How to Pinpoint a Diesel Tank Leak Without Dropping the Tank

A 2015 GL350 owner recently reached out for help with a frustrating diesel leak. The fuel tank cover is covered in drips, and the culprit remains elusive. The challenge? The tank is full, the weather is hot, and dropping a 100-kilogram fuel tank in a driveway is the last thing anyone wants to do, especially when it’s the family’s only vehicle for six.

After removing the protective covers and running the engine with the tank about three-quarters full, no leaks appear at a standstill. But during low-speed, stop-and-go driving, when the fuel sloshes around, diesel starts dripping down the passenger side, near the rear corner of the tank.

The orange collar area around the filler neck seems clean, and the supply and return lines aren’t leaking while parked, which rules out some common suspects.

The owner suspects the leak may be coming from a seal on the fuel sending unit, a crack in the top of the tank, or another opening up top. Without access ports inside the cabin, mechanics often have to drop the tank to inspect the upper connections. But before going that route, checking the sending unit gasket, the return line O-ring, and the filler neck seal from above with a mirror and flashlight can save hours of labor.

If the leak only happens with a full tank and movement, the issue is almost certainly at the top of the tank.