The tandem pump on PD TDIs can leak diesel fuel into the head, causing camshaft failure.

PD Camshaft Failure Background

The PD family of VW TDIs is known for camshaft wear and premature camshaft failure. This can lead to a variety of issues. Read our article about PD Camshaft Failure for more information.

What is the Tandem Pump?

The PD engine uses a two part pump on the back of the head (drivers side on transverse engines) where typically the vacuum pump would go. This two-part pump is both a vacuum pump and a medium pressure fuel pump sandwiched together in a tandem format and therefore it is commonly referred to as a tandem pump.

Most important to camshaft failure is the fuel pump section. The fuel pump on this unit pressurizes the fuel passage that is internal to the cylinder head and feeds the unit PD fuel injectors.

How Does the Tandem Pump Fail?

As the engine ages the fuel pump will become weaker due to internal wear. The most common symptom of this is low power or a rough/stumbling engine at low RPMs or at initial startup though typically the engine will run smoothly at higher RPMs. This is because the pump cannot maintain adequate flor or pressure to the injectors without being turned quickly.

In addition to inadequate pressure to the fuel injectors, the tandem pump can begin leaking fuel both externally and internally. Internal leaks will result in fuel being leaked into the crankcase but will have no external signs.

Tandem Pump Leaking Fuel Into Cylinder Head

While diesel engine oil is designed to handle some amount of diesel fuel, diesel fuel acts as a washing agent and will wash away the oil film on components.

In the photo above the tandem pump (circled) is located at the back of the head. The tandem pump is driven by the camshaft, and therefore the camshaft is the closest component and exposed to the most diesel fuel. This results in the camshaft being unable to maintain oil lubrication which leads to rapid wear on the camshaft system.

Signs of Camshaft Failure Due to Tandem Pump Leaks

If you have experienced camshaft failure it is vital to determine if a leaking tandem pump was a contributing factor.

• Check for the odor of diesel fuel after removing the valve cover, before removing the tandem pump. If you can smell diesel around the camshaft it is a clear sign that tandem pump was leaking.
• Compare wear on the camshaft lobes and lifters between the timing belt side and tandem pump side. If the tandem pump was leaking the tandem pump side will exhibit dramatically worse wear while the timing side may seem relatively intact.