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Author Topic: Long road to diagnosis, discovery of multiple issues  (Read 388 times)

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GusMcRae

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Long road to diagnosis, discovery of multiple issues
« on: August 29, 2016, 12:11:53 pm »

I've been trying to get to the source of some slight engine roughness at mid-range RPM since mid-April.  When I pulled the throttle back to descend for a landing, this roughness occurred between about 1200-1900, and has persisted since.  Engine has been smooth as you could imagine at take-off and cruise RPM, smooth at low RPM.  Cleaned plugs, did a few other minor things to see if we could clear it up, but nothing helped.  I have only flown the plane about 15 hours since this roughness began, and after 2 different mechanics assuring me that this mid-range roughness should not be doing any kind of engine damage.  I could not force myself to keep flying it without solving the mystery, despite their advice.  It was rough, it shouldn't be,,, why?
Hints of a bad cam lobe were being whispered, but I guess they were afraid I would open a vein if they spoke of it out loud.
Finally in the midst of trouble shooting, we decided to do a compression check after a short flight around the pattern to get the engine good and warm, and #1 cylinder compression was so low, and hissing coming from every possible outlet (exhaust pipe, oil fill tube, and the vent tube) the jug had to come off for further inspection.  All other cylinders had good compressions. 

Background: 1st annual (2012) after purchase, #1 cyl had to be removed due to compression too low. I was a naive new owner of an aircraft going through my first experience of an annual, didn't ask many questions, just said "ok".  Cylinder was sent off to Gibson for repair, but I don't know what all was done to it.  A little over a year later, and a few months after annual #2 since purchase, by the same mx that did my first annual, I discovered that only 4 of 12 exhaust nuts were holding on the exhaust stack that had been removed for cyl work done at 1st annual.

This discovery was made while we were working on a different problem on the opposite side of the engine.  I was beginning to get some roughness in the engine and fouling out plugs in the #4 cylinder.   I had begun to use a different mx, who involved 2 other mx's to get their opinions, #4 cylinder (middle cyl on left side) was bore-scoped and found to have a significant amount of oil on top of the piston.  Jug was pulled and discovered that the oil ring had been installed improperly (little half inch section of the little spring that lays in a groove on the underside of the oil ring had not been tucked in the groove).  Took it about 500 hours TSOH of wear before the oil started getting past all of the rings and thus fouling out the plugs.  Sent the jug off to Gibson to be repaired.  Twisted off an exhaust manifold stud on #6 cyl, so we had to pull it and sent it off to Gibson to extract and replace the broken stud. 

Replaced the 8 missing nuts (all 4 on #1, all 4 on #5). All four that were present were on cyl #3, which is the middle cyl on the right hand side). At annual #3 since purchase, and roughly 230 hours after #1 cyl work was done at annual #1, another bad compression on #1 cyl. I was more involved by this time and no longer using initial mx.  Assisted my new mx (same one used for #4 cylinder trouble earlier) in pulling the cylinder and sending it off to Gibson for repair.  Exhaust valve guide worn out due to excessive heat, and thus causing loss of compression. We assumed the damage was due to the exhaust nuts missing from the exhaust stack coming out of cyl #1 was not getting the engine heat away from the cylinder, and believed that the cooler position of #5 cyl (front cyl) was such that we avoided any damage on it. All other cylinders had good compression numbers.

Fast forward to now, 230 hours later, another bad compression on #1 cyl, compression good on all other cyls.
Mx situation around home base has changed, 1 moved, another is just not around here much, as well as me just not having time to do any of it myself, caused me to take the aircraft to another very seasoned mechanic a short distance from home (who is also the mx that inspected and diagnosed the #4 cylinder oil ring trouble), and just turn it over to him to pull the jug, send off for repair or replacement, and try to get to the bottom of why this same cylinder keeps going bad.
In talking to him prior to taking the plane over to him, this mx suspected improperly installed baffling, and confirmed it shortly after pulling the cowling off.  Said that improperly installed baffling has been his pet peeve for his entire 50 years of working on airplanes. Said that I'm lucky if #3 cyl doesn't have damage, but since the compression is still good on it, maybe I dodged that bullet.  After pulling the cylinder he checked out the cam lobe and said that it is not bad.  Big relief!

I'm assuming the baffling has been installed improperly since I've owned the plane, reason for the cylinder damage, and I'm assuming that the initial mx replaced the baffling exactly like it came off after pulling the #1 jug at first annual. Then when me and the next mx I used, which I was assisting, replaced the same jug at annual #3, we installed the baffling exactly like it came off, assuming that it was on there properly when we took it off.

