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!
Alex Neufeld
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.
I have Kawaskia powered ultralights and ran into this same problem with one of my two cylinders on one of the engines. I found that it doesn’t take much junk in a cylinder to bend the grounding electrode on a spark plug. The junk I found in my cylinder was a piece of carbon that got into the cylinder when I was replacing the plugs and pupping the aircraft to bed for the winter. (It gets very cold in the winter to fly an open cockpit plane) The I got the plane ready for spring take-off I had the same problem that you described and with only one cylinder working after a short engine run-up I thought that when I removed the plug in that cylinder that I had real damage to my engine. I have a close friend that is a Snapon Distributor and he has been trying to get me to buy their bore scope, but that’s another story. The results from the scope inspection was that stuck to the top of the piston was this piece of carbon. So I took the head off the cylinder and picked the carbon off and blue out the cylinder to make sure I had not introduce any other junk and now the motor has had no more trouble. This just shows how even a small piece of junk can lead you into a world of problems. My motors don’t cost as much as most aircraft motors but when you only have two cylinders to work with., well you see my concerns. I wish you the best with your problem, and I hope my testimony helps. Fly Safe and Level!
Do the COMPLETE borescope inspection, find any cylinder/valve damage and also inspect the induction system. The results of these inspections will show you and your mechanic/shop what is going to be required to fix your engine.
The primary venturi from a two piece venturi can dislodge and cause the same type of symtoms. . Had a piece of one do it and finally found it in the induction system. It migrated around the different cylinders.
Sounds like a valve problem. A valve will sometimes stick causing the early occasional miss. Then it sticks open far enough for the piston to contact it. Rarely but it can break a piece out of the valve that in turn damages the spark plug.
Sounds like a compression check is in order, and a look see with a bore scope..
have to agree with you.
Could it be an exhaust valve going out? That’s the exact symptom I had once in an IO-360 equipped Arrow.