2005 A8 D3 4.2 (BFM) - Low MPG

[Delboy]

If you didn't already know there is a thread with all the factory repair info for d3 cars.

Note there are separate pdf for engine and fuel etc so go through them and save what's relevant to your engine

http://forum.a8parts.co.uk/showthread.php?t=9347

[Mechcanico Lee]

Righty hoe .....some good info there for us to work on

Fans on .....code for high pressure switch short to ground ,I reckon this is the driver for them fans on ...look at the A/c pressure ...more than 51 bar !!! Massively over pressure for a/c high side ....no wonder it's running the fans but that pressure is a consequence of a faulty pressure sensor ...high pressure sensors normally on the air con condenser somewhere or on pipe work from compressor high side to condenser so you need to do some investigation into that .


Lamdas codes and diag
The after cat lamdas are spot on but the cats are very good at masking the real story ....remember lamdas measure oxygen .....not rich or lean conditions ,even though they can show rich or lean conditions ......for instance ....

Let's say you have an engine that's running at its correct mixture 14.7 to 1 but you have a leaking exhaust manifold or a cracked manifold or SECONDARY AIR INJECTION bleeding air in ,all of these being before Pre cat lamdas so the mixture as read by the lamdas is lean but the engine is running at the correct air to fuel mix as we know ..... we are getting ' false air ' so the ecm will respond by adding fuel to get the mixture correct but it can only increase injection duration so much before it goes out of its 'working window ' , when this happens then ecm will set ' system to lean codes '

I'm not saying it is the air injection system but we 100 % need to rule it out , they normally have reed valves to stop blow back so blowing down the pipe may not be enough to open them , can you bung the pipe up some how do we know no air can get into the manifold and affect the lamdas .

If you do this and we are still getting lean mixture codes my next tests would be to smoke test the manifold ....check that oil filler cap is not getting sucked down to hard ....eg . breather system not leaking to atmosphere or pcv pods not letting loads of vac into engine , fuel pressure tests and so on .

Are the new lamdas correct for chassis number are they the same part number or equivalent to what came out ?

Some things to have a go at
Lee

[MatthewH]

Thanks for the info Lee, very helpful.

I've looked into the air-con high pressure switch in the past, I've found the part number and it's a fairly cheap part (for these cars!). It's located in front of the condenser so it requires the front bumper to be removed for replacement - not a huge problem as I need to remove the bumper at some point to replace a couple of front parking sensors anyway. I'm assuming this is the cause of the fans running constantly but I'm not sure this alone would cause the MPG to drop so much. Of course it would be useful to fix the fan problem regardless so I'll try to get that sorted soon.

I understand what you're saying about the secondary air injection. I was actually planning on fitting a bung to the SAI pump outlet hose to rule out any oxygen entering the exhaust from that area but after blowing through the pipe I assumed that wouldn't be the problem. If there's some sort of check valve as you say this could still be an option. I'll measure the pipe diameter tomorrow and find some form of bung and try again.

I had also considered exhaust manifold leaks but I thought I'd hear any leak if it was bad enough to be causing these problems so I didn't look into it any further. There's no obvious exhaust leak noises.

I'm thinking the problem is something along these lines as there's no obvious loss in performance, the lambda sensors just seem to be picking up excess air from somewhere and dumping in fuel to compensate.

I have already replaced the PCV valve as a precaution but there doesn't seem to be excessive vacuum in the crankcase, the oil filler cap feels normal while the engine is running.

Fuel pressure appears to be fine from testing with my homemade gauge, but it does have 2 fuel pumps so it's possible this be masking an issue.

Thanks

[MatthewH]

Forgot to add: the O2 sensors were replaced with genuine Bosch ones (same part number as originals but without the Audi logo).

Thanks

[Mechcanico Lee]

Same part numbers ...great














The red coloured vac pipes to the combination valves ...diss connect from the valves and block off the rubber vac pipe with bolts or whatever .... let's see what happens.

Have a look at the pre cat lamda values after doing this

[MatthewH]

Hi Lee, thanks for the diagram.

