[Vwdiesel] Compression testing for beginners.

[H . Hagar.]

CP63-P7           Look up on net. 



    If you have an abnormally low cylinder, you should start the engine and run it on the ground or even fly around the pattern once. Test it again. If it is still low, use a length of garden hose as a "stethoscope" and listen at the exhaust of the ailing cylinder. If you hear a hissing escape of compressed air here, you have an exhaust valve that is not seating. Similarly, listen carefully with the "stethoscope" at the carb or intake airbox. A hissing sound here would indicate leakage under the intake valve. If neither of these areas is leaking significantly, listen at the breather or oil dipstick/filler tube. A leak in this area is indicative of ring blow-by. This could be ring wear, barrel wear or scoring, or all the ring gaps may be lined up. Hissing between cylinder cooling fins is bad news, possibly a cracked cylinder. Valve leakage is the most commonly found cause of a low cylinder.
     The differential compression test has its limitations but it still remains one of the best, most cost effective preventive maintenance procedures available to the builder/flyer. The method described here is simple and it works. Done every 100 hours regularly, you could save big bucks in the long run.
     If you would like to learn more about this procedure and many other cost saving tips for keeping your engine in good shape, you could not do better than to obtain a copy of "Top End" from the Light Plane Maintenance Library.
Write to: Light Plane Maintenance
1111 East Putnam Ave.
Riverside, CT 06878

Hagar.    







  A single compression test does not necessarily mean anything. A single oil analysis also means very little. No single diagnostic test should ever be used to decide the health of your engine. The key is to do these tests regularly and keep good records of what you see. Compare each test and make your decision based on several tests conducted over a reasonable period of time.
     If you have an abnormally low cylinder, you should start the engine and run it on the ground or even fly around the pattern once. Test it again. If it is still low, use a length of garden hose as a "stethoscope" and listen at the exhaust of the ailing cylinder. If you hear a hissing escape of compressed air here, you have an exhaust valve that is not seating. Similarly, listen carefully with the "stethoscope" at the carb or intake airbox. A hissing sound here would indicate leakage under the intake valve. If neither of these areas is leaking significantly, listen at the breather or oil dipstick/filler tube. A leak in this area is indicative of ring blow-by. This could be ring wear, barrel wear or scoring, or all the ring gaps may be lined up. Hissing between cylinder cooling fins is bad news, possibly a cracked cylinder. Valve leakage is the most commonly found cause of a low cylinder.
     The differential compression test has its limitations but it still remains one of the best, most cost effective preventive maintenance procedures available to the builder/flyer. The method described here is simple and it works. Done every 100 hours regularly, you could save big bucks in the long run.
     If you would like to learn more about this procedure and many other cost saving tips for keeping your engine in good shape, you could not do better than to obtain a copy of "Top End" from the Light Plane Maintenance Library.
Write to: Light Plane Maintenance
1111 East Putnam Ave.
Riverside, CT 06878