How to Replace the Diesel Filter in a Mercedes

1. Lift the hood and locate the transparent pre-filter and main filter assembly. These are on the left side of the engine. The pre-filter is below the fuel injector pump, and the main filter is mounted on the engine block forward of the injector pump. Undo the fuel supply hose connected to the pre-filter with a screwdriver. Pinch off the fuel and plug the hose with a wooden pencil or a golf tee. Loosen the front hose clamp. Remove the pre-filter from the hose and replace it with a new unit. Reconnect the inlet hose and tighten both hose clamps with a screwdriver.
2. Use a 13mm socket wrench with an extension to loosen the two bolts on the bracket holding the main filter assembly to the engine block. This allows better access to the filter can. Remove the hollow center bolt from the top of the filter housing with a socket wrench. Spin the filter can off the filter housing.
3. Replace the lower 'O' ring on the hollow center bolt with the new 'O' ring provided with the filter. Moisten the new filter rubber gasket with diesel. Slip the gasket into the groove on top of the new filter can. Fill the can with clean diesel to speed up the bleeding process described in the next step. Fit the filter can into the upper housing with the arrow on the can pointing toward the injector pump inlet. Insert the hollow center bolt into the top of the filter housing, and tighten it with a socket wrench. Do not damage the gasket by over-tightening the bolt.
4. Tighten the two bracket bolts holding the filter housing to the engine block with the 13mm socket wrench.
5. Bleed the trapped air from the fuel system. Refer to the vehicle user manual. Some models have a priming pump knob on top of the fuel pump that must be depressed several times to bleed the system and fill the pre-filter with diesel. Later models do not have a hand pump, but the engine must be turned over with the starter in 30-second bursts until the motor fires. Start the engine and carefully check the system for fuel leaks.