2005 A8 D3 4.2 (BFM) - Low MPG

[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.

[Mechcanico Lee]

Hello Matthew ........that engine vac is LOW , been working on a 1200 corsa today it pulled 21 1/2 inches of mercury no problem .......a v8 should pull that easy .

Now... engine vac loss can be manifold leaks but we are sort of saying we have ruled everything out on that side ........ hopefully !!


Cam timing has a big effect on manifold vac so we need to check cam timing ....i know this is a task but has to be addressed .

Ok ...... so we only have cam sensors on inlet cams by the looks of your data , but what ever you do to exhaust cam that will effect inlet cam as it will move also via the internal chain drive .

Them timing figures .......... on the pd diesels that kw figure is called torsion value and its normally runs as close to 0 kw as possible , i have just posted them timing figures on my other forum where there is folk with way more knowledge than me so i shall see what comes back ...........do the cams have a variable adjustment by chain tension or oil pressure .

[MatthewH]

Hi,

I will check the vacuum on one of the other cars to confirm the gauge is accurate (it's a cheap boost gauge from my Mondeo which also reads vacuum). I've ordered a new high quality vacuum gauge so I'll check again with that when it arrives to confirm the reading is correct.

Yep I'm pretty sure vacuum leaks can be ruled out - I've spent weeks looking for one with no luck, pretty much everything in that area has been checked or replaced (except brake booster).

With the timing figures, I did read that these are supposed to be around 0. From what I've read, up to 5kw +/- is "within tolerance" but this was for the 2.7T engine.

From what I can see, the small timing chains are adjusted by oil pressure. See page 22 of the following link:
http://www.kpematop.com/S6/V8-5VEngine.pdf

I know it's a common problem on these engines for the "chain tensioner slide pads" to break and cause the chains to rub. It does rattle for a couple of seconds on startup but this appears to be normal (or very common at least!).
Not my video but it sounds similar to this on a cold start - maybe not quite as bad as this though:
https://www.youtube.com/watch?v=sUUw7ze5-3o

Could this affect the timing?

The timing idea does sound a good one as I think the belt was done at a small non-specialist garage. They're supposed to be quite hard to fit properly without the correct locking tool.

The only thing that gives me doubt is that the MPG seems to have got gradually worse over the last few months.

Is the info I posted earlier about the MAF sensor of any interest? (short-term fuel trims still high with MAF disconnected)

Thanks for your time

[Mechcanico Lee]

I would of expected fuel trims to have less correction with air mass diss connected , try at 2000 rpm see what they do then .

I'm wondering why if cam timing is out why ecm is not seeing it , if exhaust cam is off a tooth so would inlet cam be off , unless for whatever reason someone has been messing in the rocker covers with the chains and the inlet cam is correct but exhaust is off so that's why no crank / cam correlation codes .

Saying that I've had corsas and bmw mini with chains stretched to death a few teeth out and they have not flagged crank / cam correlation codes .

Can you get just the upper belt covers off and take number 5 spark plug out and get tdc with a screw driver down plug hole or get the pin that goes into the web it's a bar that goes across cams to check cam alignment I believe

[MatthewH]

Still waiting for the new vacuum gauge to arrive to get an accurate reading. I'll also test fuel trims with MAF disconnected at 2000RPM.

I don't currently have the tools to test cam alignment but I'll look into this some more. I don't think these engines have position markers either to check visually.

P1128 (Bank 1 Too Lean - Mult) came up again today.

I was thinking about the low vacuum and remembered these cars have a "suction jet pump" which increases vacuum to the brake booster I believe. The part number is 058133753B (revised version is 058133753D). I did some quick tests on mine while the manifold was off and there was no obvious leaks but I may replace it to rule it out.

Will also check fuel pressure while driving to hopefully rule the fuel pumps out.

If none of these ideas give me any clues then it may be time to take it to a specialist for diagnostics.

Thanks.