#1 Cylinder Problem

dtw_rv6 · #1 03-08-2009, 03:43 PM

Yesterday I removed Cylinder #1 from my fuel injected, fixed pitch Superior IO-360.

On return flight from a 35 minute trip, I climbed to 5500 feet, leveled out and dialed in 23" of manifold pressure with the throttle. As I started to lean the engine (LOP), I watched as cylinder #2, 4, and 3 leaned out as normal. I continued reducing fuel flow until the engine began to falter quite noticeably, yet #1 cylinder never reached a peak value.

I returned the engine to full rich, waited about 2 minutes, and tried again with the same results.

Back in the hangar, I pulled the spark plugs and noted that #1 plugs were visibly leaner than the other 3 cylinders. A compression test yielded 64/80 -- not a failing grade, but certainly not to be expected from an engine with less than 200 hours total time. Cylinder #3 checked at 75/80, and I did not check 2 or 4.

One rather convincing (and disturbing) diagnosis during discussions at the shop was a broken piston ring. Sooo....

This morning I removed the #1 jug and snapped the photos you can view at
http://picasaweb.google.com/dtwrv6/D...CJLvvJKw7efOJw

No broken piston ring, but certainly an odd wear pattern on the top piston skirt only. This is certainly why the cylinder checked so low on compression.

Now the question is why?

I've run Exxon Elite since the initial break in. No other additives, and always changed on a 4 month interval with a new filter. I initially ran 100LL, but have slowly migrated to a period last summer where I ran 93 Octane mogas. I would estimate that I've put about 150 gallons of mogas into the engine. Remaining tanks were 100LL and 50/50 LL/Mogas.

I'm not sure if there is much more data I can provide, but I would certainly be appreciative of any ideas on what could be going on in this engine. The injector for this cylinder appears to be squeaky clean.

Regards,
Don Winters

Lycosaurus · #2 03-08-2009, 04:27 PM

Hi Don,

First, while I have done some work on small engines, I am a newbie to all this (so take my comments for what it's worth), I'm in learning mode as well when it comes to aircraft engines.

Your exhaust valve looks fine to me, however photo #4 shows a roughened edge at the top of the cylinder (you should take a photo of the top of the cylinder). My guess is that detonation may have caused that.

Cause? Bad batch of mogas that had lower octane rating than the 93 you thought you were buying? LOP at higher than 75%?

When you say that #1 did not peak, was the EGT still rising as you leaned the engine, or had it peaked much earlier and you did not notice? That cylinder may have been running lean all along (though you did say that the injector was squeaky clean).

I'm sure more knowledgeable people like Mahlon will chime in tomorrow and provide more pertinent questions/answers for you.

dtw_rv6 · #3 03-08-2009, 04:51 PM

Lyco: The EGT kept rising, and never started to fall. I had to quit leaning the engine because it was going to stop running.

I've updated the gallery with a photo of the cylinder top.
http://picasaweb.google.com/lh/photo...eat=directlink
Thanks for your help!

Radomir · #4 03-08-2009, 04:55 PM

I'd start here:

http://www.sacskyranch.com/eng220.htm

CESSNADON · #5 03-08-2009, 05:26 PM

Are you sure that the EGT probe is OK??

aerhed · #6 03-08-2009, 08:39 PM

That jug is scored. It has run overtemp to the point where the piston has expanded into the cyl. wall. I only know one way for it to happen unless your piston was over dimension. The valves maybe look to have a leak pattern too. Did you have any valve leakeage @ compression check?

dtw_rv6 · #7 03-09-2009, 05:57 AM

We could hear and feel air coming from the crankcase breather, but there was no sign of leakage from either valve

Piston Jug P/N: SLC 36005 Rev. F
Oil Ring: SL73857A Rev. D
Compression Rings: SL74241A Rev. D

Piston (not sure which number is the P/N): 1275 or 130700+

I'm looking for an engine BOM so I can verify that these are the correct parts.

At this point I'm prepared to order new rings and a piston, but I'm wondering if I can re-hone the existing cylinder barrel without going oversize. The piston was worn about 0.002" on the scuffed wall. I haven't been able to check the cylinder yet.

Don

1Bad67 · #8 03-30-2009, 02:15 PM

Was the scuffing on the top, bottom, or both?

Eric

dtw_rv6 · #9 03-30-2009, 02:40 PM

The scuffing was only on the top piston skirt.

Since my original e-mail, I've had the cylinder professionally honed (0.002" over nominal), and installed a new piston and rings. There was almost no evidence of the scuffed surface on the cylinder after it was honed, and roundness was within limits when checked with a dial bore gage.

I have about 5 hours on the re-assembled engine (with mineral oil and 100LL). Compression seems equal on all 4 cylinders -- not done a calibrated test yet.

What nags me now is how this happened to begin with. I'm using a VM1000 which has EGT/CHT alarms on all 4 cylinders, and I've never seen any indication of high temperatures in the 160 hours of operation.

One thing is that this cylinders CHT has always run at least 25 degrees colder than the other 3. I have installed an air diverter on the front of #1 for break in, and right now I'm running about 20 degrees warmer than the other 3. After another 10 hours, I'll try to adjust the diverter to equalize all of the cylinder temps.

Don