Fuel Tester Use and Buyer's Guide

What’s a fuel tester for?

If you’ve spent time around general aviation aircraft, you’re probably aware that a fuel tester is an important piece of equipment in any flight bag. Testing the fuel your aircraft is a critical part of the preflight process. A good fuel tester allows you to see any sediment, debris, or water that might be present in your fuel. If you see anything in your fuel that’s not supposed to be there, don’t fly until you’re certain the contaminant has been removed.

100LL avgas aviation fuel is similar to the gasoline in your car, but there are a few key differences. First, avgas contains tetraethyl lead (TEL) which prevents engine knocking. The “LL” in the “100LL” designation means that there is a lower amount of this lead. It also contains a blue colored dye to distinguish it from other liquids and different grades of gasoline.

How to test 100LL fuel

No matter which fuel tester you end up purchasing, the basic process of checking your fuel will be the same—though the equipment does vary. Some fuel testers are designed to recycle the fuel that you’re testing, making it easy to return to the tank. Others are useful for sampling at 8 or 9 different locations without having to empty the fuel tester. In almost every case, the process is simple and something that you should never skip out on doing. Clean fuel on the ground makes for a clean running engine up in the air.

So what do we look for when we’re testing avgas? First and foremost, the liquid that comes out of your fuel sump should be blue. All 100LL avgas is required to contain a blue dye. If you see any clear liquid, there is a good chance that your fuel is contaminated with water.

In addition to liquid contamination, be sure to check for any solid sediments. Any debris could potentially plug up your fuel lines or foul the engine.

Finally, make sure you’re sampling from each of your fuel sumps. Just because you didn’t observe any issues with fuel in the first two, doesn’t mean that the others are good. Water, in particular, is much denser than avgas and will always settle to the lowest point in your fuel tank.


The main benefit of the GATS Jar fuel tester is the ability to recycle the fuel. When you’re done collecting and inspecting your fuel sample, open up your gas cap and pour the fuel back into the tank. A screen on the GATS Jar ensures that particulates get filtered out—although if there’s lots of sediment in your fuel, you probably shouldn’t be putting it back in your tank, let alone flying. A reversible sump actuator gives the GATS Jar the versatility to work with either pin or petcock sump actuators.

MultiSump Fuel Tester

As the name suggests, the MultiSump Fuel Tester makes testing from many sump points easier. Fill the upper sampling cup at each sump point. After inspecting the fuel, it can be drained into the lower reservoir—just tip the jar in any direction. Once each sump has been inspected, you can empty the fuel back into your tank with the optional strainer. With the MultiSump, you can test upwards of 8 sumps before needing to empty the lower reservoir.

Classic Fuel Tester

This classic design Aircraft Fuel Tester keeps thing simple and affordable. This particular model includes a screwdriver tip as well as an adapter to access recessed drains. Becuase there isn’t an included filter, your fuel samples will need to be thrown out.

GoCheck Preflight Multi-Tool

Much more than just a fuel tester, the GoCheck Preflight Multi-Tool also includes a screwdriver, dipstick wiper, red and white flashlight, and stall horn tester. Like the GATS Jar, the built-in filter makes it easy to pour fuel samples back into the tank. This tool goes well beyond being a simple fuel tester, but if you’re looking to make the most of the limited space in your flight bag, it might be the way to go.