Coolant Leak?

You might have slight coolant weep at the water pump.  Rotax 912 water pump has wetted ceramic seals and not rubber lip seals.  Slight amount of liquid can be seen at weep hole in bottom of water pump housing.  Not uncommon... 

Over the past 20 years I've had several coolant leaks.

1. One was the pipe below the overflow bottle (not a genuine Rotax bottle). The fitting just needed a new rubber washer.

2. Another one was from one of the 17mm hoses under the cylinder head. I cannot remember whether it had one of the original sprung hose clips or an adjustable clamp.

3. The most difficult one was a tiny pin hole in the radiator that only leaked when the coolant was hot and pressurised. Unfortunately this was generally in flight and so it evaporated due to the heat and the airflow. I took the radiator to a specialist and he pressurised it and then welded it for me.

Small leaks can be difficult to find as they tend to evaporate before you have a chance to find them. You could pressurise the system when it is cold and then run your hand around the joints and radiator to see if you can find it.

I second James Stricker's comment - if the leak is very slow (judgement call - I suggest only requiring toping up at 50 hrs interval) its nothing to worry about.

If using Rotax recommended 50:50 glycol based coolant, you may find that a persistent leak will, over time, leave a chalky, white - light green, residue.

Here are two common spots for a leak that makes them hard to find. The air flowing through the cowl during flight blows the coolant away.

The first one is under the spring clamps on top and on both sides under the cylinders. Make sure the spring clamps are up by, but not over the flared end of the 17mm coolant hoses. If the clamp is way back by the end of the hose and not up by the flared end of the aluminum pipe underneath the hoses can seep and the wind blows it away and makes it harder to find. You can feel the flared end on the aluminum pipe under each hose. Slide each clamp up by it, but never over it. The clamp in this position applies more pressure to seal leaks because of the flared end and the pressure supplied by the clamp right next to it.

The next spot is usually the 17 MM 90 degree aluminum tubes on the top of each cylinder. The aluminum tubes are threaded into these small flat flanges bolted to the engine. They can leak when Loctite 243 is used. You can use and I do use it, Loctite 680 (green) on these threads and never have leaks. To unscrew the tube you need to heat it hot with a heat gun or small butane torch until the old Loctite is softened and allows you to unscrew it. Clean it with some Lacquer thinner, then liberally apply the 648 and then position the tube in the direction you want it and let it sit and cure. Before you remove the flange and tube use a magic marker to mark the tube and flange alignment so when you put it back together it is lines up in the direction you want it.

Thanks for the responses.

Are others seeing coolant collect on the top of the vented red cap for the Rotax plastic coolant bottle?

I am using the dexcool coolant and the water pump weep hole has been dry. 

Not being able to find where else the coolant is leaking from, I previously added dexcool specific dye in an attempt to identify where the leak(s) are.  I didn’t find any other leaks.  There was some fluorescent staining forward of the water pump weep hole. 

Based on the responses, my path forward is:

 - Continue to fly while troubleshooting. The risk to the engine is low due the slow rate of coolant loss and no indication that the coolant is leaking into the engine (i.e., the coolant pump weep hole is dry, the only other path into the engine is via the cylinder head casting and the engine oil level isn’t increasing).

 - Check the position of the 17 mm cylinder head coolant hose spring clamps and reposition if needed. 

 - Pressure check and check for leaks.  Is there a way to pressurize the coolant system via where the radiator cap attaches rather than the Rotax procedure of removing one of cylinder head coolant hoses?

Thanks.

Brett