The Pumpe Duse Tdi (PD) engines have an issue with camshaft wear that was eventually addressed by the change of design to Common Rail Injection. PD engines include the BEW, BRM and BHW engines produced in the model years of 2004 through 2006. For more precise injection spray patterns and better atomization, the injectors are actuated by an additional camshaft lobe. Previous VE Tdi injectors sprayed at a constant 2700 psi, and the volume was injected at a speed commensurate with fuel pump revolution. Pumpe Duse injectors are electronically controlled to allow variable pressures from 8000 to 27,000 psi and adjustable injection timing. And that system works famously, with almost no failures. But in order to fit in a third cam lobe, the lobes that drive the valves were reduced in width. To compensate, the diameter was increased. This however introduced a larger increase in surface speed, and commensurate increases in friction and temperature.
To achieve a hard outer surface with good wear characteristics, yet have a softer interior that will not crack due to the brittle nature of hardened steel, a process called case hardening is employed. A basic explanation of the process is available on Wikipedia. The hardening of a Tdi camshaft lobe occurs during quenching, or rapid cooling from high temperatures. The hardness is greatest on the surface, but if any abnormal wear occurs, wear then accelerates as it reaches the softer layers. The camshaft then wears right through the tops of the lifters. At first, the valves fail to open completely, and the engine begins to lose power. A peculiar soft popping sound is generated. Once that valve no longer opens, the corresponding cylinder has no dynamic compression. Often by that time, the internal pieces of the lifter have fallen out. Sometimes the oil pan must be removed to get these parts and the resultant metal flake out of the engine.
In most motors, a camshaft only has force applied in an upward direction as the valve springs and lifters push against it. Other force vectors are applied by the drive mechanism, such as a belt, gears and/or chain. In all of the Pump Duse Tdi motors, the lower cam bearing saddles of the head are cut round on one or both sides to allow the lifters to be installed. The location of the oil port in the head to each cam bearing is such that as the bearings wear, the cam slowly rides off center, and impedes the flow of oil out to cool and lubricate the lifters. Any copper coloration to the cam lobes or the lifter surfaces indicates that the bearings have worn completely through the surface layer or Babbitt, which is substantially made of tin and lead, though the nickel layer, and into the copper / lead layer that coats the steel bearing shell. In a Pumpe Duse engine, the rockers for the injectors push the camshaft in the downward direction, causing an oscillation in the oil film that also impedes oil flow outward to the lifters. These effects combine to decrease lubrication and cooling. This can also cause the oil in the lifters to partially vaporize. Vaporized oil allows the lifter to bounce or chatter, since the support force has moved from hydraulic to partially pneumatic, and thus has an effect like a spring. This causes the radial starburst wear patterns we find on some lifters. Any substantial wear of the cam and/or lifters will proceed rapidly to complete failure.
Occasionally, cam failure is preceded by loss of oil pressure control. The needle bearings of the roller rockers are lubricated by passageways inside the rocker assemblies. There are plugs pressed into the ends of these passageways to cap them off after boring them out during manufacture. Sometimes one or more of these little plugs drop out, possibly due to long term heat cycling, and the oil pressure and volume is severely reduced to the rockers and to the camshaft and lifters. At Karmakanix, we stake these plugs in on every job by using a center punch to expand the metal around the plug holes, securing them against future failure. Disassembly of a rocker that has lost a plug generally reveals resultant wear that makes it inadvisable to replace the plug and reuse the rocker.
Aftermarket camshafts and lifters are available that have better wear characteristics, and give better power and mileage as well. It should be noted that the “billet camshafts” generally have a larger bevel cut on the sides, reducing the width of the contact surface with the lifter by about .020″, which obviously will not last as long as some other makes of cams. Modifications can be done to the camshaft bearings to improve oiling and cooling. The best solution is prevention. Oil changes at 5000 mile intervals instead of the factory recommended 7500 mile intervals is key to camshaft survival. Karmakanix also highly recommends the use of a Molybdenum Disulfide friction reduction additive such as Liqui Moly MoS2 additive, also known as Lubro Moly.