GaRCField, unfortunately a bad plug can still pass most of your tests. A plug can still have a coil in it that will glow red and show a good circuit on a meter, so these tests are misleading a lot of times. You have to understand how a glow plug works to know why it can pass these test and still be bad. The coil wire in a glow plug is coated with platinum, and when the coil is heated the platinum has a catalytic reaction with the fuel which causes it to fire. As a interesting side note, this reaction can occur without a glow driver on, so it's possible for an engine to start without the glow driver on it. Now back to the issue. So basically the plug is bad when the platinum is gone from the wire, or the wire coil is broken so that it can no longer heat.

There is no set amount of time to change a glow plug. As DeadEye stated above, glow plugs can last a very very long time. I have on in a an engine that has over 300 flights on the same glow plug. But what can make one go bad?? There are few times when plugs are more prone to go bad. Two of which is in a new engine and when the engine is running lean. As a new engine is breaking in metal particles will mix with the fuel (this is perfectly normal) and these particles can cause the plug to go bad. So it's not unusual to go through more plugs when an engine when it's new and still breaking in. Once an engine is broken in plugs tend to last a lot longer. The other thing is that can lead to plug death is lean runs. When an engine is lean it runs hotter, and this heat leads to both the platinum coming off the coil wire as well as the coil wire burning through completely. Running an engine rich keeps the plug cooler which leads to longer life. But..... lean runs can also ruin an engine as well, so you want to keep them on the rich side to extend the life of the engine.

So when will you know to change one?? Basically when the engine dies. Many times a plug will give out in flight leading to a dead stick, and with the plane on the ground it won't restart. Replace the plug and you're good to go. Another good indication is an engine that won't start. Always check the plug when engines are difficult to start. As I stated above, even a plug that glows orange can still be bad. I keep a KNOWN good plug in my plug wrench that I can use to quick test for a bad plug. Put a good plug in and it see if that is your issue.

It's funny that you post this as I had a plug go bad today on my Kaos. I can't even remember when I put that plug in that engine it's been so long ago. But in flight it died and deadsticked. On the ground the engine would die as soon as I removed the glow starter. Bingo!!!! Bad plug. Replaced in and the plane was back in the air in less than a couple of minutes.

Hope this helps

Ken