Common faults with VAG vehicles with tandem fuel and vacuum pumps

Main parts   

This post will investigate the two most common faults that occur in the VAG group's tandem pumps.

These types of pumps are installed in those engines equipped with the pump injection system.

The tandem pump unit is a set of two pumps in one unit with very differing functions.  The entire pump assembly is driven by the camshaft.

One of the functions of the assembly is the vacuum pump; which generates the vacuum for the brake servo. The second function of the assembly is a fuel pump which sucks fuel from the fuel filter to the feed pipe located in the cylinder head and supplies the injectors.

The working pressure of this pump varies between 8 and 12 bar depending on the model.

FAULTS

The most common faults in this type of pump are as follows:

1 Loss of fuel.

2 Mixing of oil with fuel.

PROCEDURE AND SOLUTION

The repair procedures for either of the two faults are practically the same except for the additional tasks required in the case of oil and fuel mixing, which are:

• Fuel filter replacement
• Checking the engine oil level

 In both cases the problem can usually but not always exclusively be located in one of the gaskets or on the pumps main shaft oil seal.

Set of gaskets   

1- Obtain a gasket set for the repair, as shown above

  

2- Remove the tandem pump in order to carry out the repair work. Take precautions to ensure fuel is not spilled on to the gearbox as it will contaminate the clutch disc causing it to slip and making it difficult to engage the gears.

Mesh before the return pipe

Mesh before the return pipe     

3- Remove the cover where the fuel return valve is located (4 torx 20 bolts). 

• Clean the inside of the housing.
• Clean the mesh with compressed air, this mesh is not supplied in the replacement gasket kit.
• Replace the gasket.
• Refit the cover using new bolts.

4- Remove the black cover fastened with 5 torx 30 bolts.

Remove the rotor cover fastened with 3 torx 20 bolts (see the picture below)

Replace the rotor pressure springs (see the image below)

Ensure the cleanliness of the housing and repair procedure.

Replace the gasket

Fit both covers in reverse order (replace the 5 black cover bolts)

Left photo: Rotor cover bolts - Right Photo: Pump rotor and pressure spring

5- Remove the three torx 30 bolts that appear in the following picture:

Bolts for the tandem pump body

• Remove the body of the pump from its base on the cylinder head.
• The pumps main shaft and its blades is located at the bottom.
• The two gaskets that are pictured are of the upmost importance to ensure that the mixing of fuel and oil does not occur. These gaskets and/or main shaft oil seal are the known to cause issues with the cross contamination of the fuel with oil.

Main pump shaft and blade

6- Remove the pumps main shaft, which has a press fit.

Replace the pumps main shaft seal.

Left picture: Main pump shaft - Right picture: Pump Shaft oil seal

- Once the gaskets and oil seals described in procedures 5&6 have been replaced, re-assemble in the reverse order.

7- Prior to refitting the overhauled assembly it is essential that the gaskets at the base of the pump must be replaced where the tandem pump locates on the cylinder head.

Gaskets at the base of the pump

*Note: some pumps use a metal gasket instead of the two gaskets shown in the above image.

8- Refit the pump on the cylinder head. Tighten the top bolts (Allen 6) to a torque of the 20 Nm, the two bottoms bolts (Allen 5) should be tightened to 10 Nm.