Cylinder Compression test

by BERNARD BERGER in 912 / 914 Technical Questions 9 years ago

As part of my annual conditional inspection I run the engine to heat it up then started to perform compression tests. #3 & 4 were in specs, # 1 was a bit low but in spec. # 2 was low, 52/80. Suggestions made to put a small amount of marvel mystery oil into the spark plug opening an let it soak for a few day help loosen up the rings that may have aligned themselves. I flew the plane for about 6 hours but that procedure didn't provide the results I hoped for. Any other tests or procedures? I can to do? Any input accepted.

Re: Cylinder Compression test

by Roger Lee » 9 years ago

Roger Lee

126 5.13k 4.56k

Hi Bernard,

912ULS or 912UL?
Has any one worked on this engine and what is the engine hour total? What fuel and oil have you been using?

80/52 psi is is very significant and you should look for the problem. First place to look is to see if something may be stuck under a valve, do the cylinders leak around the sealed areas, then you may need to look at the rings. I would want to take a look at the top of the piston too. Are you positive the other plug in the cylinder is tight? Mystery Marvel Oil is not the right answer.
Usually when using 80 psi as a test pressure a result is usually 77-78. In these engines you usually only see a 1-4 psi drop so anything for me that is 8 psi or greater I want to know why.
If for some odd reason a ring where to be out of place then oil isn't going to fix it.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Cylinder Compression test

by BERNARD BERGER » 9 years ago

BERNARD BERGER

8 15 0

Roger,thanks for the fast reply. Engine is a 914 with approximately 558 hours. Will check the #2 lower plug. 90% fuel used has been premium fuel. Operating cross County I used 100LL with TCP. Always used road recomended oils changing them depending what was being used as fuel. In 2010 I the short block was replaced.
I had conversations with several other Rotax certified technicians indicated that the 914 rings can line up causing compression problems but running the engine usually helps to move rings in place. The last year my plane wasnt flown much which could be a contributing factor.
Bottom line. I'm at a loss. I am not about to pull the head or the cylinder. I may have to fly it down to you if we can't resolve it here.
Merry Christmas to you and your family.

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Re: Cylinder Compression test

by Przemysław Wiśniewski » 9 years ago

Przemysław Wiśniewski

65 247 3

I curious. There are any practises to avoid seizing of piston rings? Maybe from time to time we should add, make something to remove partial contamination from rings and avoid serious stuck/leakage.

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Re: Cylinder Compression test

by Roger Lee » 9 years ago

Roger Lee

126 5.13k 4.56k

Unless you are storing it for really long periods it shouldn't necessary and staying away from leaded fuel can help over the long haul.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Cylinder Compression test

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