2005 A8 D3 4.2 (BFM) - Low MPG

[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

[MatthewH]

Thanks, I'll look into the cam timing kits. Might be worth buying the tools as the belt will need to be replaced at some point either way!

I've been speaking to another A8 owner in USA who had the same symptoms as me - it turned out to be the primary fuel pump in his case. I did another fuel pressure test yesterday and the pressure does drop from ~51psi to ~46psi around 30 seconds after startup. The secondary pump masks the issue for the first few seconds as fuel pressure appears normal, but when it turns off the pressure drops. I believe the pressure should remain constant even when the secondary pump turns off.

I've purchased another fuel pump (£160) so I'll get that fitted in the next week and update this thread with the results.

[Mechcanico Lee]

Could you bring the secondary pump back in , leave it running then re check fuel trims , or recheck engine vac whilst keeping secondary pump running .

At idle when it's using very little fuel I cannot see a 5 psi drop in main system pressure affecting your vac so much , but you never know .

[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

[MatthewH]

Hi,

I tested vacuum again today with the new gauge I put together. It now measures -20HgIn which is a bit more acceptable.

Originally Posted by aTOMic

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?

There is no auxillary vacuum pump on these engines that I'm aware of. Just the "suction jet pump" that most VWs seem to have. This uses the "venturi effect" to increase brake booster pressure I believe.

Originally Posted by aTOMic

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

Luckily I'm only a 30 minute drive from Lee if my new fuel pump doesn't help things! I will update this thread once the pump arrives. Turns out I also need a specialist tool to remove the old pump and a new gasket which come seperately.

Originally Posted by aTOMic

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"!

If I remember correctly you can see the status of both fuel pumps from VCDS (on/off). As you mentioned there is no fuel pressure sensor for some reason!

Originally Posted by aTOMic

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?

The propane test didn't initially work for me as you mentioned - blowing propane into a small vacuum line did nothing. However, if I blow it directly into the throttle body or a large vacuum hose (PCV valve) for a few seconds it does give a slight reaction.

[Mechcanico Lee]

Hello Matthew , yes engine vac looks better there , is the guage nice and steady with minimal fluctuations?could you bring the other pump back in like I said to see what happens with trims ?

Fuel pressure .....does the fuel rail have a feed and return line , in most cases the pump or pumps will over supply the engine so a lot of fuel is returned back to tank , all engines are different but at idle you would get a couple of litres return flow back to tank , if there is only a dribble from return line you know you have an issue , so when engine demand outstrips flow this is where fuel pressure will go down .
I've seen fuel pressure at spec idling ,but on the road under load it can just drop away and you end up with the lean fault codes

Interesting one this keep at it , test ...rule out .....move on , every test gets you closer to the fix ....structured diagnostic pathway .....keep away from 'guess a part ' diagnostics

[Mechcanico Lee]

What's the specialist tool for pump removal ?? Is it one of the 3 legged tools to un do the big plastic nut on the tank ?? If so I've got one , if you need to use it give me a shout .

[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

[MatthewH]

Originally Posted by Mechcanico Lee

What's the specialist tool for pump removal ?? Is it one of the 3 legged tools to un do the big plastic nut on the tank ?? If so I've got one , if you need to use it give me a shout .

Hi Lee, sorry for the late response, only just seen these posts! Yes it's the tool you mentioned - it's a ring containing 3 gaps with 1/2" drive to remove the locking ring for the pump.

We swapped out the main fuel pump over the weekend and it's not made any difference unfortunately. Still averaging ~13mpg over a fairly long drive and fuel trims are at +15% (partial).

I've run the output test on camshaft adjustment tonight and it passed.

These only ideas I have left:
- PCV valve (I replaced this a few months back with a cheap aftermarket one, there was nothing obviously wrong with the original one but I've been considering buying a genuine replacement to rule this out)
- Suction jet pump 058133753D (common failure apparently, but it seemed okay when I had the intake manifold out. Still using the original part, considering replacing it for the same reasons as above)
- Cracked/leaking exhaust manifold (this would cause my symptoms - excessive oxygen fooling the O2 sensors causing it to run rich - but there's no obvious noise associated with this type of leak)

The only thing I've not mentioned is I'm using an aftermarket cone air filter (non-oiled) to tidy up the engine bay due to the amount of time I was spending removing and re-fitting the air-box while doing little jobs. The original air-box has been re-fitted several times (and codes reset) but the same symptoms still occur.

Lee: Are you free any time soon for an hour of your time? (paid for of course!)
I'm in the Bolton area so only 20 miles away.

Thanks

[Mechcanico Lee]

I have pm you