There are certain rules of thumb when it comes to learning how to bleed a diesel engine. There are dozens of different diesel fuel injection systems running around but the same principle applies to all of them. The main principle is removing the air out of the fuel system. Check out this scenario below.
So you find yourself driving down the road and suddenly your diesel engine starts to knock, lose power and pump out a bunch of white smoke. Finally it dies and after a few cranks you decide to check it out further. You find out the fuel tank is empty. The last time you checked your fuel level gauge it read half a tank, and you realize it still reads half a tank after driving 100 miles. A defective fuel gauge sender (located in the tank) is not an uncommon problem which causes more inconvenience with an engine shut down condition.
So now that you have determined the cause of the problem it is time to top up the tank with diesel and bleed the fuel system. Generally speaking diesels need a high pressure atomized spray that turns into a combustible mixture when combined with high compression. Compressed air heats up enough in the combustion chamber to ignite the mixture and provide the high power thrust that diesel engines are known for.
Step One – How To Bleed A Diesel Engine
The first thing to do is top up the tank and fill the primary fuel filter (closest to the fuel tank) with clean diesel fuel. This filter relies on vacuum or suction from the fuel transfer pump to keep it primed up and moving along to the secondary fuel filter. Diesel fuel transfer pumps are either mechanically driven off the camshaft or electric motor driven. If you are really blessed you might even have a hand priming pump built into the system.
Step Two – Removing Air From The Diesel Fuel Injection System
Any way you look at it, the main objective is to rid the fuel injection system of air. Loosening fuel lines is a common practice for older diesels. Loosen off fuel line fittings before the injection pump then work the hand priming pump until you see no more bubbles streaming from the loosened off fuel lines. This will assure that there is fuel to the injection pump.
Step Three – Bleeding A Bosch Mechanical Diesel Fuel Injection System
As an example the Bosch fuel injection pump found in Cummins diesel engines has a bleeder plug on the front, right side of the pump body and can be loosened off first while pumping the hand primer found on the driver side of the block. The mechanical fuel injection system on these engines have the fuel lines exposed for easy access. Loosen all of the injection lines at the head. Cranking over the engine will accelerate the air removal process. When the fuel system starts to pick up fuel the engine will try to start and stumble. At this point tighten the fuel lines and keep cranking over the engine, you will have fire power before you know it.
Bleeding An Electronic Diesel Fuel Injection System
Newer diesel engines went to an electric fuel transfer pump. As soon as you turn on the ignition key the pump kicks in and automatically primes the fuel system. Using the Cummins ISC diesel engine as and example, to prime you must fill the primary filter first with clean diesel fuel and install the secondary fuel filter dry. Once the ignition key is on the fuel transfer pump will prime the secondary filter with clean filtered fuel from the primary fuel filter.
Once you know how to bleed a diesel engine and go through it a couple of times you will see the importance of getting the air out of the lines. Air compresses and won’t allow any movement of diesel fuel resisting the build up of high pressures that diesel fuel injection systems require to start and run the engine.