tester123

I am having this issue come up now where both fuel banks are running lean...

Is this related to MAF or other issues? I plan to check some vacuum / breather hoses that might have been missed from the last round... in the mean time, please share your insights.

jseklund

If it's consistent across all banks, I would look for a vacuum leak first. The MAF and fuel pressure would be the next places I would look.

tester123

Would a leak from oil cap / oil cap gasket do this? I am planning to replace this as it seems to be old - still looking ok though...

tester123

Today, I checked a bit more on these vacuum lines and the one going to Fuel pressure regulator with those 90° rubber lines seemed to be a bit loose and replaced them and few others that looked on the verge to go. Still, I am coming back with same code, both banks lean. This is getting rather frustrating. I read here on of the threads about "suction valve", could this be a culprit?

Also, took one of the vacuum lines of the valves towards the front end driver's side and applied vacuum with hand pump and it is holding perfectly without leak.

There is a valve (EDIT: non return valve) looks like white and plastic discs joined and vacuum lines going on both ends. One side (white) is going to engine manifold and the other connected to rest of vacuum lines. Hand pump vacuum is very slowly dropping on one side and the other side it is open. Would this have any effect?

tester123

Could someone please post a Complete vacuum diagram for 2001 A6 4.2 Auto? I can't seem to trace the vacuum leak (if there is one at least).

Apparently, the bentley manual (ebahn reader) I have only shows a complete diagram for 2.7T...

meternerd

Fuel filter? I had an old Audi 90 and that's the code it gave when the filter was plugged.

tester123

Don't have the tools required to conduct a fuel supply test... would have to wait until weekend for that. In the mean time, I checked the MAF and it seems to be within (4.56~4.65g/s) the specs (3.5 to 7.0 g/s) at around 720rpm under normal temp. Did not road test with the MAF sensor out of the air inlet plumbing to see if the results would vary. However, disconnecting it resulted in a noticeable shudder and loss of engine performance.

Even after multiple restarts, the code didn't come back again. It seems like it would happen only after a road test - which needs to be confirmed.

Redneck Truck

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tester123

Thank you for the diagram. Won't be able to test things until weekend; will post my findings.

4Driver4

I agree. Vacuum leak is the most likely issue, followed by MAF. Fuel pressure is a distant third.

VAG-COM measuring blocks can be used to check the MAF, which may provide insight into MAF functionality or leaks.