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Jan 02, 2024

Why are the spark plugs on my Cessna 152’s engine filled with oil? — General Aviation News

By Paul McBride · January 29, 2023 · 6 Comments Question for Paul McBride, the

By Paul McBride · January 29, 2023 · 6 Comments

Question for Paul McBride, the General Aviation News engines expert: We have a Cessna 152 fitted with a Lycoming O-235-L2C engine.

Recently we opened the cylinders and serviced the valve leaks. Compression check was OK. Later we did a run-up and found everything was OK.

Today when we did a takeoff run check and mag drop check, we found full static RPM 2300 and a mag heavy drop with vibration.

I also noticed that the oil gets black very soon.

Then we opened the spark plugs and found the plugs are filled with oil.

Kindly guide us what to do further. May I do only an oil ring replacement to rectify the problems?

James Kumar

Paul's Answer: Regarding your question about your Cessna 152 fitted with a Lycoming O-235-L2C, I will offer my thoughts and hope they will get you started in the best direction to correct your situation.

First though, I must point out that I’m once again dealing with a question that offers little information regarding the history of this engine, such as total time, etc.

However, I think there is enough information to allow me to offer a few suggestions.

We’ll start with the spark plug situation and the picture provides some concerns.

If I see what I think I’m seeing, it appears that there is a mixture of spark plugs being used in the engine. I can identify the REM 37BY spark plugs, but have no idea what the others might be.

Good shop practice would dictate using all the same spark plugs and avoid mixing different plugs in the engine simply because the heat ranges may be different.

From my experience, the best choice of spark plug for the O-235-L2C has been the REM 37BY, which was developed by Champion to alleviate spark plug fouling due to lead deposits.

I noticed the oil in several of your spark plugs and after reading your information, it appears obvious that there is excessive blow-by in the cylinders to cause the plugs to be fouled.

I’m not quite certain what you meant when you mentioned you serviced the valve leaks, but I don't think that was the cause of the oil-fouled spark plugs. My suspicion of what's causing the problem is that the piston rings are completely worn out. The oil turning black in a short period of time is also an indication of this.

In order to correct this problem, I’m recommending that all cylinders be removed and inspected, including a dimensional inspection. If the cylinders are within Service Limits, they must be honed in preparation for installing all new piston rings.

Once the cylinders have been honed and new piston rings installed, you can reinstall them on the engine.

Remember, you must use a straight weight mineral base oil for engine break-in as mentioned in Lycoming Service Instruction 1014 or its latest revision. You should also refer to engine break-in procedures as set forth in Lycoming Service Instruction 1427C.

I’m confident that following these recommendations, your engine will once again provide you with good operational service.