Tips for Changing a Diesel Engine's Air and Fuel Filters
The air filter setup on most diesel engines is the same as on gasoline-powered vehicles, with the filter located inside the cold air collector box located under the hood. Most diesels have two fuel filters: a primary filter located between the fuel tank and the engine, which cleans the fuel before it gets to the fuel transfer pump; and a secondary filter up near the engine, which gives the fuel a final cleaning before it gets to the fuel injectors.
You have to take one big precaution when you change the air filter on a diesel: Always shut off the engine first. Diesel engines produce exceedingly powerful suction, and the air intake goes directly to the engine. Because almost anything can fly or drop into it — from nuts and bolts to your favorite hairpiece — you risk serious damage to the engine if you open the cold air collector box with the engine running.
Both fuel filters are usually easy to change, and your owner’s manual should show you how to do this job. On some diesels, the job is much like changing the oil filter on a conventional vehicle: You unscrew the old one, moisten the gasket of the new one with fuel, and screw it into place. Others have filters with replaceable cartridges; you just remove the old one and pop in the new one.
There’s one catch to changing the fuel filter, however: When you change the fuel filter or run out of fuel in a diesel vehicle, you must bleed the air bubbles out of the fuel system and then prime it to get a new supply of fuel circulating. Cranking the engine does the job but also wears down the battery, so most diesels include a manual primer pump and an air-bleed screw for the purpose of bleeding the system and priming it. On many vehicles, you simply pump the primer’s handle to get the fuel moving, and then you turn the air-bleed screw until a hissing noise tells you that the air is escaping. Just keep pumping until all the air leaves and the noise ceases; then tighten the air-bleed screw and replace the pump handle.