Carbon / Oil Buildup Cylinder 4

[Gary]

I have a 1.5 liter Mitsubishi Mirage, 1995 model that is burning oil and the #4 cylinder spark plug is covered in oil or carbon. I replaced the spark plugs and the engine ran fine for a few minutes, but it went back to skipping. I pulled the new spark plug and it was covered in black.

Smoke does come from the exhaust, but not all the time, it’s intermittent and the owner does have to fill the oil after a major smoking episode.

I was thinking either a head gasket or the valve oil seals for cylinder 4, but would that be a correct assumption since the smoking problem intermittent? Would a PCV system problem be more likely? I didn’t think PCV due to the major Carbon / oil buildup in the cylinder, I was thinking more a gasket / seal issue.

I would love to hear everyones opinions.

Thank you,
Gary

Topic

[Mike]

A bit of a different animal, but I just tore down a Mitsubishi Evolution that had pretty much the same symptoms you have but without the skip. This is what was wrong:

I don’t know if you have access to a borescope of some kind, but if you do, this video shows what it looked like inside the cylinder before teardown. Based on what I saw, I was able to determine it was a piston issue of some kind before teardown. In the video you can see that there is no oil being introduced thru the valve areas, and it’s almost conclusive that it’s not a headgasket issue. later in the video, you can see how the piston is bringing the oil up the cylinder walls from below.

[Gary]

I would run a compression test. Sounds internal to the engine to me.

[Mike]

I totally forgot to mention this, but Chevyman reminded me that the engine shown had equal and good compression across all cylinders. This was due to the fact that the oiled cylinder was getting a wet compression test to start, and the copious amounts of oil on the cylinder walls helping to seal made the numbers look fine. A compression test should absolutely be a diagnostic step here, but take this information into consideration when interpreting the result.

[Nik]

As already mentioned I would start with a compression test and note what you found. Then follow up with a wet compression test. If you find a significant difference from dry/wet then it’s likely that piston rings are your problem.

*edit* ONLY USE A SMALL AMOUNT OF OIL FOR YOUR WET COMPRESSION TEST

[Rereonehundred]

I forget when it was, but 1995 seems about right. Many Mitsubishi engine had ring and piston problems leading to wildly excessive oil consumption. Unload it.

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