200 TDi spill rail

My son has just finished rebuilding a defender with a 200 tdi engine, passed the MOT, now road worthy, taxed and insured but unable to use because every 5 or so miles a spill rail hose pops off and sprays diesel all over the engine compartment. It has got the local Landy Garage scratching their heads. Does anyone have any ideas?

A blockage in the return pipe which goes back to the tank?

I would make up a little test rig and work out if one of them is dumping a whole lot of diesel.

The spill rail pipe usually just allows a small volume of diesel to spill off which also goes to help lubricate the injector.

The spill lines being blocked would be an issue but it also sounds like there is quite a bit of pressure building up here.