| AIR IN THE SYSTEM
Air can cause numerous problems with the diesel engine and in particular with the fuel system. For example a massive air leak on the low pressure side of the system can deny ‘prime' to the injection pump and prevent the engine from starting at all. Less serious leaks will introduce air bubbles into the fuel system with common symptoms such as hard starting, rough running, engine not idle, loss of power, ‘hunting' in engine and sudden shutdowns.
The fuel system consists of three circuits:
Low pressure circuit - which connects the fuel tank to the injection pump and usually includes fuel filter, water separator and lift pump (also known as feed pump,supply pump, transfer pump)
Fuel return circuit - which conveys surplus fuel from the injection pump and injectors to the tank.
High pressure circuit - which consists of the injection pump, fuel lines and injectors.
Air leaks normally enter the system in the low pressure circuit and can be identified by disconnecting the return line at the tank and submerging it in a bucket of clean diesel fuel. Start the engine and look for bubbles in the fuel stream.(an alternative is to hook up some clear hose into the return line) the usual causes are finger tight filter or water separator bleed screws and loose fittings. Also a visual inspection of rubber hoses, line fittings and steel lines (where they may have rubbed) should be carried out.
Another problem which can be experienced is ‘bleedback' where air is evident after the engine sits inoperative for a period of time and the fuel drains back to the tank and has to be reprimed. The engine may or may not start but if it does, it will only run for a few seconds, die and refuse to start immediately. The lift pump (and primer) is one of the most common causes of this problem.
If the leak persists it may be necessary to pressure test the low pressure circuit. Before you do make a few simple checks - check the fuel filter for restrictions, drain the water separator, check the return line for restrictions (injection pump orifice under the pump outlet fitting).
Warning - do not pressurise the fuel tank - it may rupture. Using a maximum pressure of i5 psi, disconnect at the tank and injection pump, plug one end, apply soapy water or commercial leak detector solution to outside of line connection points and other potential leak points and then pressure test.
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