How do you know when it's time to change a glow plug without waiting for it to first fail?

Glow plug ignitors with meters built in to them will show you the condition of the glow plug. For me, if my engine does not start on the first couple of flips I know it's the glow plug, since my O.S. 46AX is the best starting engine ever

I thought the meter was so you could see the condition of the built in battery, not the glow. Does it do both?

The coil on the plug has to be in good shape to complete the circuit from the glow ignitor (kind of like a light bulb). The meter will show you if the circuit is being completed or not.

A second test is removing the glow plug and sticking it in the ignitor. It should get red hot, and if you blow on it, you should not be able to cool it down.

The first sign of a glow plug not working properly is the engine taking longer to start.

Okay, thanks,

For the heck of it I checked the owner's manual for the 46AX and it had listed:

Filament turned white.
Filament coil distorted.
Engine cuts out while idling.
Start qualities deteriorate - just like you said.

Thanks.

http://manuals.hobbico.com/osm/46ax-manual.pdf

If you tune your engines properly (meaning slightly rich), your glow plug can last a heck of a long time. I have been flying for 6 years. I have averaged 30 gallons of glow fuel a year on a dozen two and (mostly) four strokes. In that time I have replaced two glow plugs.

Thanks, I was trying to get an idea of what the life of one of those things is and how many spares to carry. Sounds like one is plenty, which is what I was thinking anyway.

I've had one sort of go in a crash. It still worked if I used an electric starter, but I don't, so I tossed it. Other then that no need for replacements in hundreds of flights, literally. I like to have 2 in my box.

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

Ok. These were the test taught to me by my instructors (checking meter, and putting plug in ignitor and blowing on it). If the plug doesn't pass these tests I would not use it. I guess there could be other things wrong too.

GaRCField,
Unfortunately if that is what you were taught then they didn't go far enough. I never said to use a plug that didn't pass your tests, because if it doesn't glow or electricity won't flow through it then the plug is bad. But, if this is all you are relying on you could very well put a bad plug right back in to your engine. Because like I said above, a bad plug can still pass ALL or your tests.

Ken

Oh. So it sounds like you can be using a bad plug and not know it, then have a dead stick as a result, and there's no way to know?

Wouldn't this bad plug give you a harder time starting, or not?

As I said in my advice above. If you have doubts try a KNOWN good plug in the engine. This should actually be one of the first things that is tried when and engine is hard to start. I can't even begin to count the number of times I have seen guys sit there and try over and over to start a dead engine. They fiddle with the needle valves. They defuel and refuel. They will do any number of things. But they never change the plug. When they FINALLY get around to trying a new plane the engine will "magically" roar to life.

Now this applies to starting an engine. An engine dying in flight will usually not restart, so it should be a very quick (if you follow my advice about trying a new plug first) fix to put a new plug in. Or in my case today with a dead stick. Then engine would only run with the glow driver on. I swapped the plug and was back flying in a couple of minutes.

Ken

Thanks. That happened at my field yesterday: guys tuning an engine for quite a while, wouldn't start, etc. Last thing they did was change the plug, and it started right up.

I rarely/never have to retune my engine at the start of a day, so if my engine doesn't start right up I think the plug is the first thing to check, not the last.

As long as you don't fly lean, don't have too much compression, don't use high nitro, don't have a lot of vibration, don't have a bearing going out, and keep dirt from running through the motor,
Then a plug will last for about 15 or 20 gallons of fuel.