"Should the car fire up if I disconnect the leak back hose from the top of the injector that I suspect is faulty?"

The leak back prevents starting because the injectors can't develop sufficient injection pressure so disconnecting won't help, previous posters have briefly crimped the return pipe whilst cranking and the engine has started, just be careful with possible fuel spray.

"When it died I think it broke that small rubber seal again, which is when the mechanic said it was the injector pump that needed replaced. What else could cause this small rubber seal to break "


Excessive high pressure?

Yes I figured it was higher pressure that was causing the seal to break, but I meant is there anything that would cause this to happen? Thanks for the other info Dan, I'm going to go out and take voltage readings from both the fuel pressure sensor connector and the cam sensor connector to see if either of these could be the cause of the fault. I can't actually get the car started at the minute though, so I'll only get readings with the ign on. I'll post my findings later.

I think the 'tuning box' has caused the high pressure to increase way beyond normal limits and the rubber seal has acted like a safety valve when it blew out. I imagine that the new seal has now also blown out.

Although you've taken off the 'tuning box' I think the HP pressure is still very high because the unit which controls HP pressure has been damaged. The unit which controls HP pressure (regulator) is screwed into the rear of the HP engine driven pump, it's basically a solenoid operated by the ECU.
If this unit has been overworked by the 'tuning box' it may now be siezed and unable to control pressure.

I thought this myself, but I've fitted a replacement pump and it ran ok for a few minutes before it died again.

I used a multimeter on the cam sensor... With the ign on, the connector on top of the engine block didn't give any readings from the 3 pins, but the connector that connects into this gave the following readings with the ign on:
Red/White wire - 9.7v
Yellow wire - 9.6v
Brown wire - 0.01v
Do these readings seem ok.

More interestingly, I disconnected the green plug connected to the fuel pressure sensor and with the ign on, none of the pins gave any reading, either on the sensor side, or on the green connector. Is it safe to assume the fuel pressure sensor is knackered? Is this easy to replace? Is it a matter of using a spanner to screw it off, then screw the new one on, or will a load of diesel shoot out when I screw it off?

Also, I have a friend with a Rover 75 with the same engine as the Td4. If I was to run his car and use 3 wires to connect his fuel pressure sensor connectors to the connector on mine, would this fool the ECU into thinking the pressure in my car was that of the Rover 75 and therefore get the car to start? Would this cause any damage to the Rover 75?

Thanks

Cam sensor harness voltage readings are about right, and the sensor itself would not give any readings whilst disconnected.

I wouldn't advise connecting the R75 to the TD4.

Regarding the green harness connector showing no voltages:

With ignition on you should find at least 5volts on white/yellow this is a reference voltage from the ECU, the others are signal return and earth return so generally don't show voltage when disconnected. Check this again and if no voltages are present then make sure the ECU multiplug is secure and the cables arent damaged.

Should the yellow/white wire in the green connector read 5 volts even when disconnected from the fuel pressure sensor? I'll check this when it stops raining outside.

Yes, with the ignition 'on', the ECU supplies 5v to many of the vehicle sensors and the returning signals are a proportion of that reference voltage this is how an ECU knows what's going on around the vehicle.

I see. If I'm still getting no reading, then I'll open the ECU box and check the connections etc., and test for the voltage of the other end of the yellow/white wire (the end in the ECU box) incase the cable has been damaged or something.

If you can prove that the three cables between the ECU and the HP pressure sensor are O.K and that the ECU is otherwise working normally i.e the low pressure pump runs briefly after turning on ignition and the MIL light illuminates briefly after ignition on then it may be that part of the ECU has suffered damage.