One thing leads to another.

Dear Half-Mast,

While inspecting M925A1 5-ton trucks, we've noticed what looks like oil in the vehicle's air filter element and air cleaner assembly.

What gives?

Mr. D.C.

Dear Mr. D.C.,

This question comes up from time to time.

No way is oil supposed to be in the 5-tonner's air filter, but there it is--something that looks like oil. So where did it come from?

Fouled air filters often start with an overfilled fuel tank.

How does that happen? Well, one thing leads to another.

The fuel tank, transmission gear ease and transfer gear case all vent into the air cleaner duet. Fuel from an ovelfilled tank can run through the vents and end up in the gear cases. Then, the fuel-oil mess from the overfilled gear cases is forced out through the vent lines ... and ends up in the air filter.

It's also possible that a leaking interlock air cylinder can pressurize the transfer and push oil into the air filter.

Regardless, if the air restriction indicator in the cab shows red, check the air cleaner. If it's oil-clogged, check the levels in the transmission and transfer gear cases. If they're overfilled, have your mechanic drain the excess. Oil in these components may contain fuel. Sniff the oil, and if you smell fuel, change the oil.

If either gear case is low, support can check the transfer interlock air cylinder for leaks.

A good reminder not to overfill the fuel tank is to paint a line two inches from the top of the tank. Then, stencil "CAUTION: Do not fill above this line to allow for expansion" in 1-in black letters as shown in Fig 20 of TB 43-0209, Color, Marking and Camouflage Painting of Military Vehicles, Construction Equipment and Materials Handling Equipment (Oct 90 with Change 1, May 91).

Also, an expired MWO 9-2320-272-20-6 installed a fuel tank ventilation kit that prevents oil from getting forced into the air cleaner element and housing.