ORIGINAL: causeitflies
Flames can also be caused by too much gas or liquid getting into the engine.
Usually big yellow (low temp) flames out the back won't hurt the engine because the flames are out in the air and not inside. However they can cause problems to the airframe if not confined to the pipe. Long blue flames mean high temps inside the engine.
Just wanted to add that damage to the airframe is not the only thing we need to be concerned about. We also need to consider damage to the pipe itself.
A pipe can sometimes do a great job of preventing a fire damaging the airframe, but become so weakened that it is just a time-bomb waiting to go off ; at some random time later, the weakened pipe collapses and the results are not pretty.
Gordon