2005 A8 D3 4.2 (BFM) - Low MPG

[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.

[Mechcanico Lee]

The electric vac pump is there just to assist servo , the engine should still make good vaccum regardless of that , all you are doing with checking manifold vaccum is ruling out any of the places that it can be lost , so gaskets , servo , carbon canister ....any take offs on the manifold , if all take offs are ruled out and vac is still low cam timing is next step

I've heard you can hire the timing kits from somewhere , basic timing needs to be checked first ,.....but in activations can you activate cam timing control advance to watch if it advances and returns to base positions , if inlet cam was stuck advanced it will affect vac .

Also you could drive car on road and watch short term fuel trims and see at what Rpms and load you get the most plus corrections

[aTOMic]

Originally Posted by Mechcanico Lee

The electric vac pump is there just to assist servo , the engine should still make good vaccum regardless of that...

Mechanico Lee, you are a saint for your thorough, scientific method of tackling this problem (should say these problems). Is there an auxiliary vacuum pump on the petrol engines? Should I disconnect it while checking manifold vacuum?

Originally Posted by MatthewH

...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.

Seems Lee is a better "specialist" than you'll find locally.

Can you see function of fuel pumps 1 & 2 on VCDS? I cannot believe considering the thousands of sensors Audi uses on this engine that there's not a fuel pressure sensor or two on this "BFM"!

I am having the same problem with my new 2004 A8L. Thrilled to have found this thread since it's so methodical and complete. Thanks, guys.
BTW, I'm lucky enough to have two '04 A8Ls; makes for easier vacuum hose routing, component replacement, and I may even try the MAF from the old one on the new one to see if that's the cause of my poor MPGs. The one with the normal MPGs won't pass smog test because of lean codes of which I cannot find the source. Now, though, I have a slew of new troubleshooting steps to perform.

When I did the leak test with propane, I could pull a vacuum line and hook the propane hose fully into the opening and the car wouldn't even shudder. Is this a symptom of something?

-Tom