There is just one small point I would add to Fisherman's comprehensive instructions:

After you remove the piping to the injector you are left with a 15 mm fitting sticking out of the Injector Pump (IP). Remove this fitting and just under it is where you will find the spring (be careful not to lose it). Under the center of the spring is the valve. You can reach into the IP with a pair of needlenose and withdraw this part from the center of the hole. Do not take out anything else...

Before you withdraw the valve thoroughly clean out the cavity. Dirt and grit in the thread will have fallen on top of the valve seating and will be sucked onto the metering piston as you lift it out. I find the best way to do this is to hold the valve firmly closed with a thin screwdriver and blow out with compressed air.

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Beagle