2005 A8 D3 4.2 (BFM) - Low MPG

[MatthewH]

Hi,

I purchased a 2005 A8 around 12 months ago and have gradually been fixing its problems.

The final issue with the car is the low MPG. I realise the MPG is never going to be great with a 4.2L V8 petrol engine but it's currently averaging ~13mpg with careful driving. This is on open NSL roads - around town can easily be 8mpg. It feels fine to drive - no rough running or misfires.

I've spoken to a few other owners of this car with the same engine who all seem to be averaging at least 20mpg in mixed driving. The figures mine returns seems far too low in comparison with that.

The only fault codes are P1128 and P1130:
P1128 - Fuel Trim; Bank 1 (Mult): System too Lean
P1130 - Fuel Trim; Bank 2 (Mult): System too Lean

I've cleared these codes but they return approximately every 100 miles.

These codes are the opposite of what I'd expect - with such high fuel consumption I would expect rich codes but these have never occured.

I've already looked at the following areas:
- MAF sensor (this seems to be the primary cause for these error codes - I've replaced it twice with new genuine parts but it hasn't improved things)
- Pre-cat O2 sensors (replaced with new Bosch sensors)
- Replaced fuel pressure regulator
- Replaced intake manifold flap links
- Replaced throttle body gasket
- Replaced intake manifold gaskets
- Checked all hoses for vacuum leaks (none found)
- Replaced all vacuum hoses
- Checked fuel pressure (3.5bar with FPR connected, 4.0 bar without FPR)
- Replaced fuel filter
- Replaced air filter
- Replaced spark plugs
- Ignition coils were replaced under recall a few years ago
- Replaced G62 coolant temperature sensor
- Replaced N80 EVAP valve
- (probably more that I've forgotten)

When starting the car and driving off, the average MPG reading usually shows a steady 15mpg, but 20 seconds later, it immediately drops to 5mpg with no change in driving style. This makes me think it may be related to the change-over from open loop to closed loop fuelling.

Both cooling fans run at full speed at all times while the engine running for unknown reasons but I don't think this would make such a difference to the MPG. All temperatures seem normal and it heats up as expected (up to 90 degrees within a couple of minutes)

Has anyone had a similar problem? I've run out of ideas. Is it possible that one of the fuel pumps may be failing? I've read that someone else had similar symptoms and it turned out to be a faulty fuel pump, although I don't fully understand why this would cause excessive fuel consumption.

Thanks.