Diesel engines made a quantum jump in power and mileage with the introduction of the VNT Tdi turbocharger. VNT refers to the variable vane technology, but in German. Inside the exhaust section of the turbocharger, a cluster of vanes between two plates surrounds the turbine. The vanes aim the exhaust at the turbine wheel and effectively reduce the size of the exhaust cavity. The turbine then spins up very rapidly compared to earlier turbochargers. When the desired boost pressure is approached, the two plates rotate, and aim the exhaust gases away from the turbine and effectively increase the size of the exhaust cavity. This boost control device is still referred to as the wastegate, although it no longer resembles the earlier toilet seat variety.
A VNT Tdi turbocharger is capable of much faster spool times, even at high altitudes. Maximum boost with a stock turbo runs around 2300 millibar. and then settles in to 2250 mbar, or around 19 psi. Vacuum is used for boost pressure control, instead of pressure as in previous engines. A more efficient intercooler was installed on the right side of the car, reducing total intake air mass, which in turn reduces turbo lag even further.
After 2001, the glow plug system went from a 2 wire to a 4 wire for monitoring purposes, but the plugs themselves and the harness were not improved.
Exhaust Gas Recirculation (EGR) was installed using a coolant intercooled radiator to reduce the temperature of the exhaust gases prior to injecting the gases into the intake manifold. The system has a valve on the intake manifold that is vacuum operated through a control solenoid.
The Mark IV Tdi marked the introduction of the dual mass flywheel in the clutch system of the Tdi. The dual mass flywheel has an outer shell with the starter teeth and much of the weight. The inner part has the clutch surface and is held by giant springs to the outer shell. This reduces vibration, makes clutch engagement smoother, an eliminates a noise generated by the diesel crankshaft that is heard through the mainshaft of the transmission. The complete clutch is a bit spendy. Single mass clutch kits are available for less, but then one gets to hear a rattling noise at an idle.
Dual mass flywheels can go wrong, making a rumbling vibration when one drives, potentially a dangerous thing if it breaks big time. These motors are prime candidates for jacking up the power output in a radical way, and the clutch MUST be replaced with a single mass VR6 type clutch to hold down the increased torque. A perfect, brand new stock clutch will slip with the extra torque from larger injectors and a Malone Tune. The Mark IV also marked the introduction of hydraulically controlled clutch actuation instead of the previous cable setup.
The path of the timing belt on the VE Tdi engines now included the water pump and some more rollers, turning the belt into a true snake. Belt intervals MUST be monitored closely. Timing belts have been known to break on Tdi’s made through the 2003 model year. Better belts with Kevlar reinforcement and Teflon coated teeth are available.