Practical Classics (UK)

Audi I get it to work again?


QMy 2006 Audi A4 Avant 2.0 DTI conked out a mile up the road and hasn't run since. There are no error codes, we've replaced all the control relays and had the ECU checked (it was fine). The in-tank fuel pump seems OK, but between there and the injectors is a so-called 'tandem pump'. These are £200 a pop. Before I buy a new one, what does it do, and should I suspect it?

John Richardson, Ashburton

Ed says

AThe electric, intank pump supplies diesel at 10psi to the tandem pump which is driven off the engine camshaft. This contains a vacuum pump for the brake servo and turbo actuator; and an intermedia­te-pressure 110psi diesel pump which supplies the fuel injectors. The injectors are directly-driven up to 27,000psi by the camshaft. The tandem pump has a single drive shaft and special seals to keep the diesel, air and lubricatin­g oil separate. They can fail, therefore, in a variety of ways - admitting air to the diesel, diesel to the oil and so on. It would be unwise to try to test the output of the tandem pump, because 110psi of hydraulic pressure is extremely dangerous. Either substitute another, or see if a garage can test it.

Incidental­ly, the 27,000psi final injection pressure almost completely atomises the fuel. This is why this generation of diesel is so efficient and appears to burn so much more cleanly than its predecesso­rs.

By comparison, a classic taxi or bus engine emits particles like lumps of coal from the tailpipe. The particles from the newer engine are so small that they can pass through the lungs directly into the bloodstrea­m. This is why diesel emissions are now the object of some concern.

 ?? ?? Tandem pump is stage 2 of 3 in the fuel system.
Tandem pump is stage 2 of 3 in the fuel system.

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