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Timing a fuel injection pump

Timing a fuel injection pump

Early injection

If the injection occurs too early on the compression stroke, it will result in high peak pressures.  This will subject the engine to unsafe stresses caused by the tendency of the pressure to reverse the rotation of the engine and evidence by excessive detonation which is known as diesel knock.

Late injection

Retarded injection or late burning gives incomplete combustion causing too low a power output and overheating.

Timing instructions

It will be necessary to follow the manufacturers instructions in the Owners Manual to time the fuel pump to the engine as different methods are employed.

Timing principle

The principle is that fuel injection commences on the compression stroke just before top dead center. With a four stroke, the piston also comes up to top dead center on the exhaust stroke. Make sure it is on the compression stroke.

As with timing inlet and exhaust valves, the fuel injection pump must be timed to inject fuel at the correct angle on the compression stroke. This means that the gear driven shaft to the pump must also be lined up in the gear wheel train. Otherwise, difficulty might be experienced in lining up the holes in the drive coupling.

Timing engine to pump

The flywheel is usually marked with a TDC and with an injection mark that is before the TDC mark when turning the engine over in the direction of rotation. Turn the engine over in the direction of rotation until its number 1 cylinder is on the compression stroke and the injection mark is lined up.

The fuel injection pump must also be lined up on number 1 element or port at the commencement of injection. The Owners Manual will identify the position of the lining up marks as brands of pumps differ. When the lining up marks on the pump correspond, the drive couplings can be bolted together.


 

Alternative method of timing

To make it easier still, some manufacturers make provision for locking the fuel injector pump shaft at a position corresponding to top dead center for number 1 cylinder. A further pin is then located in a hole in the camshaft timing gear that is top dead center for number 1 cylinder. The drive couplings can then be bolted together and the pins removed.

As the pin is located in a hole in the camshaft, it can only be on the compression stroke on a four stroke engine.

Checking the timing of a fuel pump

The timing may be checked as follows:

1.         Remove the delivery valve and spring from number 1 element in the fuel injection pump.

2.         Open the throttle to the full position. (If the throttle is left at the stop position, the slot in the plunger will be in line with the spill port and no fuel will be delivered.)

3.         Rotate the engine in its operating direction until number 1 cylinder is on the compression stroke. Keep rotating the engine slowly and when the mark on the flywheel, indicating the start of injection is lined up with the timing indicator mark, fuel will immediately start to rise from where the delivery valve was removed. (This will mean the top of the plunger has just covered the inlet and spill ports and injection is starting).

4.         If fuel starts to rise before or after the timing marks are in line, the fuel pump timing is out and will have to be adjusted.

Detroit Diesel unit injector

 On a Detroit Diesel, the cam that actuates the unit injector is on the same shaft as the cams for the exhaust valves. If the exhaust valves are correctly timed, that is they open and close at the correct angles, then the unit injector timing must be correct. It is then only a matter of adjusting the unit injector follower to get the correct height in relation to the unit injector body. A special gauge is supplied for this purpose.

Cummins PT injector

On the Cummins PT system, it is only a matter of setting the clearance between the rocker arm and the injector.


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