I have a problem with my 1997 Volvo V40 1.9 TDI. The engine runs fine above about 1500 RPM but when I it is idling it begins to make a load ticking noise. It also loses power at low RPM's and can be difficult to start. I have tried changing the injector nozzles but it made no difference and all the glow plugs are in full working order. I have confirmed that no air is getting into the system before and after the pump by splicing in a small piece of clear pipe. Could it be a problem connected to the fuel system or should I concentrate on the engine internals?