A glow plug has a coating of platinum that acts as a catalyst to the reaction between the heat (glow) and the fuel element. Often, the platinum coating wears away, or gets burned away caused by a hot mix, usually to lean a setting on the high-speed needle. And they can often just plain open up due to many causes... vibration, excessive heat, rough handling, just about anything. But, normally, if you keep the mixture set right, the plug will last a long time. However, they will go slowly, as yours probably did, and begin to show up as yours did.

That's why they are removable and easy to replace. Good ones are not relatively cheap, but neither is a gallon of gasoline now. I keep several OS3, OS8, and OSF plugs in my box, but they don't often get changed out. I think I changed one glow plug so far this year, and that's out of several glow powered planes. That's not a bad average at all.

Now, with that said, I seem to recall reading here on RCU about someone that changed the glow plug after each flight.. but he was flying pylon racing aircraft, with engines that generate very high RPM's.

CGr.