Kamaro

BizarrEftersmak

Are you saying that there is no fault codes at all?
That seems strange, since an A8 is rather sensitive and throw codes if it can't read the navi dvd.

I hope you have some kind of code reader?
Preferably VCDS or OBDEleven - which by the way is getting better by each update and since it's always in my car and i always have my mobile on me i tend to use it a lot more than the former - but even a cheap ebay-reader should work.
So buy/borrow/steal a code reader and check codes!

mixja23

Worse than a woman...!!

MP4.2+6.0

As implied by prior answers get a real diagnostic code tool. If you are only relying on CEL and feel, not close to good enough. A guess might be fuel pumps.

volvopentaman

If ´really no DTC:s are stored and the engine stalls and won't start, I would blame it on the crankshaft sensor. This sensor rarely throws DTC:s, but is the single sensor the engine can't run without at all.

Also dead fuel pump might cause the same issue, or a faulty throttle body.

MP4.2+6.0

+1, yep the flaky crank sensor is the other best good guess to me as flagged here.

IndyAudi

MP4.2+6.0

As volvopentaman identified, the crank sensor is one that may not throw any codes. It has come up on this board a bunch of times over the years now. You go back to good old (old school) diagnostics that seem increasingly lost in the app and scan era, and you go with best judgment/guess. To your analogy, the doctor with the iPad in his hand given to him by the IT guy last year is saying take a look at this screen of treatment options and tell me what looks right. The patient responds, perhaps you notice from the quaint paper chart you must have somewhere--or maybe that red tipped cane in the corner and dog on the floor--that I am flippin' blind!

The whole engine data gathering used for fuel and ignition ultimately relies on knowing where the motor is cycle wise, and without the crank sensor it is likewise blind. But car may not know it is blind sensor wise from what the field experience says. Like the stroke patient who may not realize it. Could argue they should have added redundancy and cross checking via cam sensors or some such, but they didn't. In reverse, there is cam sensor checking like that. Meantime, its a known failure point on older 4.2's, and known that a scan may not see it. Flaky start cold vs. warm is one of the classic old school diagnostic recognition points for "seeing it." As I mentioned, fuel can be another, and there likewise the sensors are not really set up for fuel flow IIRC, but rather for lean/rich as takeoffs from the O2 sensors and air flow from MAF. And again, known experience from the board says suspect fuel pumps. If it varies depending on tank fill level, suspect transfer pump; if it is flaky regardless of fuel level, suspect main one. If it is an FSI motor, then layer HPFP's on top diagnostically. Set up seems just to assume the fuel is flowing, like it assumes the motor can see its position.