sleepingvr4

Replaced the fuel pump and fuel filter today and no luck.

The car cranked and cranked but wouldn't fire. I'm kinda stumped at this point. I know other members suggested the 02 sensors. Could it possibly be the MAF? Do the D2 S8's have a MAP sensor?

Any ideas?

Oxnardo Ring

Ouch. Well, I'd only suggest the basics-- fuel and spark?

If you crank it and give it a whiff of starter fluid will it fire at all? If it does run for a couple of seconds on starter fluid (I hate using that stuff, btw) then at least you know you have spark.... going down that tree I'd suggest checking fuel pressure up at the rail, assuming the pump's running (do you hear it WHIR on when you turn the key?) and the fuse and relay are fine.

Down the "spark" path, I'd make sure the fuses to the ECU were fine and make certain the ECU was alive enough to report realtime out of the ECU parameters as you turn the key ON.

Y888

How much fuel in the tank?

jfrahm

Good point, maybe the scavenge system needs to be reprimed or the pump is not installed just right.

sleepingvr4

Yes I can hear the pump running. I was very careful when putting it back together and triple checked everything. I'm confident the pump and assembly unit and correct.

According to my fuel gauge the needle is one notch above the red empty mark? Are you telling me the pump needs to draw fuel from a higher point in the tank before it will scavenge from a lower point in the tank?

Y888

You didnt mention replacing the gaskets so it might be pulling air somewhere,add fuel half a tank at least and you will see if that is the problem.

sleepingvr4

Yes replaced all the gaskets as well. And I will definitely add fuel to the tank.

Where is the MAF located? If both if both upstream 02 sensors failed would that keep the car from starting as well?

jfrahm

Yes at that level you need to reprime the scavenge system to get it working again. Sloshing in a couple gallons of fuel should do it, some of the fuel put in via the fuel filler is directed to the fuel cup. You might need to do that 2-3 times to get all the air out of the scavenge system.

I have never seen O2 sensors keep a car from starting, you are in open-loop at startup. Maybe there is a way to miswire them to cause enough of a problem to prevent starting but I think you'd get a CEL for that. Horses not zebras, get to the point of being certain you have a working fuel pump and scavenge system first.

I do not recommend putting in tons of gas until you are sure the scavenge system is working. Just put in 1/2 gallon, maybe a gallon at a time 2-3 times if need be until the car will drive with the low fuel light on (or down near the red on the gas gauge).

Oxnardo Ring

Oddly, the 'fuel pump writeup' i was intending to do was to reiterate the view that it takes a lot of fresh fuel in the tank to ensure the "pump-in-the-cup" gets enough to re-start.Unless you put enough back in the tank to make sure the "right fuel tank lobe" (where the pump cup assembly lives) is adequate to overflow into the cup-assembly, I'm not certain you can guarantee it will re-prime.
The one I got resuscitated the other day, I ended up putting 9 gallons in-- plus "jumping the pump" from the connector to get the air out. Two rounds of that, hearing it "gurgle-gurgle-gurgle".... It started right up after that.

Part of the issue is the limited time the fuel pump is energized to run, before getting a start. If the fuel pressure on the rail doesn't get to the right level fast enough, you don't get a start and it won't sustain. I just pulled another assembly today in one of the A8Ls and noticed that the only line that is guaranteed to dump back in the cup is the REturn-Line from the rail. thus, if you are not getting a lot of pressure the rail, if you are inadequately primed, then you are likely getting low flow back into the cup. So, I'd double-down on the recommendation to make sure it's primed and make sure there's enough fuel down into the cup area.

And, yeah, this fuel pump/siphon/scavenge system is a weak point in a stellar car. I think they went to a straight lift pump scheme in the D3 didn't they?

audinutt

[QUOTE=Oxnardo Ring;25183375]Upstream O2 sensors do all the heavy lifting. The passenger side one is a PITA. It IS a long shot, admittedly, but it's easy to RESET to factory fuel trims without a lot of work. Much less work than swapping the fuel pump anyway.

/QUOTE]
I beg to differ, I was able to change both upstream sensors a couple weeks ago in under an hour. The key is patience and preparation...

I used this guide...
https://forums.quattroworld.com/a8/msgs/49929.phtml