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How did I get water in my fuel tank?


Picture of droplets of water in a fuel tank.

"You are telling me that the problem I have with my machine is water in the fuel. I only put clean, fresh fuel in the tank, and the machine is stored in a dry shed. How does water get into my fuel?"


The simple answer is condensation. But that may not explain it to you.


I was thinking about this the other day when I was strapping something down in the MAD Mowers van, and I was being rained on. Yes, I was inside the van, and the rain was falling on my head!


No, we do not have a sunroof in the van... or should I say a rain roof? And no, there is not a hole rusted through the roof either.

Condensation inside our van.

The truth is that it was not raining; it just felt like rain. It was condensation on the van roof inside, eventually dropping down onto my head.


All winter long, we see condensation in our houses and cars etc. Normally, the condensation is on the inside of windows, but this does not feel like the same thing happening to your fuel tank. But when I saw it on the inside of the van's roof, it was a great picture of what happens inside your fuel tank.


Unless your fuel tank is full, you have air inside your tank. This air will have moisture in it. When the temperature drops, especially on winter nights, this water will precipitate out onto the wall of the tank. This may drip or run down the sides of the tank and will collect under the fuel. Your fuel is lighter than the water, so the water is under the fuel and can not evaporate away. During the daytime, the air outside your fuel tank will exchange moister with the dryer air inside your fuel tank, and that night a little more water will make its way to the bottom of your fuel tank.


How to prevent water from getting into your fuel tank?


By the way, water can also get into your fuel tank from your fuel bottle or from leaving your machine where it might get rained on. Unless your fuel cap is damaged, this should not be a problem under most circumstances.


To prevent water from condensation getting onto your fuel tank, you can either...

  1. Leave your tank very full of fuel so that there is almost no air inside your fuel tank to precipitate out moister inside your tank. Though with modern E10 fuel, even the ethanol in your fuel may absorb moister from the air outside.

  2. Lever your fuel tank empty, and any condensation that may form and make its way to the bottom of your fuel tank can evaporate when the temperature rises. You may want to put a clean cloth over the mouth of the tank with an elastic band to keep dirt out but let moisture evaporate. Be careful not to lose your fuel cap, or you will call us for a new one in the spring.

But most of the time, you can just hope for the best, and if you have trouble... bring your machine in, and we will fix it for you.

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