2005 A8 D3 4.2 (BFM) - Low MPG

[MatthewH]

Hi Lee, just checked but can't see any messages - did it send correctly?

Thanks

[Mechcanico Lee]

Re sent

[aTOMic]

Originally Posted by Mechcanico Lee

Hello Atomic Tom .

Yes follow through some of the tests already mentioned , Matthews next step with the new pump will be the tell tale .

I've never done the thing with propane myself , I normally do vaccum checks and use a smoke pro machine for finding manifold leaks , I'm assuming the vaccum line that you are feeding it in is only a small pipe ? If so its probably just not taking in the volume to affect it so much

Here's the daddy for manifold leaks ....hehe

Not a small pipe, in fact the "Hose to Nowhere" connected to 1/2"ID fuel line connected to a regulator set to max on a 30lb propane tank! Some think I fried the O2 sensors, but I told them you crazy *******s convert the cars to run on propane instead of "petrol". I do not think they believed me.
Re the "SmokeDaddy"; I've seen (less elegant-looking) testers in that style built on youtube and had considered that option; however I do occasionally smoke cigars and thought this would be a good excuse to have one... except I've had a cold for a couple weeks, and prior to that my times to experiment (smoke) are limited because I don't want my young kids to see me smoking anything...

How much pressure do you find necessary to find a leak? I'm guessing 20 PSI couldn't hurt. I can only blow about 2PSI IIRC from elementary school. Still worth a try. What I lack in presssure I make up for in volume (TWSSLN). Sorry, couldn't resist.

Thanks again.

[MatthewH]

Update: I have purposely not reset the engine module since the weekend I replaced the main fuel pump. I checked the figures again today after ~200 miles of driving and the long term fuel trims are now both around +7% and the lean codes haven't come back. MPG still hasn't improved though.

I recently remembered that at my last MOT there was an advisory for brake calipers sticking slightly (both fronts, one rear from what I remember). I was meaning to have a look but completely forgot, until last week that is. I was about to drive home and the electronic handbrake wouldn't disengage - the light was flashing and the motor didn't sound right. The offside rear caliper was seized and wouldn't free. I had to drive it home like this (stopping every couple of minutes so that brakes would cool down - the wheel was smoking and too hot to touch). When I got home I left it overnight and it seemed to work correctly again the next morning - the handbrake now disengages properly and it hasn't happened since.

I'm thinking this may turn out to be the cause of my MPG problems. I've never dealt with a sticking caliper before - would it be possible that I've been driving all of this time (sticking enough to cause 13mpg rather than 25mpg) without noticing it? It's never really felt like the brakes were dragging (other than the incident last week) but I'm not sure how much they'd have to be sticking to cause a big drop in MPG. It will roll down a hill in neutral when coming off the brakes.

I ordered a new set of discs and pads (front and rear) so I'll try to get them fitted this weekend and see if this improves anything.

[Mechcanico Lee]

Hello Matthew , the fueling trims are much closer to where they want to be now .
I would of thought for the loss of mpg the brakes would have to be sticking a fair bit but any dragging brakes will not help .
When you do the job give the brake pad hangers a good fileing get rid of all the rust so the pads fit nice , when you use vcds to open the rear calipers in basic settings the Pistons should be able to be pushed back quite freely ...check for corrosion behind the piston rubbers , same with fronts the Pistons should be able to be pushed back easy enough , sometimes the pads can get really tight in the hanger so as the disc warms up the bind gets more progressive .

[MatthewH]

Originally Posted by Mechcanico Lee

Hello Matthew , the fueling trims are much closer to where they want to be now .
I would of thought for the loss of mpg the brakes would have to be sticking a fair bit but any dragging brakes will not help .
When you do the job give the brake pad hangers a good fileing get rid of all the rust so the pads fit nice , when you use vcds to open the rear calipers in basic settings the Pistons should be able to be pushed back quite freely ...check for corrosion behind the piston rubbers , same with fronts the Pistons should be able to be pushed back easy enough , sometimes the pads can get really tight in the hanger so as the disc warms up the bind gets more progressive .

Thanks. I didn't get chance to do the brakes over the weekend but I did have the wheels off. They're definitely wearing unevenly so this won't be helping things but I don't think this is the main cause of my MPG problems. To cause the drop in MPG I'm getting, I think the wheels would be very hot after any journey, but they're just slightly warm.

I did another test with my vacuum gauge last night. It seems to sit at around 17HgIn with a slight fluctuation. (Video attached).

Does this give any clues? From what I've read online about interpreting vacuum readings, this could point towards an ignition related problem. (Or possibly bad cam timing but this would seem to cause a much lower drop in vacuum to around 10HgIn).

Thanks

[nakata]

Originally Posted by 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.

Sorry, i'm not read the whole thread. Only will give you a clue. Camshafts adjuster. Very common problem on 4.2 40V engine.

[MatthewH]

Originally Posted by nakata

Sorry, i'm not read the whole thread. Only will give you a clue. Camshafts adjuster. Very common problem on 4.2 40V engine.

Hi, I have considered those. I get the 2 second rattle on cold starts but they sound fine after that.

If these had failed would it cause a cam timing fault code?

Thanks

[Mechcanico Lee]

The vaccum is not as good as it should be Matthew , like i said back there you need to check cam timing first port of call .

If cam timing correct next would be to have a look at variators to see if foot has worn through or fell off .......and the inlet cam timing mark
Cambelt drives exhaust cam ........ exhaust cam drives inlet cam with internal chain .

Cam / crank correlation codes .......... sometimes you get them .....sometimes you dont .....if the cam that is off does not have a cam sensor the ecm does not know that it is out , it can only reference from the cam that does have the sensor ......does inlet and exhuast have cam sensors ?? .

I seen chains stretched to death on vauxhall engines putting cams out loads and still not had cam /crank correlation codes .

In some cases the ecm can only see cams that have gone retared (chain stretch) it can not always see cams that are advanced .....as in someone doing cam timing incorrect whilst doing the cambelt job

[nakata]

Originally Posted by Mechcanico Lee

The vaccum is not as good as it should be Matthew , like i said back there you need to check cam timing first port of call .

If cam timing correct next would be to have a look at variators to see if foot has worn through or fell off .......and the inlet cam timing mark
Cambelt drives exhaust cam ........ exhaust cam drives inlet cam with internal chain .

Cam / crank correlation codes .......... sometimes you get them .....sometimes you dont .....if the cam that is off does not have a cam sensor the ecm does not know that it is out , it can only reference from the cam that does have the sensor ......does inlet and exhuast have cam sensors ?? .

I seen chains stretched to death on vauxhall engines putting cams out loads and still not had cam /crank correlation codes .

In some cases the ecm can only see cams that have gone retared (chain stretch) it can not always see cams that are advanced .....as in someone doing cam timing incorrect whilst doing the cambelt job

From what you write, only show is not familiar with this engine.
And yes is not have 4 cam sensor, only 2.

And no not will give you a fault, but will have rough idle and some time missfire without reason.