P0304 - Cylinder 4 Misfire (random)

Maintenance/Repairs
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1

2007 Jeep Compass, 2.4L 4 Cylinder, 140K miles

At idle, I randomly get a P0304. Sometimes a month will pass before I get it again. I have swapped the plug, coil, and injector with another cylinder. Still throwing a misfire from cylinder 4. I can certainly feel the rough idling and know when it’s about to throw the code. I never feel it during acceleration nor has it happened during acceleration.

I currently have the throttle body and intake manifold off the engine. TB was dirty, so I cleaned it with some carb cleaner. Noticed oil in the intake manifold, but apparently this is normal if it’s just a little. I plan on changing both gaskets and the PCV valve (although the valve seems fine). Once I reassemble everything I want to run some SeaFoam through the intake because the valves have some carbon build up. Not NEARLY as much as I expected to see from 140K miles, but I want to try and clear anything out.

Other than this, anything else I should be looking at while I have the intake and TB off the block? I inspected the head gasket, and I don’t see anything wrong.

2

When you get this code, you should note the conditions it occurs in, like weather, road conditions, length of the trip, last time the gas tank was filled, etc. Look for a common occurrence and you may find out why this is happening.

3

Here’s the very next step . . .

perform a compression test on all 4 cylinders

Please post the results

We’ll let you know if everything is okay or not

If the results for #4 are low, then the next step is to perform a wet compression test

4

I agree;a compression check. What can happen if a compression drops in a cylinder the spark plug may have a tendency to foul somewhat due to that cylinder running a bit rich because of lowered compression.
Hopefully that is not the cause.