2005 A8 D3 4.2 (BFM) - Low MPG

[MatthewH]

Thanks Lee. Yes, I thought it may have been a problem that occurs under load only - I just thought bad cam timing would cause rough idle (and bad emissions) so I was hoping that was not the case! It's still worth checking as you mentioned. I'll get a photo of the timing-related VCDS values you mentioned later on today.

People have also mentioned the hydraulic camshaft adjuster valves as these can be a common problem on this engine - I think the plastic "shoes" wear down and cause the chain to become loose. This uses causes a loud rattle though which I don't have.

I actually have a spare fuel rail with injectors so I'll probably start with these. I was planning to rebuild the injectors with new seals and microfilters. I tried to remove my injectors when I had the intake manifold off but the fuel rail wouldn't lift off (with only the injectors holding it in place). It felt like I was going to bend the rail if I pulled any harder so I ended up leaving it attached.

[Mechcanico Lee]

Iirc the cam timing actual and cam phase position should be as close to 0 kW as possible ...kW just means degrees of cam rotation , so if these data blocks are off it sort of proves that cam timing is off ......and the low engine vaccum supports this also ....yes this could also be a fault with the cam chain variator units ( yes known issues on these ) but you also have to know that your base cam timing position is correct also .

[MatthewH]

Photos of all cam-timing related values attached.

Photo #1 is idle
Photo #2 is ~2000 RPM
Photo #3 is ~3000 RPM

I think the "phase position" values relate to the hydraulic camshaft adjusters but there's not much info about it online. The info online mostly seems to be related to TDI engines.

Also, I checked fuel trims again today. They're both at +7.0% (partial), 0.5% (idle), so still within tolerances. Replacing the main fuel pump fixed my P1128 lean code but didn't improve MPG.

[Mechcanico Lee]

Mmmmm ....phase position not doing what I thought it would be doing , actual timing is advancing with speed as I thought it would ......we could do with another member looking at there settings to compare readings on that phase position , just in case we are going up the wrong pathway .

The idle reading fuel trims are spot on , you would think if injectors where over or under fueling you would see it at the idle fuel corrections , on the partial throttles there's some correction but not so bad .
If you watch short term fuel trims on the road is there any areas of throttle where they go widely out of range .

What would be interesting.... the camshaft position sensor on the drivers side bank ...the easier one to get at , looks like it's held in by two 10 mm head size bolts , if they are slackened is there any movement clockwise or anti clock ...like turning an old type distributor , I just want to see if moving it what it does to the phase position kW readings and in turn what that does to the partial throttle short term fuel correction values .

For how heavy it is on fuel you would expect the trim values to be showing a rich condition or is it a case of its leaning out and the engine becomes in efficient and that's using the fuel , do the spark plugs show any signs that's it's rich ....bit dark in colour ?
Interesting one this

[MatthewH]

The phase position always seems to stay the same regardless of RPM, when not moving at least. I've not testing while driving but I'll record a log tomorrow.

Back on post #36 on this thread I checked the phase positions and they were almost the same as they are now (-5.0 and -9.0).

If I get chance tomorrow I'll also remove a spark plug as you mentioned. I changed them around 12 months ago.

I'll also have a look at the camshaft position sensor - it's fairly easy to access from what I remember. What would you expect to see from turning it?

Thanks

[Mechcanico Lee]

By turning the cam sensor on that bank I want to see what it does to the corresponding actual cam setting and the phase setting values

Back in the day before we had cam sensors petrol injection used a system called block .....all the injectors would fire twice every engine cycle , then came along sequential injection ....so a cam sensor was needed so the ecm knows where the cam is and knows the time to inject ...a period just before inlet valve opens .
In most cases if you unplug cam sensors on a sequential system it will revert back to block injection

Where my head was going if phase sensor was off perhaps injector opening time was slightly off time ...coming to early or late which ever affecting overall fueling
It's just thoughts mind .

Hey there's a thought how about un plugging cam sensors and run it in block just for diag purposes to see what happens to fuel correction values , very often in block the car feels better ...more punchy , smoother makes power easier , got to be worth a try ...will put management light on but that doesn't matter.

Keep at it ....! !