Intermittant P0300

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Ksat

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having an intermittent p0300 code on my '06 3.7 liter. Not sure what triggers it, but the few times it's happened the car's been at a stoplight or otherwise idling when engine is warm. I thought I fixed the problem after changing the plugs about a week ago, but the problem continues. Engine runs rough/wants to stall out and CEL flashes. It happened again today and this time I could barely get the vehicle to move under its own power. I shut it off for a few minutes, started it up and it ran fine after that.

Aside from the P0300, there's a downstream O2 sensor code that comes and goes, but it's been doing that long before this problem cropped up.

I did some digging on the site here and on Youtube and it looks like my problem could possibly be at least one of 3 things: 2 or more coils are bad, the automatic shutdown relay (ASD) could be malfunctioning, or an upstream O2 might be faulty.
 

LibertyTC

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What brand of spark plugs did you install?
jeep likes NGK gap at .040 inch
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Mopar coil packs are best and could be considered a tune up item.
Next would be cleaning the IAC- intake air control /throttle body- it controls the idle and often can seriously be carboned up.
A flashing MIL means serious misfires & pull over and turn off jeep.
some members have seen multiple misfire issues requiring new PCM modules.
 

Ksat

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Thanks for the reply. I installed Champion copper @ 0.040". I'll try cleaning the IAC and see if that helps. Is there something I could look for on my scan tool to see exactly which cylinders are misfiring? Not sure if I'd want to go through the expense of replacing all of them when it might not be that problem at all.
 
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LibertyTC

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With a high end scanner like a snap on ultra, you can see the misfire count and cylinder locations.
It is always recommended that you use the factory NGK plugs to start with.
Dirty injectors can also be a source of misfires, but use the NGK plugs.
 

jp7794

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I doubt multiple coils would be intermittently bad. Agree 100% on the NGK plugs. Lots of plug threads here and they always circle back to use the NGK factory plugs.
 

Ksat

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Thanks for all the great replies. I ordered the NGKs and they should arrive in a couple days. I got around to cleaning the IAC, but it hasn't helped improve the problem.

Though I don't have any of those high end scanners, I do have one that was able to give me some some freeze frame data at the time of the fault which might be prove useful. Here's some of what what it said:

- Fuel system 1 CL (closed loop?) fault, Don't know if that's referring to bank 1
- Short term fuel trim bank 1 = +32.8%; bank 2 = -32.8%
- Long term fuel trim bank 1 = -0.8%; bank 2 = 1.6%
- Manifold pressure was 8psi
- Coolant temp at 197 F

I wouldn't expect the STFTs between left and right banks to vary as much as it's showing there. In addition the -32% is super lean leading me to think the info the PCM is getting from the upstream O2 on that side is faulty
]
 
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Ksat

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I probably should've asked before ordering, but are the NGK 7100s an acceptable one to use?
 

KJ05user

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How did everything work out ? How was your issue resolved?
I'm currently going through similar problem.
 

Ksat

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I replaced both upstreams and it fixed the problem. NTK brand is mandatory. The reason the car ran better after the restart (some times it took several restarts) is it bumped the ECM into closed loop fault and was probably ignoring 02 inputs at that point. I figured out that the car ran fine cold, but after a shutdown and restart- from either a few minutes to several hours, it ran like crap (with blinking CEL) right as it went into closed loop. It's still a mystery to me why one bank's fuel trim went way negative while the other went way positive simultaneously like that. Driving with the CEL blinking unfortunately roasted bank 2's cat on me.

Try unplugging both O2s on yours and see if the problem goes away.
 

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