The current mx got it all back together about a week ago, and was still getting roughness at mid-range RPM.  Upon looking for the source of the roughness, he discovered an "impulse stop" on one of the mags was worn such that he didn't think it was functioning properly.  He happened to have a new one in stock (which was a small miracle), replaced that, still had roughness at mid-range RPM, and the engine backfired pretty good during one of these run-ups. 

Pulled the cowling off again and finally discovered the hole where an injector would be installed (if it were a fuel injected engine) on #4 cylinder was completely missing the little plug that goes in the hole when it is a carbureted engine.  He had one of those in stock as well, replaced it, engine ran smooth.  This is the cylinder that was sent off a few years ago for repair after discovering the improperly installed oil ring.  I don't know if the screw just wasn't torqued down good initially, if they loosen over time and they all need to be re-torqued periodically, or what.  But after this I think it would be a damn good idea to do so at annual!
I'm including a diagram but not sure it will show up right here, at the top or bottom of this post.  Look for the "injector boss bushing".

Mx thinks the plug has been sitting in the hole, loose.  And at higher RPM, there is enough suction going on that it sucked the loose screw down in the hole tight enough to seal off the hole.  He said if I had been running the engine for very long without it in there, the cylinder would definitely have been damaged and would have eventually seized up.  That's pretty scary!  He thinks when it backfired, it may have finished blowing the loose screw completely out of the hole. 

Picked up my plane last Friday, short trip back to home base, smooth engine at all RPMs. 
Even after gaining new confidence in a very seasoned mx that could probably do half of what he did with his eyes closed, I found myself a little leery to just jump in it and go.  I feel that I need to pull the cowling off and look for missing exhaust nuts, check to make sure a spark plug is not sitting there just hand tight, check all these little plugs that might have worked themselves loose.  Did he torque the cylinder nuts back according to the book? 
I also find it disturbing as to what some mechanics will do (leave off 2/3 of the exhaust stack nuts), and what some will say is ok to go ahead and do (it's ok to fly it because just a little roughness at mid-range RPM won't cause the engine any further damage) , when it's not them or their families that are crawling in the cockpit. 
Really need to spring for an engine monitor so that I can pinpoint problems as they arise. 

Through it all, I've never experienced this engine acting up so bad that I was even slightly concerned about it making more than enough power to get me to my destination.  I'm pretty impressed with the O-470's ability to keep on producing ample power when things are not exactly right.  I guess that's why you hear 470's mentioned in discussion of bullet proof aircraft engines. 

 
« Last Edit: August 29, 2016, 12:24:35 pm by GusMcRae »
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gotyacovered

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Re: Long road to diagnosis, discovery of multiple issues
« Reply #1 on: August 30, 2016, 03:05:59 pm »

that might be the craziest engine story I've ever read.

bet looking back on that ole pucker factor when that sucka got rough...  ;D
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GusMcRae

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Re: Long road to diagnosis, discovery of multiple issues
« Reply #2 on: September 19, 2016, 11:52:25 am »

I need an engine monitor so bad.  My CHT gauge has quit working since all of this work was done.  It pegs as soon as I crank the engine, or just jumps all over the place.  Need to be monitoring every cylinder, not just 1.  Continental O-470 with ~1000 hours needs monitoring closely at this stage of the game.  Just not a good time for me to fork over the $ to do it.   
Have flown about 12 hours since getting out of the shop.  So far so good. 
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gotyacovered

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Re: Long road to diagnosis, discovery of multiple issues
« Reply #3 on: September 20, 2016, 09:13:28 am »

I need an engine monitor so bad.  My CHT gauge has quit working since all of this work was done.  It pegs as soon as I crank the engine, or just jumps all over the place.  Need to be monitoring every cylinder, not just 1.  Continental O-470 with ~1000 hours needs monitoring closely at this stage of the game.  Just not a good time for me to fork over the $ to do it.   
Have flown about 12 hours since getting out of the shop.  So far so good. 


gus - but i posted a WTB on red board and got a EI FP-5 (fuel flow) for $283 (inc shipping) and a EI UBG-16 (engine monitor) for $826.75 (inc shipping) and had them installed at annual for a cost of $400 and i can definitively say it is the least amount of dollars invested on an upgrade and by far the best money i have spent. may be worth a shot, i may have just been lucky.
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