Loss of coolant

I've had very small leaks in the past that have been difficult to detect. One was at the base of the coolant reservoir and the other was one of the cooling fins on the radiator. The latter was only found when I took it to a radiator specialist who had to pressurise it to find the leak. This leak was only visible on the ground when the engine was fully hot and then it was only seen when the engine was stopped otherwise the prop blows the drips away.

You may be able to pressurise the coolant circuit on the ground with cold coolant and look for a leak.

If you don't have an external leak then you might have a porous cylinder head, I'm not sure how to diagnose this other than by elmination.

If the reservoir keeps going down and the expansion tank might be low then you may have a leak. Most of the time the expansion tank is okay unless you let the reservoir go dry.Where I find leaks are on top of the engine where the 17mm coolant hoses are. Sometimes you get a small leak where the 90 degree hose fitting screws into the small flange that screws on. The air coming through the cowl blows the coolant off and makes it harder to detect. Look at the top four 90 degree flanges for ANY residue right where the pipe screws in. If there is residue then you need to remove that flange, unscrew the fitting (you'll need a heat gun to loosen any other Loctite) and re-apply some Loctite. I prefer Loctite 648. It is a green retaining compound and is stronger and seems to seal leaks better than Loctite 243. If you do this and before you unscrew the fitting use a Magic Marker to mark the direction angle between the flange and the fitting so when you screw it back on the fitting is facing where you need it. Replace the part and let it full dry. 

Sometimes during hose changes and you use a little too much force on this fitting it can cause an ooze by loosening the factory Loctite. 

Other leaks can come from a constant compression hose clamp on any of the 17mm hoses. If the clamp is too far back by the end of the hose you can sometimes get a leak. I always slide the spring clamp up to, but not on top of the flared end on the metal tubing. With the clamp up by the flare it applies a little more sealing pressure right at that point and seems to stop leaks.

One last place to look is under the engine at the weep hole on the bottom of the coolant pump.Is there any coolant on the floor?

Hi Alejandro,

"I am observing a small loss of coolant in the expansion vessel,"

"The overflow vessel maintains its level"

Check that your "radiator cap" meets the current Rotax pressure standard,  is clean, the rubbers seals/valve are undamaged. Replace if in doubt.

Ensure your cooling system is full to the top of the inner lip, in the, so called ,expansion tank.

Rhythmically squeeze coolant hoses to try and expel any trapped air. Top up until all air expelled.

Your reservoir/overflow bottle should be filled (with coolant) to about 1/2 capacity when cold (the level rises when hot). Mark cold level on reservoir.

Go for a fly. Pay close attention to coolant temperature - if unusual rise/fall occurs, land immediately .

Put aircraft away until engine cooled to ambient (preferably overnight).

Inspect system;

Coolant in reservoir may have dropped  a lot - if there had been air in the system this is normal - just top up. NOTE: If all coolant drained out of reservoir, air may have entered cooling system - repeat above.

Expansion tank, coolant level - There may be some residual pressure, SLOWLY open radiator cap. Coolant level should be at or slightly above inner lip - no air gap between bottom of inner cap and top of coolant.

Reservoir level should remain almost constant - a small "top up" between services is acceptable. Frequent top ups, indicates a leak which should be addressed.

I would speculate;

Symptoms: Reservoir level constant - Level drops in expansion tank.

Return of coolant, when engine cools, not working - possible defective cap.

Pump seal or hose connection,  fails under negative pressure, allowing air to be drawn in to system, in preference to coolant from reservoir.

There is no coolant on the ground.
With all your contributions I have work to carry out verifications.

I want to thank everyone for your help.

It is an honor to belong to this forum.

Regards

Alejandro Zapata

Spain