914F3 coolant leak/sticking wastegate

This sounds almost surely like the failure of the expansion tank cap. It looks fine, but the hidden vacuum valve has failed. BMW offers the exact same cap for half the price: Look for "BMW 17112345074 radiator cap" on the Internet for the nearest dealer.

Yet another problem can be the O-rings sealing the elbows at the cylinders. The leak can be so subtle that it is hard to detect. Details are here:

https://pointsforpilots.blogspot.com/2017/07/vanishing-coolant-of-rotax-912-engine.html

Klaus

"I'm also dealing with a sticking wastegate."

Hi Tony - I have no experience of Rotax turbo- charged engines but have been around turbo diesels most of my working life.

With this statement in mind I offer the following:

The wastegates I am familiar with are not lubricated.

"Sticking" is usually caused by misalignment due to mechanical damage and or wear

The wastegate is subject to very high temperatures - most lubricants will  flash off very quickly - one exception that comes to mind, is graphite. The problem with graphite is getting it to the place it will be effective and then keeping it there. Rather than purchase a graphite lubricant, I usually make a "paste" of graphite and light oil. I then force the paste into the hinges with a spatula (flat screw driver will do). There is fair amount of wastage as much of the paste will not get into the hinge. The oil will flash off leaving the graphite, which will persist for a while but also eventually dissipate.

The only durable solution is to find out why your wastegate is sticking & fix it.

The 914 wastegate axel has a specific lubrication hole.  The specified lubricant is Loctite 8151, which is actually an anti-seize compound containing a suspended powdered aluminum film that is very good in high heat situations.  It’s worth getting the correct lube, spray it into the hole using the supplied straw.  

Hi guys...thanks again for all the helpful information.  KLAUS - you've been through a lot with that cooling system; that link was very educational, much appreciated.  Yesterday we inspected the airplane and found that for sure, the leak was coming from the overflow tank area...the airplane was dripping coolant even while cold and sitting in the hangar. We removed the overflow bottle and the inlet tube on the bottom, and tested that...no leaks.  We replaced the tube anyway (the tube between the overflow bottle and expansion tank) and rerouted it....and also replaced the radiator cap with the proper 1.2 bar cap (the old one was .9 bar). We then filled up the tank, added coolant to the overflow bottle, and ran it on the ground for 20 minutes.  No issues, no leaks.  We will flight test in the next day...and get her up to altitude.  Also, all the other connections, the water pump, etc showed no signs of leaking coolant.  

Regarding the wastegate - my turbo was completely rebuilt by a shop recommended by Rotax, less than a year ago.  It has maybe 30 hours on it now.  It does not have the small lubricating hole on that flange adjacent to the shaft - apparently, earlier model turbos didn't have that hole, but they added it later on.  We can see now why they chose to make that change.  So I still have the problem of lubrication - but ultimately, we definitely need to understand why it binds a bit.  Maybe after the rebuild, and after several temperature cycles running the engine for a while, there was some expansion/contraction that is causing this.  Also, Loctite 8151 looks like a good option because we can "spray" it.  I see that "LB 8151" is readily available; I'm not sure how that's different than straight 8151.  But for sure, we need to find out what's causing this - to have to remove the cowling and lubricate the wastegate before every day of flying seems excessive.

Thanks again for all the great comments guys....super helpful.

"It does not have the small lubricating hole on that flange adjacent to the shaft - apparently, earlier model turbos didn't have that hole, but they added it later on. "

Hi Tony - you could easily add the hole yourself.

I have often drilled a hole in a hinge that would benefit from lubrication. I use a 2mm drill bit, finish off with a brief countersink to make a small conical top to the hole. Using a chainsaw grease gun (they have a pointed application end) pump LB 5151 or any other high solids content, high temperature lubricant, into the hole until lubricant appears around the joints.