Need Rotax 912S Help To Get The 7-Oh-Fun Back Home

I need some Rotax advise as my plane is stranded at another airport.

The coolant overflow bottle failed in flight. The first sign was some red fluid in the cockpit a few minutes after take off. At first I thought it was brake fluid.

I noted the EGTs were high, but the CHTs were in range, and the oil temps were slightly high, but not in "the red zone"

A minute later I started smelling something in the cockpit and turned back to the field at reduced power.

A few minutes later I lost one of the CHT probes, but the other was reading fine. A few minutes after that I lost the other CHT probe. Oil temps at this point were still just into the yellow zone.

On the ground I pulled the cowling and found the coolant bottle had melted a hole at the point closest to the oil reservoir. The shrink wrap that holds the CHTS in place had melted and the CHT connectors had come out of position.

I was able to get some more DexCool and field replaced the coolant bottle with some attempts to heat shield it.

Runup and pre-flight were normal. The engine made normal power on take off. After climbing above the airport I noticed the CHTs were still high.

The engine was also only making 5100RPM in level flight with full throttle. I normally can exceed 5500 with my prop pitch.

I landed back and found a hangar to tuck it back in at.

So now the question is, what should I be looking for? The engine ran smooth. Shut down was brisk as always. Based on this I do not think one of the pistons is damaged.

The current plan is to compression test it, check the plugs and pull the oil filter to look for metal.

Does anyone else have any ideas?



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Comment by John Marzulli on January 9, 2013 at 8:49am

The good news is the cylinder bodies checked out. No the heads are out for a hardness test to see if they lost their temper. The bolts for the heads were loose, so there was probably a good amount of compression loss from that.

After the hardness test then we check the valves, springs, ect.

Comment by Chris Aysen on January 9, 2013 at 7:44am

John - I presume by now you have checked out everything. Did you have piston ring or valve problems?

Comment by Jeff Francis on December 30, 2012 at 7:20pm

I've got a compression tester if you'd like to borrow one (I'm over in Seattle for work at least once a week, would be happy to drop it off).

Comment by Robert Stacey on December 30, 2012 at 5:42pm

John.. When was the last time you checked the coolant bottle. The purpose of the bottle is to catch the coolant which leaves the system when the heat and expansion drives the pressure above the radiator opening pressure. It then just returns the expanded coolant when the engine cools. The real question is how much coolant did you have to add into the top tank?. If it was less than a few ounces you probably never flew with insufficent coolant to do any damage. If you had to add a quart you may have been gradually loosing coolant and have to consider the possibility of overheating the heads. 

If the compression checks out OK I would have a look at the other mechanical items like a partially open choke or throttle not opening all the way. Check the fuel lines for kinks or signs of overheating.

If this occurred right after refueling I would have a close look at contaminated fuel. Maybe some Jet-A or diesel got into your tanks. Gas engines will run on a mix of diesel and gas. They loose power and get hot depending on the ratio of diesel to gasoline.

The Rotax 912 is one tough little engine. Let us know what you find out.

All the best.


Comment by John Marzulli on December 30, 2012 at 1:40pm
Thanks for the input Rick. The CHTs were in the normal range right before I lost the sensor, but they may not have been reading correct at that point. There was coolant in the system and I filled the reserve bottle "just in case".

My Rotax specialist will be making it out to the plane on Wednesday, and I was able to get a ride back home in a Long-Ez.
Comment by Rick George on December 30, 2012 at 1:06pm


When the potting that holds the CHT sensors melt - this is a sign of cylinder head overheating. Did you add dexcool to the engine as well as the overflow bottle when you replaced the bottle? How much did you have to add? Make sure there is no coolant in the oil.The concern is not being able to achieve full power after replacing everything. Cylinder compression check and oil filter check will be most helpful to determine any damage. It is not unusual for the head bolts to loosen when the cylinders over heat - I would check the torque on the cylinder head bolts. Would be good to place this on the Rotax owners forum - if you are not a member  - you should join it. Please keep us posted - Being stranded is no fun.

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