I have blanked off the vacuum hoses to the combi-vales (red in diagram) and also blanked off the 15mm air hose (blue on diagram) to prevent any air reaching the exhaust if the combi-valves were stuck open.

I've just finished a 15 mile drive since making these changes and checked the results:

Engine Block 032 (Fuel Trims):
Bank 1 - Idle => 1.4%
Bank 1 - Partial => 15.4%
Bank 2 - Idle => 0.9%
Bank 2 - Partial => 10.2%

I reset the engine module and checked the short-term fuel trims again. The values at idle were:
Bank 1 => fluctuating between +18% and +24%
Bank 2 => fluctuating between +10% and +15%

As you can see, both banks are reading lean but bank 1 always seem to read slightly higher.

I also checked oil temperature as you mentioned a few posts back. A couple of minutes after the above 15 mile journey, the oil temperature was 89 degrees C. After a quick drive up the road it increased to 91. What was your theory regarding oil temperature?

Thanks

[ainarssems]

Looks OK for the idle but a lot for partial throttle and as the both banks are affected I would be more inclined to look at fuelling rather than air leak.

You say fuel pressure was 4 bar without FPR and 3.5 with FPR. Is in not supposed to be about 7-8 from pump and 4 with FPR? Any possibility to measure it while driving to see if it drops at load?

It could still be air leak in exhaust as it has had work previously done to it, could be a hole or maybe air leaking in through the O2 sensor thread. Leak can even be downstream from O2 sensor as the exhaust pulsing sucks some back higher in the exhaust. I would expect it also to affect idle but maybe at idle there is not enough pulsing to suck it upstream.

[MatthewH]

Hi, the fuel pressure reading seem correct according to this page:

http://workshop-manuals.com/audi/a8_...ing/page_5971/

I did think it may have been some sort of exhaust leak but the emission results from the last MOT came back perfect so I think the welds were okay. It was also a bit quieter after the flex pipes were repaired and no noise seems to have returned since.

As the car has 2 fuel pumps I haven't yet ruled out a fuelling problem. I think there is a main pump and a secondary pump. The main pump runs at all times, and the second pump runs for the first 30 seconds and then only at high load, although this is sometimes disputed depending on where you read and some people say both pumps run at all times.

I don't fully understand why a failing fuel pump would cause low MPG as you'd assume it would actually be the opposite - maybe a low fuel pressure could cause an inefficient spray pattern on the injectors.

Thanks

[Mechcanico Lee]

Ok , so air injection ruled out if we are sure there is no other exhaust leaks we can move on .
If fuel pressure correct to the spec , like airnassems says there drive it on the road with gauges taped to windscreen make sure fuel pressure stays constant no drop offs .

At idle the trim is pretty near 0 correction so engine side of butterfly must be pretty much sealed as air leaks show more at idle as the closed butterfly is keeping the vaccum up ....the Air is looking for a pathway out in this condition open the butterfly then you allow manifold vac to go more towards atmospheric pressure and air leaks don't show there face as readily .

Can you give me the kg/h or gramms per min that the air mass is reading at idle then at 2000 rpm .
Then drive car on the road and where you see the fueling trims going the biggest plus correction make a note of the rpm and what the air mass is reading at that time .

Pipe work from throttle butterfly to air mass good and sealed ?&
Pipe work to evap canister good no leaks ...if in dought blank off at manifold but leave fuel filler cap loose to allow tank to vent if your driving around .

Does vcds give injection milli second reading ?

You could try driving with air mass un plugged this should default it to a richer map at the lower throttle areas then see if your fuel trim values start to go more to 0 correction or minus readings .


Oil temp seems ok , I cannot see any radiator outlet temp on the engine diagrams .

[ainarssems]

I don't fully understand why a failing fuel pump would cause low MPG as you'd assume it would actually be the opposite - maybe a low fuel pressure could cause an inefficient spray pattern on the injectors.

If the mixture becomes too lean engine starts knocking and ECU retards ignition timing to prevent damage. In result burning starts very late and more of the energy goes in the exhaust and less to the wheels. So that another thing you could look at is measuring blocks for knock retard and knock sensor voltage.