Rotax 912UL oil overflowing? Why?

[tigger_na]

Dear Forumites

After the last oil change of my Hawk Arrow 2 the oil tank was overfull, at the very top of the flat part on the dipstick. I left it at that. Today, after landing from an uneventful 2hr flight through light rain I noticed the smell of oil. I found that the engine lost at least 50ml through the oil tank vent. It was in a puddle underneath the plane and all over the tail. The tank was brimful. I obviously didn't add any oil. Where does the extra volume come from? To matters a bit more of a challenge, I'm three hours from my home port.

[justin.schoeman]

Not sure about Rotax engines, but oil level increase is usually one of two things - both very bad:

1) petrol dilution - popped seal in fuel pump allowing fuel into the sump.
2) water dilution - popped head gasket.

Water dilution leaves the oil milky, so is easy to detect. Fuel dilution you can pick up by smell...

[tigger_na]

Good info, thank you very much. The fuel pressure was low, but I put it down to the fuel pressure sensing setup that is always erratic, but not that low. It felt like the engine revs were unstable, but always willing to return to full power.
Temperatures were a low normal.

The oil wasn't milky but I didn't pick up a smell of petrol. But I wasn't trying to either.

[KFA]

What you are describing is normal behavior for an over full Rotax oil tank. Clean out the oil that leaked. The level it is at now is where it wants to be as a max level.

[tigger_na]

I'm not questioning your expertise, I thought about the same, but where did the additional volume come from? The engine is standing still and the oil level is a the brim of the tank. And this "coincidence" of the low (0,1 bar) fuel pressure? Then again, the fuel pump is new, with less than 70hrs on it.

[justin.schoeman]

KFA has experience on Rotax engines - most of mine is on car engines - so he probably has a better idea what is wrong.

Have you always 'burped' the Rotax correctly before checking oil level? If not, then there could well be some extra oil in the sump, which could overflow when running. I am not sure if there are any other Rotax specific quirks that could increase the oil level - but there are probably some that I don't know of.

[tigger_na]

My installation doesn't burp easily. I later ASSumed that the oil return in the tank doesn't go deep enough into the tank to make it burp.

I forgot to mention, the oil change was 20hrs ago, any excess oil should have been in the tank long ago?
Water in synthetic oil (I use Fuchs, can't remember the type) seperates much easier, perhaps masking the milky colour of water in oil?
The new type petrol pump (not sure about the old type) has a drain pipe to drain petrol out of the pump in case of a ruptured diaphragm?

[tigger_na]

Diesel in the fuel. I traced back the petrol supply. It was drained from a diesel car that was wrongly filled with petrol.

[Turbo]

I have always had a different oil level reading with a cold and warm engine - specially with a dry sump setup. The Rotax engine is no Different. I always measure my oil level when the engine is warm, typically AFTER a flight. I try keep the level around 3/4 way up the flat part of the dip stick - and never had trouble.