Differential pressure test trouble shooting

You don't really need to listen too hard. If the compression test is normal and everything else checks out with no abnormalities then you should be good. If a valve or anything in that particular area is leaking or bad then it should show up during the compression test. Don't use static test. I think that's a waste of time. Use the differential leak down test. This is why I ALWAYS do a compression test every annual or 100 hr. I want to see trends or find something wrong before it becomes a big issue.

Remove the oil tank cap.  If you hear the gurgling sound you normally get when burping -- or the sound of air hissing into the tank if it's already been burped -- this would indicate leakage past the piston rings.  If you keep the air connected long enough, you're probably guaranteed to hear something eventually.

There are a couple of good articles on AOPA's website, by Mike Busch and Jeff Simon, about compression testing.  Spoiler: Mike believes that it's nearly useless.  Jeff is more sanguine, but points out that borescope inspections are much more useful for determining cylinder health.  In other things he's written, and in his EAA webinars, Mike also evangelizes in favor of borescope usage.

Mike's article: https://aopa.org/news-and-media/all-news/2023/july/pilot/savvy-maintenance-unbelievable-compression

Jeff's article: https://aopa.org/news-and-media/all-news/2023/february/13/aircraft-maintenance-a-modern-approach-to-compression-testing

For borescopes, Mike recommends the Oasis Scientific VA-400: https://www.oasisscientific.com/products/vividia-ablescope-va-400-v5-usb-rigid-long-articulating-borescope-with-180-degree-one-way-articulation

Specific brand or model of borescope isn't important; just that you can articulate the business end of it to clearly photograph the valves.

If you're an EAA member, you can watch two of Mike's webinars about engine condition inspections, both of which contain good info about compression testing and borescope inspections.

"Ending the War on Jugs": https://www.eaa.org/videos/webinars/6350266355112

"Borescope Initiative": https://www.eaa.org/videos/webinars/6352195828112

Finally, there's an upcoming EAA Webinar about compression testing by Bill Ross from Superior Air Parts: https://pages.eaa.org/WBN2025-03-26AircraftEngineCompressionTesting_LP-Registration.html

Hi Eric,

If you do a differential pressure test and everything is lined up properly you will hear an oil tank gurgle. This is how the oil gets returned back to the tank. 

So the air pressure has to come from somewhere to return the oil.

Hi Eric

Most have noted that the air pressure will give you oil tank "burping", this is normal.  The other places to listen at are the exhaust outlet.  Leaking here is a sign of the exhaust valve not fully seating.  The intake also can show if an intake valve is not seated, this is however less likely to be an issue.  

To do this correctly always bring the engine up to operational temperatures before you do the test.  This allows the rings to be in the operational range for temperature and the seats to have cleared any light debris that can happen when an engine sits for even a relatively short period of time.  

As for results this varies with the engine you are testing.  The legacy 9 series engines have stainless seats and stainless valves, they are called out for no more than 25% differential leakage from one to the other.  While this is a max experienced technicians get rather worried on a Rotax if this difference is more than 10%, usually they will look deeper to find out what is going on.  In the case of the 915 and 916 this differential test jumps to a staggering 45%.... and it alarms many technicians.  This was the new installation of sodium filled exhaust valves and a change in the stainless seat to a bi-metal seat on the exhaust with wider sealing area.  (jumps from 2mm up to about 3mm)  The new seat is improved for heat transfer but this also accounts for more small debris to accumulate.  Again almost all of that is taken care of if you bring the engine to operational temperature before you do the compression differential test.  Debris is cleared out once the engine reaches temperature for the most part.  

It is worth noting that all new Rotax have not switched in production to the newer sodium exhaust and by-metal valve seats on the exhaust.  I expect that the newer edition of the manuals will catch up and change the values published to match the 915/916 engines so. 

Cheers

All:  The total production of all 9 series are currently all running sodium exhaust valves and the new head with the wider exhaust seat (new material)  This will be seen in the data as the manuals are updated.

Cheers