Random Misfire Conundrum

I've got a unique misfire problem. And before anyone shouts out "have you checked out the dozens of other misfire threads?" Yes, I have. There were a a couple that were fairly similar, but lacked some variables that I'm experiencing...and nobody had a definite answer for them.

So to start off, I have an 05 renegade 3.7L with about 125k miles. I bought it last October and it ran like a top up until winter hit. The misfires usually happen on cold, damp mornings (usually below 40F degrees and either raining, snowing, foggy, or right after a storm while everything is still wet) It always starts right up and usually idles just fine. It isn't until I get about a mile down the road that it starts to randomly misfire. Along my 15 mile highway commute to work, it will usually misfire only a handful of times, bogging down between 2-10 seconds, and then boom, takeing off on all cylinders. After the engine completely warms up, the misfires completely stop and usually don't come back on my way home from work unless I'm leaving work in the before mentioned conditions. Whenever it misfired enough to keep the CEL on, I would always hook my scanner up and only get the useless p0300 code. Could never get it to give me a specific cylinder. I just kinda dealt with this problem all last winter and once spring hit, I didn't have a single misfire for months... until this winter hit.

So that's where I'm at now. The only thing I've done so far to try and remedy this problem is replacing the spark plugs (which all looked okay-ish) but nothing has changed. I know there are a lot of variables that could be causing this, but given the backstory on the problem, where do you think I should go next?

Change your cam and crank sensor. Most likely your crank. Guessing it has rust around it and has cracked the sensor housing. This lets moisture in. As it warms up the moisture starts to evaporate and causes it to seed eradicate signals just enough to cause a misfire. Or moisture in the plug end going to the crank sensor.

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moisture in the PCM connector can also cause this, as long as the moisture is frozen it stays nice and compact, once it melts from engine heat it causes a problem until it finds a happy place

see a lot of this sort of thing in the winter, moisture, cracks in plastic sensors, weak batteries(knocking off at stoplights) etc

I had a misfire at idle until I fitted new coil packs

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If you replace coil-paks use OEM ONLY.




Dave

rjkj2005 said:

Change your cam and crank sensor.

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This is where I would start, also.

rjkj2005 said:

Change your cam and crank sensor. Most likely your crank. Guessing it has rust around it and has cracked the sensor housing. This lets moisture in. As it warms up the moisture starts to evaporate and causes it to seed eradicate signals just enough to cause a misfire. Or moisture in the plug end going to the crank sensor.

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belvedere said:

This is where I would start, also.

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If you do the crank and cam sensors (and I'd start here too), use Mopar like Dave said about the coil packs. I've heard several stories of aftermarket crank sensor not fitting correctly and hitting the crank. Probably not a bad idea to swap the coil packs too, but if one or two were bad it seems you'd get a specific cylinder code. Hopefully not the PCM itself.

Alright, I'll start with the crank sensor first and see what happens. If that doesn't work, I'll try a new cam sensor. And I'll follow your advise and spend the extra money for mopar sensors. Ill let you know what happens.

I used "Standard" coil packs done about 5000km now runs brilliant.

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UPDATE - Installed a new crank sensor 2 days ago and all went well until today. Misfired for about 10 seconds while accelerating onto the highway on my way home from work. The interesting thing is that it wasn't a wet or cold day. Outside Thermometer was reading 57. With that being said, do you think I should still move onto the cam sensor?

Shotgun approach it seems...

I'd be more inclined to think ignition coils.

Either which way, whatever part(s) you want to throw at it, they all can be tested off the vehicle. I'd keep that in mind.

ANOTHER UPDATE - Misfired really bad the whole way going to work yesterday on a cold and very wet morning, which triggered a constant CEL. Now I've hooked my scan gauge up to it several times before since this all started and could get nothin more than the P0300 code. Today on the other hand, I hooked it up and the P0300 was accompanied by P0153, B2S1 O2 sensor slow response. Finally! A lead on what's going on! Ordered a new sensor so will see what happens.

10 days since the last update. Have you resolved your issue?

Sorry, I have the sensor but have been bogged down by the holidays, work, and the weather. Gonna be putting it in as soon as I catch a break.

Okay, got the new sensor installed today. Supposed to be getting a lot of cold wet weather all this week which has been the prime condition for misfiring, so let's see what happens.

before you throw new parts at it, i would pull the fuel rail and check the condition of the injectors. this will give a random p300 misfire code. I had a lot of crud in my fuel rail, although i checked the operation of my injectors while they were out, the dirty rail was the problem. This is not hard to due, will only cost a can of carb cleaner and worked for me ! I agree that these jeeps have problems with bad crank sensors, but i would try this before i bought any new parts. good luck

It's been about a full week of driving with lots of rain and snow and so far I haven't had a single misfire. Not even a hiccup. It's looking like the O2 sensor has been the culprit this whole time

If you're using regular gas that may be your problem. High octane ONLY IN THAT ENGINE.

The manual says 87, ok to use 89 if you're towing in hot weather, but it actually recommends against 91+.

4x4Husky said:

It's been about a full week of driving with lots of rain and snow and so far I haven't had a single misfire. Not even a hiccup. It's looking like the O2 sensor has been the culprit this whole time

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Glad you got it fixed 4x4Husky.

HoosierJeeper said:

The manual says 87, ok to use 89 if you're towing in hot weather, but it actually recommends against 91+.

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Yup, that is what my manual says also.


Dave