistance test of the IAT harness. When I hook it back up, it still looks OK, no short to ground. When I drive it and it gets to operating temp, it fails again and I have the short back. Soooo, I'm thinking about getting it up to temp on a cool day (when it fails) and hitting the PCM with a hair drier or heat gun for a bit and see if it changes. Not sure what else to do here... Thoughts?
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Old 11-12-2012, 02:09 PM
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Re: '04 WJ, bizarre misfire of sorts

how are you measureing the continuity to ground?

you would be supprised what leaving connectors connected can do to trick you into believing a false reading. if you want to measure continuity to ground with the circuit intact you need to measure voltage drop on the circuit with the circuit operational... never trust an ohm meter on a completed circuit.

I was taught a quick check of a simple circuit like this is to unplug it which should give you an open or "voltage high" code, then short it to ground which should give it a voltage low or shorted to ground code. If you get both codes you know your circuit is A) intact and B) able to see when it is wrong so one could assume fully operational... if you see both codes it needs an IAT sensor.
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Old 11-12-2012, 02:48 PM
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Re: '04 WJ, bizarre misfire of sorts

I guess my question then would be, is the chassis ground and sensor signal ground (pin 4 on PCM) supposed to be the same potential or should there be a difference? I'm under the impression that they should be different, with the sensor ground being somewhat isolated from the chassis ground. Please correct me if I'm wrong though, I hate chasing up the wrong tree. Thanks!
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Old 11-13-2012, 07:42 AM
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Re: '04 WJ, bizarre misfire of sorts

Sorry for the slow responses. Been a lot going on making it difficult to get into the garage to do any testing. I appreciate everyone's input...

OK, the pin to pin voltage at the IAT sensor with the sensor unplugged is 5.3 volts, same result with the blk/red wire on the connector to chassis (alternator case). The voltage from chassis ground to sensor ground (blk/blu wire on the connector) is 0 volts. I'll have to borrow my friend's auto scope and see what actually happens when I short the connector out and maybe even try a pot in the circuit to see if there's a difference. Thanks!
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