Exhaust Valve(s) Blow

[Ragsf15e]

3,500 hours of flying and it finally happened.  First "real" engine issue in 25 years of flying.  Happened over big mountains just outside McCall, Idaho with the whole family on board too.  Worst part was, I didn't even realize it happened until the next day!

We went to KMYL (McCall) for a $100 hamburger lunch on a beautiful winter day.  1968, M20F with IO-360A1A.  800 hours SMOH (1998), top overhaul with rebuilt cylinders after prop strike (prev owner) in 2004.  

Flight down there was normal.  Departed about 2pm so the kids (1 year old twins) were right at nap time for the way home.  Chosen route was up the Little Salmon river valley over highway 95.  There are dirt strips about 40 miles apart in the canyon, but there's basically nowhere to land northwest of KMYL for the first ~30 minutes of flight.

Leveled off at 8,500' below a scattered deck and leaned for 20 LOP.  Maybe a minute or two later I noticed my speed decreasing and autopilot telling me to trim up.  I had lost ~10 knots from normal cruise.  First thought was that I set the mixture too lean and was at too low a power setting.  Rich mixture, surge forward pretty good, reset to 20 LOP and everything seemed fine.  Then I decided to climb above the clouds, adjusted mixture, climbed, reset mixture and it happened again.  Huh, interesting.  By now we were 30 minutes from home and I didn't really think anything was wrong.  Flew along ROP for a while, checked TAS and it was normal, JPI 930 showed normal engine indications.  Reset LOP and noticed a very slight vibration.  Actually thought about looking into balancing my prop.  Landed uneventfully.

Thought about the flight and didn't feel right... took off the cowl the next day and checked for leaks around the injectors.  Everything seemed normal.  Downloaded from EDM 930... didn't like that at all.  Pictures below.  Watch #3 EGT after leaning.  The last picture is right before the decent to landing.  They are at a different time scale, so they look a little different, but clearly #3 EGT was not behaving.  It was going above it's leaning "peak" and then by the end of the flight, it was pretty unsteady.  It had only climbed about 100 degrees from peak and was not noticeable on the EDM unless I had written down the peak values which I don't think anyone does.  The fluctuating EGT wasn't noticeable without staring at the EDM which I didn't do.

Sent the data to my mechanic and put it in the shop the next day.  #3 exhaust valve was shot.  The valve guide is worn excessively and the valve and guide have deposits causing it to stick and wobble.  While he had that side of the engine apart, looked at #1 and it was not as bad but going down that road.  Pulling 2 cylinders to send to an engine shop for new valves and redo the guides, etc.  $$ Ouch.  Oil samples done at each change and all normal (last one in December).

Thankfully caught early enough that it didn't stick and fail catastrophically over the mountains.

Any idea what causes this?  Am I doing something to the engine to cause it?  

I guess I'd just feel better if I could see this coming a little before it starts affecting power.  Don't want to have any power loss in the mountains, on takeoff, high DA, or all three!

Appreciate your thoughts...

 

[1964-M20E]

good catch knowing what you plane should do and noticing that it isn't tells me you listened to the plane.  Just like in chitty chitty bang bang.

Glad to hear no major issues like complete engine stoppage.

 

 

 

 

[kortopates]

Glad you came out ok and you did the right thing by going ROP to lessen the power imbalance between the opposing cylinders.

If you'd like share data in a meaningful way I suggest you upload it to SavvyAnalysis.com and share it. If you don't already have one, an account is free and you have multiple levels that you can share your data such as an individual flight like this.

Perhaps the best method you can use to see these issues as soon as possible is to set your monitor in Normalize mode right after set up for cruise power. That way you'll immediately notice any divergence in EGT before it becomes real obvious.

If you had a sticky valve that would show a cold EGT at startup which actually sounds unlikely here.

Wobbly and sticky valves are one of the areas few pilots and maintainers are pro-active about. I suggest you read up on Lycoming SI 1425A and SB 388C.

 

[Ragsf15e]

I have been reading about normalize mode and will definitely try it next time.  Yeah, I got into both of those Lycoming documents in the last few days as I belatedly educated myself on exhaust valves...

[carusoam]

Rags,

Paul is our inside guy at Savvy.  Take him up on the advice if you can.  Send the data. 

Nice catch. Way to use the engine monitor to avoid deeper challenges.

Best regards,

-a-