Following on from an earlier post in January we are still trying to chase down an engine vibration/surge just between 35 - 40 mph in top
and equivalent revs in lower gears. We have now replaced nearly the whole fuel system: injector pump, injector nozzles, fuel filter
& fuel lines but no improvement. Local diesel experts do not believe it's mechanical as the engine runs smoothly before and after the vibration.
MPG and performance is the same and no smoke when warm.

On checking the official workshop manual I note that at the top of the fuel tank there is a check valve that appears to be connected to the
diesel return pipe. I believe it mightv also be known as a float valve. The valve is shown on the exploded diagram but there is no reference
to it in the text. Does anyone know what this valve is for?

Any info greatly appreciated.