Stain in the overflow bottle

Hi Fabricio,

I stand to be corrected; 

Coolant blowing out of the cap is a sign of overheating OR to much coolant in the expansion /overflow bottle - not a defective cap (assuming cap correctly vented).

60C(140F)  is far to low for Cruise temperature. Ideally your oil/coolant should have a similar Cruise temperature.  I suggest low to mid 90C (203F)  for the Coolant  mid 90C to 100C for the oil.

A "ring" around the inner surface of the expansion/overflow bottle, is usually indicative of coolant contamination, most often oil (could be machining oil from factory). It could also be the coolant breaking down - usually when its been in service too long.

😈

Thank you Sean. I did take a closer look now and appears to be indeed contaminated. The coolant is pink, from the factory, but now I see some tiny brown spots, besides the ring I mentioned earlier. I don’t think there is a way for oil from the engine to get into the cooling system, is there? These brown stains were not there 50h ago (engine has 100h now). The oil tank is right below the coolant overflow bottle, and there were few times where I left the oil cap open for few minutes once the airplane was in the hangar cooling down before checking the oil level post flight. Wondering if oil vapors could have been sucked into the coolant reservoir. The engine has been running well, smooth, etc. The temperatures are normal for the Pipistrel. I will flush and replace the coolant, but wondering if there is anything else I could check or might be missing.

Congratulations on owning a Pipstrel (SW?). Terrific aircraft one of only two aircraft, in their class, that have an astonishingly wide flight envelope - 27 knot Stall combined with a 135 knot Cruise

"I don’t think there is a way for oil from the engine to get into the cooling system, is there?"

Since this is a new engine, I was thinking oil from the factory building process. 

I suppose you/your mechanic may have inadvertently contaminated the coolant. Was the container of 50:50 coolant completely clean?

"The temperatures are normal for the Pipistrel."

My friend, the Pipstrel doesn't care what temperature the coolant runs at😁  Rotax on the other hand have a valid opinion - From Rotax 912 Operators Manual

Applicable for engine S/N without Suffix -01.
Coolant temperature: (coolant exit temperature)
Max. 120 °C (248 °F)
Cylinder head temperature
Max. 135 °C (275 °F)

Note: Your instrument panel is likly to be indicating Cylinder Head temperature

I could not find a Rotax recommended minimum coolant temperature, however I feel strongly that the engine cooling liquids (oil/coolant), should not be too far apart, when in Cruise. It is best for the engine that there not be large temperature differentials. To me 60C is to cool. It should not be to hard to partially block the radiator air flow, using something like adhesive foil, air-conditioning tape. Do a small area at a time  - go fly, check Cruise temps and increase/reduce blocked area as required. Aim for low to mid 90C. If Climb - Out temperatures are approaching the above Max 135C, a compromise will have to be made😈

It is indeed a nice aircraft, I really like it :)

Mine is an Alpha Trainer. Not as fast as their siblings, but still very cool for its mission.

Here’s a picture of the coolant (pink from the factory, you can see the brown spots). It is possible it was contaminated at the production… or also when I had the leak at the cap (Pipistrel sent a new cap under warranty at that time, fixing the problem). At that time, the system was sucking air into the radiator because the cap was not sealing properly, so maybe that could have also caused some contamination. When I replenished coolant at the time, it was about 200ml or so, not much, and I did dilute 50/50 with distilled water, using a brand new cup. Wondering if this contamination could be some sort of micro-organisms, or even impurities from the air.

I will flush the system, replace the coolant, and then monitor.

Thanks for the help, Sean!