Stumped on a misfire diagnosis. Could use ideas…

[Chris]

I’m working a a 2009 Toyota Carolla/Matrix with a 1.8L 4CYL. I have gotten a P0304 code from it. I am chasing the problem and can’t find it. Checked for spark and it has strong spark, fuel injector is working and all have been removed and flushed. Mechanically the engine is sound but I still have this misfire that seems very bad at idle but clears up during accel. Once or twice during accel. The customer has stated that the car began to stumble badly and lack power but I have been unable to recreate this symptom. Aside from the couple times it has stumbled during accel. It has only been cyl. 4 and it only shows at idle. Any ideas would be greatly appreciated.

Topic

[Andrew Phillips]

Have you pulled the spark plug? I would check that out before chasing other demons that may not exist. There could be a carbon track on the plug causing the spark to go to ground.

[Its me]

May want to check for a thrown rocker arm
..

Karl

[Chris]

Yes I have. Brand new spark plugs were installed a day prior to the car being brought to me.

[Andrew Harris]

Has this vehicle had any major engine work done? Have you pulled the valve cover and checked the valve train. Did you do a compression test? If you know fuel and spark are 100% good then it has to be something mechanical with the engine.

[Its me]

These engines were known for throwing rocker arms if over-revved… Problem manifests as a cylinder misfire… The first thing I would do (as suggested earlier) is to remove the valve cover and look for a thrown rocker arm… I’m interested to see what he finds…

-Karl

[Author]

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