A bit of history - VW launched the Pump Duse engine as an alternative to Common Rail as VW did not want to pay the licensing fee to use common rail, so they invented their own alternative, a very torquey fairly refined engine but seemingly with one major design flaw...
The Cams are very prone to wear in these engines resulting in knocking/rubbing noises, misfires, black smoke, loss of power, reduced fuel economy, and damaged valves/lifters.
The reasons for the wear:
The PD in TDI PD stands for Pumpe Duse - this basically means the injection system is 'Unit Injection', a low pressure lift pump draws the fuel from the tank to a second injection pump, this pump called the 'Tandem Pump' increases the fuel pressure sending it to the 'Unit Injectors', the Tandem pump also provides the vacum for the brake servo.
The injectors each have their own 'pump' if you like built in, the timing of the injectors is set by the Camshaft, as the Cam rotates it pushes down on the Injector telling it to fire, the ECU then controls how much fuel is injected.
Because of this the PD engine has much thinner Cam lobes than a normal 4 Cylinder engine meaning the Cam is put under a lot more stress.
This often causes the Cam to wear prematurely damaging the Lifters, and the injector rockers. The Cam shell bearings are also known to wear.
Now it is worth noting that 'Unit Injection' has been around since the 30's and is a good system that works well, however due to a seemingly flawed design the VW PD engine isn't quite as reliable as it should be.
From my research and now sadly experience this seems to be down to a number of factors:
The main one is the use of either the wrong oil or insufficient servicing. The TDi PD MUST be serviced using VW PD oil meeting specification 505.01 for 10k servicing or 506.01 on longlife, the viscosity must be either 5W30 or 5W40 - some say 5W40 is better as it it thicker. The non use of either of these oils will more often than not result in premature Cam failure, likewise not servicing in accordance with VW's recommendations will also likely cause Cam wear. My advice is if you are keeping the car for a long time - change the oil every 7.5K and avoid longlife servicing.
Sustained high speed driving.
There seems to be a link between sustained high speed driving (75mph plus) and the cam wear, it seems at sustained higher revolutions the engine cannot lubricate the cam and bearings enough resulting in premature wear and failure.
Longlife Servicing.
There also seems to be a link between longlife servicing and premature Cam wear, Because the oil is in there that much longer more particles build up damaging the Cam. As above personally I'd change the oil every 7.5K.
Heres some photo's of the cam in my PD, this has done 79K, each lobe is showing signs of the start of serious wear (the hardened coating has gone as shown by the Copper colour) and the lobes on CYL3 have severe scoring to them like they have had insufficient lubrication, there also seems to be a lot of large particles in the oil in the head. Luckily (you cant see them in the pics) the lifters are ok for now, however the cam is getting noisy and has maybe 20k at best left in it.
The cost to repair for the average man in the street is around £800 from an independant (using genuine VW parts) and £1500 at the main dealer (advantage of 2 year warranty on the parts).
Its worth noting it seems to be a lottery which ones fail, a number of people service them exactly to VW specs and the cams still wear, other people have their og cams at 200k. Furthermore it seems to be much worse on the PD130 and PD150 engines than the PD100's fitted in our bugs - however it does still happen.
Its also worth noting that VW are slowly dropping the PD engine and switching over to Common Rail, this is because a PD engine cannot run a DPF because of the injection system meaning VW cannot meet the new legislation for Diesel emissions.
