Ask Paul: Why is my spark plug oil-fouled?

Q: I have been noticing an intermittent miss on the left mag during mag checks on the O-235L2C in my 1981 Cessna 152 (about 670 SMOH that included new Lycoming cylinders and pistons). About 30 hours ago when I cleaned, regapped, and rotated the plugs, I found that the lower plug on the #4 cylinder was filled with oil around the inner electrode and ceramic insulator. The plug I rotated into the lower #4 position was clean and in good condition. I rechecked the plugs after 25 flight hours and found the lower #4 plug was again oil-fouled.

In addition to these symptoms, my last five oil samples show increasing copper, iron, and, now, aluminum traces. The engine uses about a quart of oil every 10 hours, which has not changed since I bought the plane with 200 hours SMOH. The compressions at last annual were in the range of 65/80 for all cylinders. Other than the occasional miss at run-up that can be cured by leaning, the engine runs fine.

My guess is that one of the #4 cylinder valve guides is deteriorating, although I understand oil could be bypassing the rings. Do a leaking valve guide or rings require immediate repair? Could there be some other source for oil in the cylinder? What causes valve-guide wear?

PETER SCHMEELK, Arlington, Va.

A: Peter, I can assure you this is not the first time I’ve heard about this type of situation and I’m just as certain it won’t be the last, not only on this engine, but just about any engine currently in flying status. This is not unusual and can be dealt with rather easily in most cases.

Peter didn’t mention the specific spark plug he was using in his O-235-L2C, so the first place I’d start in attempting to solve his problem would be to install a set of Champion REM37BY spark plugs, if they are not presently installed.

The high compression O-235-L series has had a history from its introduction of experiencing higher fouling problems than other Lycoming engines. While most of this is caused by lead fouling, we may be able to relieve Peter’s problem by using the REM37BY spark plug. Normally, the REM37BY spark plug is the best all-around plug for the O-235-L2C engine application. However, even though they may be more resistant to lead fouling, there is a chance they may not deal as well with oil fouling if it’s bad enough.

Another consideration might be moving to a hotter spark plug in an effort to overcome oil fouling. The hotter spark plug offered by Champion for this specific engine model is the REM40E. Before anyone runs off and begins to think about just putting a hotter spark plug in the cylinder experiencing the oil fouling — forget about that right now! It is not good shop practice nor recommended by either the engine manufacturer or spark plug manufacturer and may end up causing more problems in the long run.

From past experience, I’d suggest cleaning, gapping and rotating the spark plugs every 25 hours. While this may seem like overkill, it gives you the opportunity to check the condition of the plugs and could contribute to a longer spark plug life.

The oil consumption you mentioned is certainly very good. You might want to observe how long it takes for the oil to turn black. Oil that turns black in a few hours of operation may be an indication of blow-by the rings. Of course this also can be a cause for the oil fouling that you mentioned.

Neither blow-by nor valve guide wear should require immediate repair. There are ways to track the condition of the one cylinder you suspect and all of the others, as well. This can be done by completing a good “hot” (engine at normal operating temperatures) differential compression check. You stated your compressions were 65/80 at the last annual, so you’ve got a good reference to compare when you perform the same check now. As long as the loss is no greater than 25% or no lower that 60/80, no corrective action is required.

If I were to take a guess about what you might find on your #4 cylinder and if the rings are allowing blow-by, you should hear this loss coming out the dipstick/oil filler neck.

While this fouling may be a slight nuisance, your method of leaning to correct the problem is an acceptable practice prior to takeoff. The important thing you must remember is to always return the mixture lever to the full rich position before commencing your takeoff roll.

Finally, regarding oil analysis: I’d attribute the increase in copper to the fact that AeroShell 15W50 typically raises the copper content slightly, but this is nothing to worry about. The important thing to keep in mind regarding oil analysis is that we are looking for a trend. If a certain component in the oil shows a radical spike, such as iron going from 100 ppm to 198 ppm during a normal 25-hour oil change interval, this would be a reason to recheck in a short period of time.

This should give you a few things to consider, but at this point, I’d say fly it and keep your eye on things, as you apparently do anyway.


Paul McBride, an expert on engines, retired after almost 40 years with Lycoming. Send your questions to:


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    Hi Paul,
    At present, I work on the IO-360-MIA Lycoming engines. My problem is that the right mag has a mag drop of 220 RPM. I have done internal timings, engine to mag timings, ignition leads check and clean the plugs.
    I have done nothing to the left mag, and now the left mag is now dropping to 210 RPM.

    • George Coiner says

      Ditto. Experiencing a similar issue. Three weeks ago we did an annual on a Schweizer 300C with Lycoming HIO036-D1a engine. Found a sticking valve on #4…..10 psi only! I didnt notice the engine running rough while flying however so maybe it released during hot operation conditions? Anyway after the annual we did a test runup. Fine. Then I flew for 1.5 hours. Again everything fine. I then parked for three weeks as we experience our normal fall rainy weather beginnings in Seattle. I went to startup and found the engine began running rough with muffled pops….then loud backfires. Tried the mags …drop was about 125 to 150 max. Ok.
      We shut down and pulled my Champion fine wire plugs…..lower plugs were oil damp. Top plugs dry. We cleaned and reinstalled the did runup test. Again engine running rough and backfiring. Pulled plugs again. This time they were saturated with oily runny liquid. Again all four plugs. Poured oil/liquid out. We then pulled the intake pipes and noted oily accumulation in manifold chamber. I sent for dye penetrate testing. Nothing found cracked at least at the oil drain area in the oil sump nor at the bottom of the sump chamber. Am going to try sitting alcohol in the sump and looking for any leak thrus to intake chamber but doubt i will find anything. I sent my mags in for their 500 since almost due. Will know about those in a couple of days. I have never had this running rough/backfiring in the two years since I bought this helicopter. Coincidently this all began after this last annual. Have called Lycoming (Mark in technical assistance) but they are as yet a loss for what is going on and are contemplating. My next step besides double checking the sump for leaks is to vacuum out any oil sitting in those lower piston chambers with a flexible hose where the plugs screw into. then putting back together the sump, mags and cleaned/tested other Champion fine-wired plugs and try to run up again. Any other ideas? What about checking for intake leaks somewhere but then that doesnt address the oil in the plugs???

      • George Coiner says

        Also….my oil consumption is a quart for every 2.5 to 3 hours. I use Aeroshell 15W-50.
        Our annual weather is cool so not running engine in overheating type conditions. Hottest this summer was about 107 at engine air filter temp probe.

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