2.5TD idling fault

[Seth]

My 2.5TD Estate has 140,000 on the clock and has an intermittent idling/racing fault. I've done a FULL service, but this problem remains, although it's running much better now. A local specialist suggests two diagnoses, either a fuel-line fault, or the pump potentiometer -cost to rectify the pump is around a cool grand!!!!! There's also a 'flat spot' on occasions - can you guys give me a pointer, before I start shelling-out the dosh?

[TheBoy]

The pot in the pump may be cleanable with care.  Cock it up though, and thats a grans worth of pump down the drain

[Seth]

Another query, (sorry), when looking downwards at the tank pump, which of the two pipes is which, as neither have markings on them.
I've also found that the fuel filter casing looks damp - even though it's assembled correctly. Do you think there could be a hairline fracture here - could this be causing my idling/racing fault?

[Dazzler]

Any airleak on a diesel line will give poor running if at all, you must rectify any leaks as a starting point.