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Hello. I need help diagnosing a misfire code P0303 on a 2004 izuzu rodeo

I’m having trouble diagnosing a misfire on cylinder 3 on my Isuzu Rodeo. I’m getting spark, new coil pack and new plug. Cylinder compression dry test is 150 psi. Which is within Specs. OBD freeze frame data for the p0303 trouble code show a 10% positive change in the LTFT bank 1 and 5% positive change in the STFT bank 1.

The trouble code is being recorded only when the engine is under load at about 30mph

Fuel pressure is being reported

bank 2 fuel trims are -.08%. I do have a leaky valve cover gasket on the left side of the engine.

I have a 3.5l engine. Direct injection. This has happened before, I fixed the misfire before by pulling the injectors and manually cleaning them, I also replaced the valve cover gasket and plug seals at that time. Intake and exhaust Valves were cleaned thoroughly.

What could be the problem? Should I just replace all of the injectors on that side? and the valve cover gasket, I don’t want to keep throwing money at it. I’m not sure why the valve cover is leaking again. It was replaced only 4 years ago.

The problem might be caused by dirty intake valves.

Since direct injection doesn’t allow fuel to come in contact with the intake valves, carbon deposits can form on the intake valves.

Seafoam makes a product that addresses this problem.


I did an inline fuel injection cleaning with GM top engine cleaner to clean the injectors and put some techron fuel system clean in the tank. But I will try cleaning the valves with the seafoam to see if that is the issue.

Could fouled oxygen sensors cause a lean misfire condition? I’m not getting a code for the O2 sensors.


If there were a problem with the O2 sensors, there would be a code indicating so.


Sea-foam made no difference. Would a severe leak in the valve cover gasket cause a misfire?
There is no oil leaking onto the spark plug tube, but it drips from the gasket onto the exhaust manifold. It has worked well for a few years since being replaced in 2016, but is leaking again.

What are the useful fixes for the valve problems on some of the DI engines?

Oh In case it matters, the misfire makes the check engine light blink.

Stop driving the vehicle.

A blinking Check Engine light indicates a major misfire is occurring. Which can damage the engine and/or the catalytic converter(s).


Yep. I know. I’m still not any closer to a diagnosis though. Does anyone have any suggestions?

Try moving your injector and coil to a different cylinder and see if the trouble code moves.

It’s got a new coil and plug. Moving the injector is a 2 hour job. :confused:

If you have good compression, try moving the coil just to make sure is good…if the the problem does not change…you may need to replace you injector…you could have a weak injector, clogged or leaking.

Try some starter spray in areas where a vacuum leak to that cylinder could occur. If the engine rpm increases noticeably, you’ve found the leak. I have to say however that this sounds like a valve or piston ring problem of some kind, given everything you’ve already done. Might wanna try a cylinder leak-down test. What are the compressions in the other cylinders?