Moved to Technical.

Lots of posts on the InterWeb about this. It's a common fault on the Fiesta 1.4Tdci. On mine, the fault was a crack in a plastic fuel pipe near to the priming bulb and this in turn was caused by the pipe vibrating because the priming bulb was not firmly fastened to its bracket (it is now). As I've pointed out on other discussions about this, I was surprised that there was no obvious fuel leak. With the engine running, the fuel pressure seemed to hold the crack in the pipe closed, but when parked up, air would get in and the fuel would run back to the tank. A local garage sorted this out by using readily available standard size fuel pipe which is slightly wider than the specialised pipe fitted by Ford. He cut the cracked pipe at the crack and slid on the standard size pipe which he first dunked in a mug of hot water, then sliding the other side of the cut pipe into that. Well, the car's done two years and 20K miles since then and the repair is good! Another cause which I've read about is one make of fuel filter where the welded plastic join around the 'lid' of the filter (where the two nozzles are) starts to leak, and again, air getting in whilst parked causes the fuel to run back to the tank.

One other thing... There's a tap at the bottom of the fuel filter. It's to run off a bit of diesel if there's a chance of water in the fuel. (Water sinks below oil). Make certain that the tap is tight, because if you open it with the engine off, you'll just hear a gurgle as the fuel runs back.

Whatever happened to 'Hungarotex'? When I wuz a kid, so much stuff in the shops seemed to be made in Hungary. My first tent was a two-man bright orange Hungarotex!