2003 Grand Cherokee - 4.0L Code P0300

[hank]

We have a 2003 4.0l jeep with missfire codes. Cylinders 1,2,and 3 have higher counts than 4,5,and 6. scanner shows injector milliseconds on front bank much higher at times than rear bank.short term adaptive goes positive with injector ms. Fuel pressure is higher than spec at 65 psi. ( spec is 44-54 ). o2 sensors seem normal. Another shop replaced the fuel filter/regulator. Thanks for your time!

[fortknoxx]

how many coils?

[steven kiser]

pressure at that level will cause a problem. i would look into that before i anything else. was the pressure at that level prior or not? probably no way to tell but an improper install can cause this. a kinked return line maybe the culprit. that much pressure can give you some of the symptoms you're dealing with. good luck.

[hank]

This is not a normal missfire. First take off on test drive it runs ok, until you load it. Front injectors start to rise ms counts to around 25, while rear ms counts are around 12. At the same time this happens the short term adaptive on front bank shows positive #s around 20 to 30. We belive the ignition is ok because it always cleans out from the rich mixture. Replaced Wix fuel filter with O.E filter, pressure dropped from 68 down to 48. Spec is 44-54 . Changed contaminated oil and filter, and all is well. ( EXCEPT FOR A FEW MORE GRAY HAIRS! )

[steven kiser]

thanks for posting the fix.

[Tim Martin]

That's a rather strange one now isn't it? I do not doubt the fix but I am trying to comprehend the meaning of it. Is there a flaw with the WIX filter in that application? Or perhaps there was an issue with that particular filter. I am trying to understand how a filter could actually increase the pressure.

Am glad it is fixed - that is the bottom line.

[steven kiser]

i thought the same thing until i looked it up and under location it showed the regulator and filter as one unit.

[Tim Martin]

i thought the same thing until i looked it up and under location it showed the regulator and filter as one unit.

Ahhhhh. That explains it. I also looked it up and sure enough. The regulator and filter are one unit. Ok.

What they don't come up with these days.

[Tim Martin]

I am really curious. I happened to have a WIX 33750 fuel filter - what it calls for and so I got it out and I really wonder if there is a pressure regulator with the filter. So I sent an email to WIX company requesting info on this. For my own info. Will post the results when I get them unless someone beats me to it.

[Tim Martin]

WIX Filters just sent me an email saying that yes the fuel filter for that application does have a fuel pressure regulator set at 50psi. Interesting. In that event, it musta been a faulty filter/regulator.

[steven kiser]

it seems that we have to be careful when installing these. i know i would grab a loose on off the shelf if it looked right.

[Tim Martin]

it seems that we have to be careful when installing these. i know i would grab a loose on off the shelf if it looked right.

I expect we all did that already.

[steven kiser]

speaking for myself here, i know that i've grabbed filters that were out of their package that looked like the one i took off. need to be careful i guess.