All engines use closed nozzle, hole-type injectors. How ever, the injectors can have different part numbers for
different engine ratings. The last four digits of the Cummins part number are used to identify the injectors.
During the injection cycle, high pressure from the injection pump rises to the operating (pop) pressure which
causes the needle valve in the injector to lift. Fuel is then injected into the cylinder. A shimmed spring is used to
force the needle valve closed as the injection pressure drops below the pop pressure to seal off the nozzle after
injection.
Failure of the needle valve to lift and close at the correct time or needle valve stuck open can cause the engine to
misfire and produce low power. Fuel leaking from the open nozzle can cause a fuel knock, poor performance,
smoke, poor fuel economy and rough running.
To find which cylinder is misfiring, operate the engine and loosen the fuel line nut at one injector and listen for a
change in engine speed.
A drop in engine speed indicates the injector was delivering fuel to the cylinder.
Check each cylinder until the malfunctioning injector is found.
Be sure to tighten the fuel line nut before proceeding to the next injector.
Remove the malfunctioning injector to test or replace it.
If the engine continues to misfire after replacing the injector, check for leaks in the high pressure line. Also
check for a defective delivery valve that lets the fuel drain back into the injection pump.
Carbon build up in the orifices in the nozzle will also cause low power from the engine. Remove and check the
spray pattern or replace the injectors.
