A6 2.5TDI starting problem solved

[nikw]

Jumped into my A6 2.5TDI last week and started it up, only for the engine to die moments later. Repeated attempts to start the car failed until I became concerned about the battery getting low.

The immobiliser symbol on the speedo wasn't illuminating so I wasn't surprised when trying the spare keys didn't work. The glow plug light was illuminating and anyway the weather currently isn't cold so I didn't suspect faulty glow plugs.

Interweb research suggested all sorts of scary causes such as a failed injection pump (£1k+), the pump having jumped teeth on the drive belt (£400+) and so forth. In the end I called the AA man out to diagnose it. He spotted that there was air in the transparent fuel line running into the injection pump located at front of the engine (immediately under the silver plastic engine cover that has to be removed for access). He then disconnected the fuel line which supplies the fuel filter (highlighted red in 1st pic below) and switched the ignition on expecting fuel to pour out, which it didn't. This lead him to believe that the fuel pump in the tank was at fault. He couldn't figure out why the pump wasn't pumping even though we could hear it working.

To cut a long story short, after much investigation of the pump and level measurement assembly under the rear bench I satisfied myself that the pump in the tank doesn't actually push fuel up to the engine, it just swirls it around the pick-up. The aforementioned injection pump pulls the fuel up from the tank. With this knowledge I went back to the web looking for other fuel supply weak points and came across this thread, in particular the post by Andyn501 referring to the fuel return valve on top of the fuel filter (2nd pic below).

In the hope of a cheap fix I bought a new Bosch filter from Europarts (£25) expecting it to come with the valve, which it didn't. The valve on its own is £25 from Audi which is a little excessive for a small moulded plastic part. However, I noticed that the new filter came with a new o-ring already installed on the valve fitting and wondered if this might be the critical bit.

So, I disconnected the four fuel lines (crimping the one which feeds the injection pump in an attempt to prevent air getting in) removed the existing filter and valve, carefully cleaned the bottom of the valve where it fits on to the filter, filled the new filter with fuel (very important, no fuel = no lubrication for injection pump), fitted the valve to the new filter, fitted the filter and reconnected the four fuel lines.

After a generous application of Easy Start (sprayed in after the air filter but before the MAF) and some churning the engine started and quickly settled to a smooth idle. Woo-hoo! :D

I've yet to drive the car any great distance, but the fact that it started, plus the subsequent absence of air in the transparent fuel line suggests that I may have successfully resolved the problem.

If you have the same symptoms (engine won't start, glow plugs OK, air in transparent fuel line) I'd recommend changing the the fuel return valve and filter, or at least the filter. It's an easy job for anyone who can work a screwdriver and spanner and at less than £65 even if you use genuine parts it's a cheap first step towards a fix.

[Chippy757]

Good post!

Perished/cracked fuel lines also add to the problem, harder to notice as no fuel escapes but air is sucked in to lines.

This also could lead to lack of fueling under high rpm. High mileage pumps usually show this problem quicker.