An extraordinarily bizarre airspeed mystery that no A&P can solve

[bflynn]

Guess what? I wasn't crazy. Turns out the engine was experiencing a loss of power/torque, despite everything appearing completely normal. It was build up behind the exhaust valves and in the exhaust manifold that reduced exhaust flow and thereby effectively reduced the displacement of the engine. And it became a self-reinforcing cycle: The ECU still “thought” it was moving the expected volume of air, so fuel delivery stayed the same = effectively richer mixture, lower power, more deposit formation. So the engine was running rich ever since the problem started and the richness only continued to add more deposits. This rich condition was undetectable because this engine consumes such little fuel to begin with that we're only talking about a difference from maybe 4.2 GPH to 4.5 GPH, which could have been easily justified by temp/air/seasonal differences.

We did an exhaust leak-down test early on, but that only told us it was sealed properly, not that there was a partial blockage.

This was discovered after I finally decided to have a top overhaul done and the cylinder heads and valves were pulled apart. The cylinder heads and valves themselves appeared fine (which is why it always passed borescope examinations), but once the exhaust valves were removed from the cylinder heads, a significant amount of stuck sediment fell out. Now that a top overhaul was done and the cylinder heads and valves reconditioned, the engine can breath properly again.

To ensure this condition isn't repeated, we have added an Annual checklist item to remove one of the exhaust headers and borescope inside the exhaust manifold to check for and track build up.

It only took two years (and about $10,000 of troubleshooting), but mystery solved.

Posting this in hopes that this may help someone in the future.

So your mechanic gets the Starbucks card?

 

[bbchien]

"Ma'am, your sink's backed up!" what a painful journey.

 

[Racerx]

Can you let us know what initiated the sediment or debris in that location?

I've never heard of it.

I'd think you are not the only one (unless your engine was being operated differently than the thousands of similar engines out there - which I have no reason to suspect) - but Rotax has not SDR or other guidance on it??

So, a one-off, with no real explanation?

Spitballing...what if he had a partial exhaust blockage? Flame tube coming apart? Rodent or bird crawl up there that has since been incinerated.

 

[Capt. Geoffrey Thorpe]

once the exhaust valves were removed from the cylinder heads, a significant amount of stuck sediment fell out.

Carbon? Lead?
What fuel do you usually run?
I would assume that the Rotax heads run cooler than "typical" aircraft heads.
What kind of cylinder temperatures do you see? Mine tend to run on the cold side. One of these days, I am going to have to invest in a thermostat...

 

[Crashnburn]

In engineering, if you do something and something changes, you undo what you did and see if the problem goes away.

Here, just for grins, undo the rudder trim adjustment and see if that makes a difference.