Differential pressure test trouble shooting

I had to jump in here, not the bickering but for the information.

FYI, the info you experts share helps a lot of people that only read and do not post. Your information and knowledge is useful to a lot more people than you know, don't stop sharing your knowledge.

I have a recent bad leakdown test on #1 cylinder, 65/80. I do leakdown tests every 100 hrs which occurs at about 5 month intervals. This cylinder was 79/80 5 months ago. The oil tank burbles like during a "burp" on the other 3 cylinders but on the bad one it blows steady. I don't notice any air coming from the intake or exhaust during the leakdown test. A regular compression test has the good cylinders in the 180# range and the bad one at 150#. Adding oil to the top of the piston does not improve the compression test. My suspicion is a broken ring although the experts say this is unheard of. A bore scope inspection reveals nothing unusual, no scores on the cylinder wall or discoloration pattern on the valves. The crankcase pressure at 190f is within tolerance at 6.3psi.

The engine is 912ULS, 1600 hrs, runs fine and produces good power. The only thing I can notice is the right side EGT averages 70f hotter than the left where a year ago it only ran 35f hotter.

Next week I will inspect / lap the valves and look at the rings, then maybe learn what has happened.

EDIT, I didn't put this in the original post but it was part of my conclusion. To eliminate the posibility of the rings being lined up I did 3 separate runs on the ramp, and a 30 minute flight. I tested leakdown on all cylinders after each one with no change. I have since flown 10 flights acumulating 8 hours, testing afterwards again with no difference.

Walt, I wouldn't call 65/80 a "bad" test result.  Look in the 912ULS Maintenance Manual Line, section 12-20-00, page 8 (see attached image).  The limit is a 25% pressure drop.  For an 80 psi test, 65 psi is only a 19% drop (65 psi is the lower limit for an 87 psi test).

If it were my airplane, I wouldn't pull a cylinder off of a good-running engine -- especially with a clean borescope inspection -- without flying it for an hour and re-testing.  Even if I got the same result I would just increase test frequency and keep an eye on it.

One man's opinion; possibly worth what you paid for it! 😆

Hi Walt and Eric

On a Rotax for those who work on them a lot, 65 over 80 is an anomaly.  Let's remember that this engine will most likely pass the 25% even if the rings are all stuck.  This is because the piston to cylinder clearance is about .001 inch, yes it is that tight.  When you see more than a 10% difference from one cylinder to another most Rotax technicians will start looking for what is changing.  

You are on the right track, lap the valves and be sure that you have no stuck rings.  Remember this can also happen if all 3 rings line up on the ring end gap from rings creeping slowly around inside the bore.  While not common it does happen on occasion.  In that case you can ignore it for 25 to 50 hours and it may go away.  The other way is to remove and inspect for this condition.  A borescope will not be able to help you with this problem.  if you listen to the intake and exhaust when doing the leakdown it may tell you if one or both of the valves are slighty open when they should be both seated closed.  

Just my view.

Cheers

Eric, after the first “bad” reading I ran the engine to operating temp and 5100 RPM on the ramp, then retested. I did that 3 times with the same leak down results. I then flew it for 30 minutes with the same results. Then I put Seafoam in the oil 1.5oz /qt and the remaining 11 oz in the gas tank. I flew that for 8 hours and retested with the same numbers. This engine runs on unleaded so there is no reason to expect lead related issues. I have given it enough time to redeem itself. 

Hi Walt

Consider that this took 1500 plus hours to get this result.  i doubt that a 30 min run and Seafoam will solve this.  Do the listen test to find the excessive leakage, please don't put your faith in a chemical solution.  Personally Seafoam is perhaps OK in the fuel to help clean up a few surface deposits but I would not run it in your oil.  

Obviously others may have another opinion.

Cheers