Fuel Pump Blues, take TWO.

[adjuster]

Originally Posted by ainarssems

You can get new VDO pump same as original on aftermarket for around £120, no need to buy whole housing.

Have that pump sitting on my desk.

Problem is, the long bolt that secures the siphon cup to the pump housing, just spins the captured nut way down in the tank where nobody can get at it.... Without destroying the pump housing in the process...

Thus my problem/question/possible solution thread.

And I tried a simple test to see if the pressure regulator was failed, or allowing too much fuel to just flow back to the tank? Easy as can be, two short sections of hose, slipped over the jaws of a needle nose pair of vice grips.. and clamped the return hose down, and after a few tests, completely shut. (So all the fuel, only could go to the fuel rail.)

No dice my friends. The pump is failing to move enough fuel to supply the injectors, even with the return line clamped shut, so it's not the fuel pressure regulator, filter, or injectors.

IT's the pump. (That I can't get to without cutting up the housing...)

What's left of the pump still works pretty good however, I drove about 450 miles yesterday, at 82mph, with the check engine light flashing, and the engine running like crap until I slowed down, shut of the engine and re-started it again, then it runs fine, smooth as can be around town....

[Dezzy]

I'm sure Graynada replaced his sometime ago, i don't remember if it was a second hand unit from A8 Parts or some other repair. Might be worth asking him

[ainarssems]

Just a thought. If it's going around you can try turning bolt while pulling delivery unit up, it might pull the nut out of bottom adapter plate if its already loose.

Also here is pdf on testing pump delivery rate if you want to verify that it is definitely pump not delivering, it does require special tools but you can always improvise with whatever is available

[adjuster]

Thanks for the test proceedure. That's very interesting that they don't go by pressure, but by volume to an open hose.

Every other one I've ever done was with a pressure meter, and specific pressure range.


So, let's say it puts out the correct amount of fuel? What else could be the problem with my car?

It runs fine around town, but if I try and go faster than 65/70mph for more than about 10 seconds, it flashes the CEL, and then cuts injectors and the engine runs rough.

It also flashes the CEL if I go WOT through the gears for more than a few seconds. And I've noticed that this engine pings, and sounds like it's lean when you get on the throttle, like it needs more fuel. It is however very fast, and smooth around town, even if the CEL flashes on WOT, as soon as I back off to normal cruising speed throttle, it stops flashing, and just remains lit.

The RossTech scan indicates muliple random misfires. I've cleared it, and it comes right back.

And when I bought the car it ran fine up to 120mph, or on the very top end of WOT, it would flash the CEL a very short amount of time, but then went off, and stayed off. (I've seen that happen on my 2004 A8L too, so I figured it was perhaps the MAF? I've seen MAF codes come and go, but did not have the RossTech tool to check this car before I bought it.

If it's not the pump, what could it be? (I've changed out the filter, had the injectors cleaned and tested and I've replaced the bad vac lines etc. There are no vac leaks, and I've gone around the intake with carb cleaner, and there are no gasket/manifold leaks either.)

I'm stumped, but think it's the fuel pump starting to fail.

[ainarssems]

The amount is measured at the return after pressure regulator so it is flow at full working pressure whatever it is on these engines 3.5 or 4 bar.

[graynada]

I replaced mine with a new VDO pump but I am struggling to think of a way to get the old one out if the captive bolt is no longer captive, beyond what ainarssems has suggested in putting some tension on it. Plan B would be a replacement tank from A8 Parts, salvage whatever you need from the pump housing by brute force from the old one, fit you new pump in there, but given the 'saddle' nature of the tank I can't imagine they are much fun to change

[Goran]

I dont know if this is a rubbish suggestion as I dont know how the bolt set up looks, but would it work if you cut the head of the bolt off then somehow push the remaining threaded part through?