Engine barely runs (MAF/O2/Relay/Pump) issues

[jamesb]

Hello! I am a long time lurker, but only just registered.

This forum has been a great help in helping me diagnose issues with the A8 and felt it was time to actually sign up!

I am trying to diagnose a fault with the A8 - basically it cut out one day and wouldn't restart. Got the car recovered and suspecting a dead fuel pump, installed a new unit.

After installing the new pump the car ran briefly, idled nicely and even revved up a few times before starting to stumble, barely tick over and then stall.

So potential causes -

Bad 372 relay?
Bad MAF sensor?
Dead o2 sensor? - A precat sensor had failed, maybe the other one has gone as well?

Is there anything else on the A8 that may cause these issues that I haven't thought about maybe on the ignition side, additional engine control sensors etc?

Any insight is appreciated.

Regards

James

[Ronin]

fill the car up well past half mark and see if it starts
if it does you have a badly seated fuel pump or a pump thats seated ok with bad seals.

It would make sense to make an introduction post to say where you are based and what car exactly you have.

[MikkiJayne]

Welcome As Ronin said, chuck a couple of gallons of fuel in it. If you have less than half a tank, a newly installed pump won't prime itself and will need some help.

[jamesb]

Thanks for the responses.

Will add an intro post shortly.

The A8 is a 3.7Q AKC engine.

After switching the pump I poured some fuel down the neck and jumped it off the battery to push the fuel through to the fuel rail, its not at 1/2 tank but fuel is getting through the the engine.

When I changed the pump I checked the seals on the assembly and they were all in good condition.

Will top the tank past 1/2 and see if that solves helps.

The only reason why I think there is a fault elsewhere is because after removing the old pump i tested it and it worked fine (maybe should have done that before changing it but there you go!).

[MikkiJayne]

The symptoms say insufficient fuel pressure. A failed pump often can move fuel with no load, but can't maintain enough pressure for the engine to run.

Did you use a genuine VDO pump?

Is the vacuum line to the pressure regulator intact?

[jamesb]

Thanks all for the advice.

Threw some more petrol in the tank (a fair bit actually as it was 1 notch above the red), got the gauge just under 1/2 and gave it a try. A few hesitant cranks, and a pump of the throttle and the engine came to life. Left it running on the drive for 20 mins and no issues, lumpy running or codes coming up. Multiple starts since and all appears ok.

Is there a recommended way to check how well the pump is seated? I was thinking about pulling the sender unit out and using a small extending inspection mirror to see if I confirm its seated right.

Seals were present and looked fine when I pulled the assembly, so if I can check the seating then it may mean the issue is resolved.

Regards

James

[Ronin]

If the long bolt screws in with the smallest amount of resistance then it's not seated right, that's what I found in my experience. It should just go in perfectly on first try. Depending on how full the tank is you may have a small chance using a mirror or some inspection camera. Also have a look if the piping at the bottom of the tank didn't come off

[MikkiJayne]

If it drops in to place for the last 2cm without having to force it, its in the right place.

The failsafe test is to run it until the warning light comes on, with a can of fuel in the boot just in case

[jamesb]

Thanks for confirming - after a bunch of jiggling and some swearing it just dropped into place so I am hopeful its seated right. The long bolt dropped in and threaded in without any effort. So its looking good.

Will keep a can in the boot for the next few trips.

Regards

James