From time to time someone who is having a hard time running a glow engine will go to an online forum or Facebook group to ask for help. Sooner or later someone will suggest that the trouble may be a result of bad fuel. While this may be true, it appears that the majority of RC pilots do not understand why fuel goes bad.

The most commonly quoted cause for bad fuel is moisture. The way the story goes, because methanol is very hygroscopic it absorbs water from the air. A little bit of water doesn’t hurt, but after a while your engine starts to run poorly, until eventually you can’t get it to start at all.

While it’s true that methanol is quite hygroscopic, this line of reasoning does not explain why this problem can sometimes occur even when the fuel jug remains unopened. I have seen some forum contributors explain this by claiming that methanol can absorb water right through the plastic jug. This sounds reasonable, but it’s not true. I have personally used fuel that was stored for 30 years in closed plastic jugs in a very humid environment.

There is an even bigger problem with the water theory. Quite a few years ago there was a guy who decided to conduct a scientific study to determine what effect water would have on glow engines. He added water directly to the fuel and the engines ran well until the fuel consisted of a few percent water. I think this lays the “your fuel has water in it” theory to rest. If you do a little bit of online searching you can find references to this study. It’s very interesting.

Even if water is not a cause for concern, sometimes fuel really does go bad. But what is the cause? The answer is sunlight. It is a well known fact that sunlight causes nitromethane to decay, which is why glow fuel is supposed to be stored in a the basement or a closet. One gallon plastic jugs of glow fuel can also be stored in a cardboard box or have a thick blanket draped over them. Sealed fuel stored in the dark will theoretically last forever, but sunlight will cause the nitromethane to decay, producing CH3 and NO2, which further decay into methyl nitrite, formaldehyde, carbon monoxide, nitric oxide, and water.

Now that we have that question cleared up you may be wondering how to tell if your fuel has gone bad. There are some very specific problems caused by bad fuel, which you should take note of to avoid a lot of hassle. The first symptom you will notice is difficulty tuning the needle valve. When your fuel has gone bad the acceptable needle valve range becomes very narrow, so just one or two clicks in either direction will cause the engine to die. When the problem is bad enough you will spend a lot of time starting, restarting, twiddling the needle, trying again, etc. It can be tempting to suspect that the glow plug has gone bad, but a bad glow plug usually creates a simple no-go situation. When things just don’t make sense, your fuel is probably bad.

The next major symptom to watch out for is overheating. Under normal circumstances you should be able to touch a running or recently stopped glow engine without getting burned. You can’t just lay your hand on it and leave it, but a quick touch should not burn your finger. But with bad fuel your engine will usually be smoking hot when running and shortly after it stops. It will also require a richer needle setting, which causes more oil residue in the exhaust, but somehow it will still run hot anyway.

So, that’s pretty much it. It’s one of the most frustrating problems because it’s usually not a 100% failure. When your fuel has gone bad you get really frustrated because you have to start the engine a hundred times, and when you finally get the plane in the air the engine quits.

Do yourself two favors. Don’t believe the guys who say your fuel has sucked up a bunch of water, and keep your good fuel in the dark.