Heavy-duty Mercedes vehicles generally come equipped with diesel engines. The fuel system of the Mercedes diesel engine can lose prime if the fuel filter is not completely filled up when you replace the fuel filter. If any air gets into the fuel system, the air takes the place of the diesel and causes vapor lock. The vapor lock prevents the engine from cranking unless the fuel system is primed or purged of the air.
Open the hood of the Mercedes and locate the diesel fuel pump on the right hand side of the engine bay. The fuel pump has a hand pump on the side of the fuel pump that is used to prime the fuel system.
Loosen the handle on the hand pump by turning it counterclockwise until it is completely out of the fuel pump.
Push the hand pump handle in and out of the fuel pump until the handle becomes almost too hard to move because of the fuel resistance. Push the hand pump handle back into the fuel pump and turn the handle clockwise to lock the handle down.
Slide a fluid catch pan underneath the center of the engine. Locate the fuel injectors on the top side of the engine. Loosen the locking nuts out from around the fuel injectors with a 17 mm open-end wrench.
Turn the ignition key on and off five or six times, then back to the off position. Inspect the area around the fuel injector locking nuts for any diesel seeping around the nuts. If the diesel is seeping around the nuts, tighten the nuts back down with the 17 mm wrench. If not, attempt to crank the engine in five second intervals until the diesel is seeping around the nuts. Then tighten the lock nuts back down tight.
Crank the engine and let it run for about two or three minutes to completely purge the entire fuel system of any air. Turn the engine off.
