Sudden High EGT’s on Rotax Motor
What would cause the exhaust gas temperatures on an otherwise perfectly running 503 or 582 engine to suddenly climb sharply?
Exhaust gas temperature (EGT) is affected by the fuel air mixture ratio, that is the volume of air coming into the cylinder compared to the volume of gasoline. A very lean mixture, that of too much air combining with a small amount of gasoline, will cause the EGT to soar.
EGT is a critical engine temperature. If the EGT gets too hot serious engine damage will occur. An engine failure is imminent.
Typical EGT’s in Rotax 503 and 582 engines run between 950 and 1100 degrees F. 1150 degrees F is the yellow zone. 1200 is the red zone. Over 1200 degrees and damage will occur. The engine will seize and stop.
What can cause a lean mixture to suddenly occur?
An extra lean fuel situation can be caused by the pilot when pushing the aircraft into a high speed dive and at the same time reducing the throttle setting to idle or near idle so as not to exceed the airplanes maximum speed. This forces a lean fuel mixture into the engine.
How? The high speed dive unloads the propeller making it spin faster than it normally would at a given throttle setting. The fast running engine continues to suck in a large volume of air into its cylinders. The reduced throttle setting, however, means that less gasoline is being admitted into the engine. This combination of a large air volume with a reduced gasoline flow is a very lean mixture. High EGT’s result. The engine will seize.
One owner pilot encountered another situation wherein his 503 engine suddenly began producing continuous high EGT’s in all flight configurations.
After checking his fuel system and carefully examining his air filter and carburetors, he discovered an unusual source of the sudden increase in exhaust gas temperature.
Although his Rotax engine was not equipped with an oil injection system (he pre-mixed the 2-stroke oil into the gasoline by hand) he discovered that the engine had been fitted with oil injection fittings.
The oil injection fittings are located on the round carburetor bosses on the engine block just inboard of the rubber carburetor mounts. The barbed fittings had the oil injection lines installed but, in his case, they had been cut short, crimped closed and left in place.
Now, years later, the clamps that had closed off the unused oil injection fittings had fallen off. This opened up a new air passage way into the engine cylinder. The engine began sucking in extra air through the oil injection fitting. This produced a lean air fuel mixture that caused a high exhaust gas temperature situation.
This was a difficult problem to trouble shoot because the fittings and tubes are small and are located in a hard to see location. Kudos go to the owner pilot who discovered the cause of the problem.
This experience gives us a clue to what might be some other potential causes of a sudden increase in EGT. Anything that might suddenly cause an increase in the air volume to the cylinder, perhaps a loose or damaged air filter.
Another cause of high EGT’s is a large drop in outside air temperature. Cold air is much denser and it too affects the fuel air mixture. It will produce the seemingly odd result of hot EGT’s when the weather turns cold. For more information on this situation, see the article Seasonal Carburetor Needle Adjustment Procedure on this web site.