Drastic Compression Loss??

[mikerocosm]

I've had my new-to-me 1966 M20E since October, at which time compressions were in the mid 70s all around, and the past several annuals showed the same thing, with a pound or two of wiggle from year to year, but nothing remarkable. The airplane runs great; it's fast, smooth, and cool (low CHTs, normal EGTs shown on the EDM700 installed a few months ago), cylinder 1 running about 20 degrees warmer (CHT) than the rest.

 

So yesterday (during the annual inspection) my A&P/IA guy calls me to say that cylinders 1 & 3 show just over 20 pounds on the compression test (done cold: he figures if it shows good cold, it'll show good warm). Discussion ensues. This morning, he runs the engine to get CHTs over 300, then tests again. This time, cylinder 3 goes to 64 pounds (and stays there even after cooling), but cylinder 1 remains at 22 pounds. I can hear hissing from the releasing presssure in cylinder 1 from the oil-filler tube, which surely indicates leakage past the rings, no? No hissing in the exhaust pipe or intake; so no reason to suspect intake- or exhaust-valve leakage. I can hear just a whisper in the oil-filler tube when cylinder 3 is pressurized, but nothing like cylinder 1, and at 64 pounds, it can wait.

 

So my question is: What could cause such precipitous loss of ring condition that compressions would fall like this in just a few months?

 

I've read and listened to Mike Busch's columns and webinars, and John Deakins and whatever I can find on AvWeb, and I'm not panicked about the actual compressions, but about what has caused  the loss in compression. I'm a believer in getting a solid diagnosis before surgery, but it looks for all the world like sloppy rings, and I just can't figure out how they got that way so quickly. My guy doesn't have a borescope, but I'm not sure what it could show that would make any difference anyway.

 

Any ideas?

 

Mike

[FloridaMan]

http://www.avweb.com/news/savvyaviator/savvy_aviator_56_before_you_yank_that_jug_197497-1.html

[N601RX]

One of the rings may have broken.

[Fly By Night]

Did your mechanic simply run the engine up on the ground or did he fly the aircraft to normalize all engine parameters? When I plan to check compression on my Long-EZ or my Mooney, I do it immediately after flight.

[sleepingsquirrel]

My 0-360 has two compression rings. If the two gaps are lined up compression will temprorarily  be low . Run the thing and the rings will probably rotate and give better compressions next check.  My mechanic said put a 1quart of MMO in the oil and run it. I don't think that is necessary . I'm running mine and have plenty of power and oil is clean/not blowing out.

[KSMooniac]

sounds like rings to me, but likely not a cause to overhaul the engine.  Depending on the history, time SMOH and your desires you could do everything from re-ring to a top overhaul or something in between.  I had a similar situation at 1650 SMOH and opted to IRAN all four cylinders and am happy with that decision.  Modest cost, a lot of my labor, and great results in almost 400 hours since then.

[Mooneymite]

Two additional causes I can think of:

.Running the engine (CHT's) too hot, glazed the cylinders and the rings no longer seal properly. Remove the cylinders, have them honed, reinstall, and sin no more. Your engine guy will tell you if the rings will need to be replaced as well.

.Rust has formed on the cylinder walls and ruined the ring seal. You see this phenomenon (dramatic loss of compression) on engines stored (not run for extended period of time) without proper preservation measures.

Since both low cylinders are the same side I'd suspect a baffling issue caused an over-heat.

-

[N201MKTurbo]

It really won't hurt anything to run it that way, except for contaminating the oil a bit faster.

 

Run it for a few more hours and if it doesn't get better, you will need to pull the cylinders..

[BigTex]

You can always scope the cylinder... That might tell you something.

[Riq]

I ran a low cylinder in hopes of trying the same thing in my cherokee.

 

Long story short the prop seal blew and black oil coated the plane, loss of oil pressure ensued and forced what seemed like an imc emergency landing.(the windshield was useless, I had to use the storm window and slip to see anything.)

 

Im no a/p, but I will never treat a low cylinder as airworthy again.

[Bill_Carter]

Just wondering, if it is a broken ring, couldn't that cause a lot of damage if you kept running it?

I was thinking too that when the ring gaps lined up it might cause the low compression. I think I would be scared to run it until I knew the problem for sure. But then again, I am a rookie. I do know automobile engines though.

[1964-M20E]

I ran a low cylinder in hopes of trying the same thing in my cherokee.

 

Long story short the prop seal blew and black oil coated the plane, loss of oil pressure ensued and forced what seemed like an imc emergency landing.(the windshield was useless, I had to use the storm window and slip to see anything.)

 

Im no a/p, but I will never treat a low cylinder as airworthy again.

I agree with finding the source of the low compression but how is the prop seal related to low compression?

[Riq]

According to the a/p, blow-by loaded up the crankcase pushing out the 150hr old seal. I would think the breather would relieve enough pressure, but thats what the mechanic told me.

[KSMooniac]

Yes, I've heard that excessive crankcase pressure will potential blow a crank seal since the breather hole isn't that large and cannot vent a "lot" of pressure.  The breather could also be blocked by ice or an insect nest or other debris as well.

[N201MKTurbo]

A valid concern. You could have someone run the engine up while feeling the pressure at the dip stick.

[thinwing]

Some really great advice...just about covered every trick I know...Not sure what to make of the suggestion for a cylinder flush///thats a new one...how do you suppose that is done??

[jetdriven]

a cylinder flush is done by putting a piston at the top of the compression stroke add 100ll in sparkplug hole, put plug back in and work it past the rings. Drain plug is pulled so fuel goes straight through the engine and out drain.

 

i know of several owners who picked up 5+ lbs of pressure on their compression test by using this procedure.

 

ymmv, i've never tried this.

some Beech guys have done this with MMO on the compression stroke. Set the piston just short of TDC on the compression stroke (valves are closed), fill cylinder with MMO, then put in top plug, and then pull the engine a little further until it is hydrolocked with MMO.  Let it sit overnight or longer, then pull the bottom plug, drain, and go fly the heck out of it for 30 mins, then test it.  One guy's compression improved from this method. It flushed the carbon and varnish from the rings.

[bumper]

some Beech guys have done this with MMO on the compression stroke. Set the piston just short of TDC on the compression stroke (valves are closed), fill cylinder with MMO, then put in top plug, and then pull the engine a little further until it is hydrolocked with MMO.  Let it sit overnight or longer, then pull the bottom plug, drain, and go fly the heck out of it for 30 mins, then test it.  One guy's compression improved from this method. It flushed the carbon and varnish from the rings.

I used to pull the prop through each of the compression strokes before flying. One day I canceled a trip when I had zero compression on #2. Hooked up a differential compression tester, and could hear the air leaking at the oil filler.

 

Did as Jetdriven writes only used paint thinner (it's what I had handy). After doing the hydro locked thing, I hooked up compressed air directly to the cylinder using about 10 or 20 psi or so. Allowing piston to go to bottom of power stroke, Then, when you pull backward on the prop, it'll make like a choo-choo train (grin) with piston first loosing all pressure then moving back towards you, building pressure and moving back away from you, repeat. If you put a bungee cord around the blade and pull back with that, it's "automatic". The back and forth action of he rings in the grooves, along with the solvent, may help break up gunk in the rings. Don't know if the compressed air back and forth added any fix-it value . . . I was bored waiting and having fun playing (sort of). In any case I got full compression back and flew for several hundred more hours before replacing engine (it was burning a quart of oil every two hours at that point).

 

bumper