1994-1998 12 valve Cummins Engine Troubleshooting

Posted by Diesel Conversion on Oct 16th 2020

94-98 Fuel system troubleshooting

Engine won’t start:

Bleed the air out of the system

If you are starting your Cummins for the first time, it is required to “bleed” the air out of the injector lines by loosening the injector line nut just enough to allow fuel and air to escape the line while you are cranking the engine. It is usually required to do at least 3 injector lines. Tightening the line nut while you are still cranking the engine is the best way. This is also a good way to verify that the injector pump’s shut down solenoid is allowing fuel to the injector. If your supply fuel line is loose, it is possible to draw air into the fuel system at that point, so if you suspect you are not getting rid of the air in the system check your supply line connections. These engines should have between 18-30 psi or more of lift pump fuel pressure. 

Good supply pressure/fuel at injector

If you have fuel at the injector, good fuel supply pressure, and you are fairly certain you have no more air in the system, you may have a weak injector pump or incorrect injector pump timing. You may especially suspect this kind of problem if you have never heard the engine run before, or you replaced the injector pump.

Good supply pressure/no fuel at injector

If you have no fuel at the injector, but you have good fuel pressure, then your shut down solenoid is not working properly. The shut down solenoid lifts the shut down lever on the injector pump to allow the engine to run. Start only power lifts the solenoid, and key on power holds it in the up position.

Poor supply pressure

If you have poor supply pressure, check the overflow valve. This is a special banjo bolt with a spring and ball inside that regulates the fuel pressure inside the injector pump. It is located between the engine and the injector pump.
Take the valve out and shake it. If the spring is broke it will rattle. If it looks like this one, you can take it apart and add or remove shims to adjust the pressure. We don’t recommend pressures greater than 60 psi. If there is nothing wrong with the overflow valve, then you may have a lift pump problem- or a restriction or an air leak in your supply line between the lift pump and the tank.

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