2005 A8 D3 4.2 (BFM) - Low MPG

[paulrstaylor]

Have you measured actual fuel consumption, i.e. how many miles per tank?

DIS is not always a good indicator, especially real time. Easy to go from 100MPG to single figures as you go from coasting to WOT in my S4

[MatthewH]

Originally Posted by paulrstaylor

Have you measured actual fuel consumption, i.e. how many miles per tank?

DIS is not always a good indicator, especially real time. Easy to go from 100MPG to single figures as you go from coasting to WOT in my S4

Hi Paul, it's the average figure (average no. 1 - this seems to reset every time the car starts) that drops.

This may not be relevant though as even over a long drive it's averaging 13mpg (according to both DIS and actual calculated value)

[Mechcanico Lee]

Just reading where you say at the low down areas the car seems hesitant .
Is the idle sweet ? Any burblyness to it ?

Can you do activations for variable length inlet manifold flaps solenoids ?

Would be good diag info if you could get a guage on manifold vaccum , on a v8 should be real steady at about 21 inches of mercury .

Has it ever had a cambelt done ?

This engine is 4 cams one on each bank driven by the cam belt and a small link chain between the cams , does it have cam sensors on all cams or perhaps just on the inlets .

[MatthewH]

Idle feels okay to me, maybe a very slight vibration but nothing I thought was worth mentioning. Certainly nothing like the vibration you get from a major vacuum leak (eg starting the car with PCV valve disconnected)

Intake manifold flaps seem okay. I replaced the linls a few months ago as they were broken, but the arms weren't seized. The arms are pulled back at idle, one opens up around 3000RPM and the other one opens at around 5000RPM. I filmed this while pressing the throttle to ensure they were working correctly.

I tested vacuum values a few months back. Can't remember the exact figure but I think it was 17/18. Seemed to be on the low side but I read online that it's just about acceptable. I've replaced several hoses and gaskets since taking this reading so it may not be this low now. I'll take another reading tomorrow morning.

Cambelt was apparently done at 130k miles according to the service book (not at Audi). I bought the car at 140k miles. I'm not sure about the cam sensors - I'll have a look. If the belt was fitted a tooth or 2 out could it cause these symptoms? Any way to check this with VCDS?

Thanks

[Mechcanico Lee]

Yes check engine vac again 17 / 18 is a bit low ( not enough vac ) engine vac is a good tool for under standing cylinder fill which cam timing has a big influence on .

Why did I ask was there 4 cam sensors on the engine .....

Let's say the cam sensor was on inlet cam and the cam timing was off by a tooth on that cam ,the ecm could see this and flag ....' Cam to crank correlation codes


But what if the exhaust cam was off and that did not have a cam sensor .....the ecm would never know this but the issue could be seen by missfire detection or lamdas as the cylinder would not burn as well which will leave higher oxygen levels because of poor burn

4 gas reading before cats is a good diagnosis tool also as cam timing being off will make more hydrocarbons ( un burned fuel ) the cats are good at masking this that's why testing before cats is key , but obviously you need a gas analyser for this .

Let's just re try that vac test and see what we have .

[MatthewH]

Hi Lee

I've just checked vacuum again. My gauge shows -0.6 bar (this converts to 17.7 inhg)

I'll check the 09X measuring blocks for camshaft adjustment values.

[MatthewH]

Some more figures (all taken at idle):

Engine Speed => 680 RPM
Cam Adjustment - Intake B1 (act) => -1.0 KW
Cam Adjustment - Intake B2 (act) => -1.0 KW
Phase Position - Bank 1 Intake => -5.0 KW
Phase Position - Bank 2 Intake => -9.0 KW

I couldn't see any exhaust cam related values, only intake.

I then unplugged the MAF sensor, reset the engine controller and took some more readings:

Mass Air Flow => 0.00 g/s
Fuel system 1/2 status => Closed loop, using O2 sensors
Short Term Fuel Trim - Bank 1 => 15.6%
Short Term Fuel Trim - Bank 2 => 10.9%
Long Term Fuel Trim - Bank 1 => 0.0%
Long Term Fuel Trim - Bank 2 => 0.0%

For the first few seconds after starting the engine without the MAF, the short-term fuel trims seemed to be cycling correctly (between -2.0 and +2.0, this also happens with the MAF plugged in) so we know the O2 sensors can read rich conditions. The values then went back to the +10 to +15 range as usual.