It Happened. The Big $$$ One. Now What?

I'm still in shock.

Got some bad news today.  Low compressions on all 4 cylinders.  All 4 exhaust valves are leaking.  One really badly.  Worst case--complete top overhaul.  Huge $$$'s.  After only 600 hours on the engine.  I still can't believe it.

Paradoxically, I was just re-reading a blog from a couple of years ago, where I was musing that in 28 years of aircraft ownership, I've never experienced "the big one", i.e., a completely unforeseen and hugely expensive maintenance issue.

Well, that all changed this morning.

In my last post, I talked about the bad Costco gas and how I thought that had fixed the engine roughness that I had experienced in flight.  I really thought it had.  But during subsequent flights, I could sense that something was still amiss--the engine just didn't feel right.  It seemed fine in the climb and at cruise, but there was a disconcerting low vibration at lower rpm, particularly noticeable when I reduced power when turning base.  Something wasn't right.

A further clue was a noticeable "softness" in one cylinder when I pulled the prop through by hand.  I had had this happen before, but it went away after several more hours of flying.  This time, even after multiple flights, it was still there.  Hmmm.

Last week, Jim, the guy who has a Thorp T-18 in the hangar next to mine, came by and I asked his opinion.  Jim is a true homebuilder, super experienced, very technical and an engineer's engineer.  He pulled the prop through and frowned:  "Yeah, you got a soft cylinder all right.  I've got a compression testing rig at home.  Why don't we meet next Saturday and I'll check all four cylinders for you."

That was this morning.  I drove to the airport with a gnawing sense of dread in my gut.  Hoping it wouldn't be that bad.  Maybe just the one cylinder.  Maybe a simple stuck valve.  Maybe an easy fix.

Nope.  Compressions were:  #1--70/80, #2--40/80, #3--low 60's, #4--low 60's.  #1 was passable, but the rest were horrible, especially #2 where the air hissing out the exhaust pipe was clearly audible.

Complete and utter bummer.

But why?  How?  A relatively new engine.  Flown regularly.  Never abused.  Lots of x-country time.  My last two engines went well past TBO.  I hadn't changed anything in my technique of engine management.

Car gas?  But I had run both of the previous engines on car gas.

Costco gas?  Poor gas quality leading to valve failure?  Maybe a remote possibility.  But I've got to believe that the bad gas incident was a one off.  Maybe a couple of jerry cans worth.  This was a five years in the making problem.

Too aggressive leaning?  This is the red herring that often gets blamed for burnt valves.  But I've scrupulously followed the POH guidelines--lean slowly to the first sign of roughness, then richen back to smooth engine operation.

Now having said that, and thinking back to the previous engines, I used to run them richer.  My mechanic at the time, Frank DeJong, warned me about leaning too much so I'd only lean to peak rpm and then add back in a couple of clicks for good measure.  Clearly running rich of peak (ROP).

But today's thinking says that running lean of peak or LOP is good for the engine.  As long as the cylinder head temps don't get too high, it actually produces cleaner combustion and stops the valve guides from getting fouled with the byproducts of incomplete combustion.  Or so the thinking goes.

Could I have inadvertently over temped the engine and smoked the valves?  I don't have a cylinder head temperature (CHT) gauge.  No C172 does.  They aren't considered necessary.  But could my little bit of extra leaning have pooched the engine??!!!!!!

Again, conventional wisdom says that the Lycoming 0-320 is such a small, de-tuned, low compression engine that it's virtually impossible to hurt it by over leaning.

At the end of the day, who the *(^%$%  knows for sure?

Jim said that it was weird that all 4 exhaust valves were leaking.  Usually, it's just one cylinder that goes bad.  Could it have been a bad batch of valve guides from the factory?  Improper machining of the valve seats?  Bad exhaust valves from Lycoming's supplier?

Clearly at some point, I will need to have a conversation with Jason, the president of AeroTech, the firm that overhauled the engine 5 years ago.  He's a nice guy and there maybe something he can do from a warranty standpoint, but I will need to have all my facts straight first.

Next steps?  A borescope exam of each cylinder to see what's really going on in there.  Hopefully Chris my mechanic will do that next week.

Stay tuned. 

And if you're feeling charitable, please go ahead and start a "Go Fund Me" campaign for my poor, ailing exhaust valves.  They're gonna need it.