Q: I’ve been plagued with engine trouble on my Cherokee 140 for nearly three months now (O-320E2A). It began missing briefly every five seconds or so in-flight. After landing, we pulled the plugs and cleaned them. But it did no good, she still wasn’t happy.
The magnetos were over their rebuild time, so we replaced both with factory-rebuilts and had the harnesses tested. After putting everything back together, it ran better, but still not like it was originally, so we decided on new spark plugs. We went with Champion’s new high-reach spark plugs with an extended electrode which made a world of difference, and the motor ran better than ever….for about an hour. Then it began running very rough consistently.
We pulled the plugs again and the electrodes were completely closed up on the #3 cylinder. Then, we put new massive-style plugs back in and they didn’t close up but it still doesn’t run well. I’m starting to run out of ideas on what could be causing this very confusing and frustrating issue. Any help would be greatly appreciated!
A: Alex, DO NOT start your engine until you complete a thorough borescope inspection of all four cylinders! Let me tell you why.
From the information you furnished regarding the spark plug fouling problem on your Cherokee 140 with the Lycoming O-320-E2A engine, I’d say you’ve got an interesting situation. I was reading along happily and thought this would be an easy one to answer until I got to the point where you mentioned the spark plug condition after removing the plugs from #3 cylinder. My mind raced and I didn’t even have it in gear yet!
To back up a little, I’d say the engine miss you experienced inflight prior to having the magnetos overhauled may have simply been a tired ignition system, possibly paired with some lead or oil fouling of the spark plugs. Then, after having the magnetos overhauled and installing the Champion REM37BY spark plugs, all was good for an hour or so before things turned bad again. When I said things turned bad again, I am referring to the condition of the spark plugs you noted after removing them from the #3 cylinder.
If I had to lay money on the table about this condition, I’d bet that a foreign object of some kind passed through the #3 cylinder causing the ends of the spark plug to be hammered shut. The reason the new massive spark plugs that you installed next didn’t close up was because whatever passed through the cylinder causing damage to the previous spark plugs and was no longer in the induction system and probably was blown out the exhaust stack.
My immediate recommendation to you is, before starting the engine again or flying the aircraft, conduct a complete borescope inspection of all cylinders. By conducting this inspection, I believe you may find that one and possibly other pistons will have been damaged by a foreign object through the cylinders. This may have come from something that was dropped into the cylinders accidentally during routine maintenance, or something that passed through the engine induction system.
I realize all of this is not what you wanted to hear, but I’ve given it my best shot drawing on my past experience and all I can say is your situation doesn’t sound good. I hate being the bearer of bad news like this.