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sprinkles08, ASE Certified Technician
Category: Jeep
Satisfied Customers: 21436
Experience:  Chrysler/Dodge/Jeep Master Certified, ASE Master and Advanced Certified, Trans and Hybrid Specialist
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Liberty Renegade: Getting P0300 OBD code when CEL on. CEL sometimes

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Getting P0300 OBD code when CEL on. CEL sometimes flashing. Have changed plugs, coils, PCV valve, and done one SeaFoam application thru brake master vac. line. P0300 persists. What should I check next? 2005 Jeep Liberty Renegade 3.7l w/ 106k mi.
Thank you!
Hello and welcome to JustAnswer!

Although it should set a fuel system lean code, low fuel pressure may be causing the misfiring. It would be a good idea to verify fuel pressure using a fuel pressure gauge and make sure it's not dropping off under acceleration.

A restricted catalytic converter could cause this. If this was the case I would expect at least the spark plugs on one bank to be a bit fouled, if the plugs were clean this isn't as likely.

With everything that you've already done I'm questioning valvetrain integrity. I've seen valvespring issues on these, rocker arms that would fall out of position, sticking valves and valve/seat wear that causes sealing issues. If you have a scan tool I would first try to see if you can pinpoint suspect cylinders and doe a running compression test on all cylinders to compare them. If you have more than 5 PSI variance between cylinders on a running compression test then you have valvetrain issues.
Customer: replied 3 years ago.

I doubt fuel pressure issues; acceleration is superb, smooth and responsive. Would there be other symptoms? The plugs were not fouled at all so I doubt the cat is bad (fingers crossed on that one!). I've read a lot about valve issues with these engines, but that diagnosis is beyond my abilities/toolbox. I'll have to take it to the shop I guess, as I feared. I'll pass along your suspicions to my mechanic as a starting point. Thanks for your time.

Low fuel pressure would often cause hesitation and lack of power under load. I sort of doubt fuel pressure myself but it only takes a minute to rule it out, plus at the same time you'll be able to see and smell the fuel to make sure there isn't a contamination issue.

With everything that you've already ruled out I'm afraid that you may have a valvetrain issue of some sort. Plugs and coils are probably the most common cause of misfires, but you've already ruled out an ignition issue.

Another thing that comes to mind is injector issues. If you had multiple injectors bad or they were plugged with debris that made it past the filter then it could cause this. If suspect cylinders can be identified on a scan tool then those injectors can be moved to other cylinders to see if the miss moves.
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