When a diesel fuel pump is switched off, it does not just stop pumping fuel, as many people assume. It also begins to reverse its pumping action, which can cause a vacuum or suction in the fuel system.
This is because the diesel fuel pump is actually two pieces of equipment in one: a fuel injection pump and fuel inlet. The inlet (intake) is designed to let the diesel fuel in, while the pump impellers force the fuel through the outlet (exit) and into the combustion chamber. This is very similar to an ordinary fuel pump on a car.
When the diesel fuel pump is switched off, the inlet and the outlet valves close. But because the impellers are still rotating due to inertia, they create a vacuum in the inlet. This vacuum then draws fuel back in, causing the pump to reverse its direction and generate a vacuum in the diesel engine. This suction can increase idle speed and can even be enough to start the engine.
The vacuum created by the reversed pumping action also affects the flow of diesel fuel from the fuel tank. The fuel tank is pressurized, which pushes diesel fuel into the pump and out of the injection system. But when the diesel fuel pump reverses, it can draw in fuel from the tank, creating a vacuum on the outlet side. This will slow the rate at which fuel is delivered to the engine.
To prevent the diesel fuel pump from reversing, some diesel engines have a fuel shut off solenoid. This solenoid is a small electrical device that is activated when the engine is stopped. It moves a small valve into the fuel line and shuts off the fuel supply to the pump. Without the return flow of fuel, the impellers no longer create a vacuum, preventing the diesel fuel pump from reversing.
In summary, when a diesel fuel pump is shut off, it reverses its direction due to inertia and creates a vacuum in the inlet, which can increase idle speed and even start the engine. To prevent this, some diesel engines have a fuel shut off solenoid to shut off the fuel supply to the pump and prevent it from reversing.
