I took a boat down to the Florida Keys for 2 weeks this February. I had a couple of issues that needed addressing. This article is about one of them: the fuel sending unit.
I suspected something was up with either the fuel gage or the sending unit prior to leaving on the trip. When last using the boat I did my best to burn through the remaining gas in the tank. It always showed a quarter of a tank. When I filled it, it showed a quarter of a tank. I launched the boat in Marathon, Florida and motored it around to its dock at the house I was renting.
The next morning the fuel gauge showed empty. This should not have been possible. I removed the tank access panel and the fuel sending unit. There was at least 3/4 a tank of gas! The wiring from the sending unit didn’t look like it was in the best shape and since I couldn’t trace it through the harness, I simply ran 2 new tinned wires. I modified the connection to the sender so that I could quick disconnect it if I had to. For all of the connections I use ancor marine grade terminals with integrated heat shrink. I tested my new wiring with the old sender and the gauged showed the entire range when the sender was manually actuated. I also tested the sender float by putting it in water. It floated fine, so I dried it and put in back in the tank.
The fuel gauge now showed…. EMPTY!!! What the ??? I pulled the sender. It once again floated in water and activated the fuel gauge correctly when in water. In stead of reinstalling it, I bought another and installed it. With the new sender, the fuel gauge showed full, as it should have. But what was the issue?
I turns out that the sender float had a tiny hole in it that allowed the gasoline to fill it. Once full, it was no longer buoyant and would sink to the bottom of the tank. Why did it float in water? Gasoline’s density is less than that of water, so a float filled with gas will, well, float. If I had a spare can of gas I could have tested that immediately.