got the code p0303 and replaced the ignition coil, then about 4000km got the code p0303 again but found oil in the cylinder, open the other cylinders but they were clean except for cylinder 3 which had blacked up. car is running fine it been 1000km, need help

 

Cylinder 3 is well known for often having a loose sparkplug. Maybe it was just that. Beyond that time for a compression test.

 

More precision required.

Oil in the cylinder, on the electrode end of the spark plug? Or oil all over the coil, without removing the spark plug?
Were all plug torques set the first time you were in there after the code?

 

I only changed the ignition coil both times, it has the original spark plug and coils in all cylinders, not in 3 ignition coil is changed. there was burned oil on the coil and the cylinder was blacked up, which I thought was common but when I opened cylinder 1 it was clean and had a full silver color. I never touched the spark plugs for any of the cylinders.

 

Are you sure it was oil and not old dielectric grease?

 

I am super sure it was oil.

 

Then I wouldn't just change one ignition coil. Change them all and the sparkplugs. Make sure the plugs are tight, then check again in six months if you have no other issues. Beyond that like I said a compression check is all you can really do.

 

Leaking valve cover?

 

OK so you've never "opened the cylinder", so don't describe it that way to folks online when you're describing what's going on. The part of the engine where the coil is located is not "the cylinder" - if you are are reading information from other sources working with this definition you're going to be mixed up about a lot of things. It's not "normal" operating condition for there to be any oil or blackened soot on the ignition coil, but one or the other means very different potential causes.

If you had the coils out, I would have thrown a socket on the plugs and made sure they were tight.

If there was liquid oil all over the coil, the first thing I would be thinking was leaking valve cover gasket / spark plug tube seal.
If the coils were blackened with soot, the first thing I would be thinking is combustion gases blowing past the spark plug (out of the actual "cylinder") because the spark plugs have loosened off.

 

If the spark plug tube has oil in it, it would be from the valve cover. If it entered the cylinder via a failed head gasket or piston rings, it would burn during the combustion part of the four stroke cycle. I guess there could be potential for oil to come from within the cylinder if the spark plug was loose/not sealed via crush washer -- but I'd expect a miniscule amount of oil in that case, and for it to be more of a greasy mist since it will have been pressurized and forced upwards into the tube between the spark plug threads. It seems substantially less plausible that the cause would be anything other than a valve cover leak, which is common due to the mating of dissimilar materials (plastic to aluminum sealed by a kind of rubber gasket which will deteriorate over time).

The great thing about fluid leaks is their predictability. Once they escape a pressurized environment, gravity pulls them downnnnnn. So always start at the top of the engine when diagnosing them 😸