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  #13  
Old 07-16-2015, 08:05 AM
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Re: HELP! P0300 multi misfire out of nowhere

Quote:
Originally Posted by Frango100 View Post
Maybe some water went into the tank?
But that it started up normally and only after warming up a bit, still makes me think that it has someting to do with the O2 sensor itself. When you start the engine, it will run in open loop (not using the O2 sensor intput) and only after warming up a bit it will go into closed loop (using the O2 sensor input to trim the fuel injection pulse).
But lets see, maybe after all it was just water, where ever it was.
I drove 25 miles to work this morning, running it pretty hard and no issues so far. I'm down to 3/4 tank now. I plan to run it down to 1/2 tank, then refill with premium gas from a different station and see how it goes from there. BTW- I filled up with Sunoco last week, not some cheapo brand. I'm crossing my fingers that it was just a bad batch of gas. Time will tell. In all my years of driving I've never had something like this happen due to bad gas although I've heard about others having issues after filling up at off brand stations.

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  #14  
Old 07-16-2015, 12:03 PM
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Re: HELP! P0300 multi misfire out of nowhere

I dont see how bad gas would cause the fault codes you had, would more expect moisture in the O2 sensor connector. Ofcourse the misfire could be, but the O2 sensor???
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  #15  
Old 07-17-2015, 10:45 AM
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Re: HELP! P0300 multi misfire out of nowhere

Quote:
Originally Posted by Frango100 View Post
I dont see how bad gas would cause the fault codes you had, would more expect moisture in the O2 sensor connector. Ofcourse the misfire could be, but the O2 sensor???
I tend to agree with you, but the only thing I did was put a bottle of Techron and 1/2 bottle of seafoam in the full tank and run it hard. It's still running great, if not better than before. The engine feels more responsive. This morning I was down to 1/2 tank of the 'old' gas so I stopped to fill up with 93 octane and put another 1/2 bottle of seafoam in. Hopefully it continues to run well through the weekend and beyond.
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  #16  
Old 07-30-2015, 08:43 AM
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Re: HELP! P0300 multi misfire out of nowhere

Well, some anomalies have cropped up over the past two weeks. Every once in a while, while in drive, stopped in traffic, the idle will waver or even drop 100-200 rpm's- like it's going to stall, but then recovers. No check engine light or codes thrown. Then last night at a stop sign, I pulled up to check traffic before pulling out, idle dropped and engine stalled. I turned key off, then restarted just fine, drove home without further incident. Now this morning coming to work, the idle was wavering maybe 50 rpms when stopped in drive.

Oh, and a week ago, while descending a long grade of a couple miles, in drive, the idle was randomly dropping maybe 100 rpm's. Coming down the long hill, I barely needed to apply gas, just enough to prevent engine braking and maintain 55 mph down the grade. I haven't experienced this again, but haven't been around that particular type of terrain since either. Again, no codes are being set.


The random misfire has not happened again since I ran the fuel system cleaners through.

Could this be a crank sensor going bad? I have a new one ready to install. I might just replace it this weekend to be safe and eliminate this part as a possibility.
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  #17  
Old 07-30-2015, 10:07 AM
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Re: HELP! P0300 multi misfire out of nowhere

If a GM car I'd say a gummed up idle air controller (IAC) but am not sure if your Jeep has one. See here.

I'd also check to see which cylinders are showing a misfire (P300 is a catch-all for misfires and usually means multiple cylinders - With Torgue Pro on my Cell Phone and a $15 dongle I can display both current and long term misfires by cyl.)

Sometimes just a few cans of SeaFoam will cure a lot of ills.
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  #18  
Old 07-30-2015, 11:02 AM
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Re: HELP! P0300 multi misfire out of nowhere

Since you already have a new crank sensor, i would try that first. If the fault continues, i would run some seafoam through the brake booster line. Carbon buildup on the valve stems can make them stick.
The 5.7 has an electronic throttle control, so there is no IAC motor anymore.
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  #19  
Old 07-31-2015, 07:13 PM
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Re: HELP! P0300 multi misfire out of nowhere

I replaced the crank position sensor today. Wasn't too bad of a job except I had trouble getting the old one out. Space is limited. I could get it to spin, but couldn't get a good grip to pull it out. I used some PB Blaster to loosen things up and eventually got both hands snaked up in the space to twist and pull at the same time. Eventually it wiggled out. The new one went in without any issue.

Fired it up and everything is running well for now. Ran some errands, did some shopping and the truck runs good. Time will tell if any problems arise again.
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  #20  
Old 08-11-2015, 12:56 PM
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Re: HELP! P0300 multi misfire out of nowhere

Truck is still running well, no misfires or anything so far (knock on wood)
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  #21  
Old 08-11-2015, 01:01 PM
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Re: HELP! P0300 multi misfire out of nowhere

Good to hear, happy jeeping.
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  #22  
Old 10-10-2015, 02:06 PM
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Re: HELP! P0300 multi misfire out of nowhere

Update- no misfire issues after two months! Don't really know the root cause, but after running two bottles of Techron and seafoam through, didn't have any further issues after that. Maybe a ghost in the machine
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