Fuel Pump Blues, take TWO.

[adjuster]

Okay, So since I bought my 2001 S8, the CEL would flash on WOT, but then go away, then it flashed at speeds over 120mph, and was fine at "normal" highway speeds.... (Around here, it's common to drive 82/83mph for hours and hours and hours.)

Well, I ran a few gallons of toulene in there to see if that would clean out any gunk, and it sure did. BUT it also took some life out of the pump too it appears, as now it's not happy if I try and go WOT or drive faster than 70mph...

So, I bought the wrong pump, then got the right pump, and when I went to remove the long bolt... POP, it's just turning the captured nut down on the venturi/siphon cup assy....

So, I have a car that runs great around town. And even at 55mph. But want to faster than 70? No dice. Want to acclerate at WOT for the onramp? No dice. (But it's plenty fast the rest of the time, and actually goes just fine, just flashes the CEL lamp.)

Here is my dillema.

The new housing is 800.00 or more.
The used ones are about 350.00 to 375.00.

I can get a in-line 255lph walbro pump for less than half that. (About a third actually.)

The pump in the tank/housing and siphon system all work just fine.

It's just not moving enough volume/pressure to feed the engine's needs at higher RPM, and under sustained load from what I can tell.

The in line Walbro would "suck" on the feed line to the injectors, I figure I'd mount it just after the stock Fuel Filter, in a section of "hose" there where it can be spliced in with minimal fuss.

Hook it up to power with a relay wired up to the stock system so it's only on when the stock pump is on, and it SHOULD be able to keep up with this engine's needs no problem, and I'd hope, the lack of "resistance" now on the old/stock pump in the tank would be able to allow it to feed the new in line Walbro just fine for many years to come.

Any ideas on this?

Has anyone done anything like this, or even considered it?

In my Supra days, we used to put twin in-tank Walbro's, and a few people did external fuel pumps with custom pick-ups etc. (All of them feeding 650rwhp engines needing far more fuel than this 360crank hp rated V8 will ever need.)

I know the in line Walbro's don't like to suck on the tank much. (IE: You want them close to the tank as possible, and have a easy source of fuel, but I've seen them draw through existing fuel pumps/systems with pretty good results, and where the stock pump on this A8/S8 is such a over-engineered mess, I'd rather just use it as the feeder system for a high quality external pump if I have to build my own, or spend 500.00 or more to try and get what's in this car to work correctly.

Thanks in advance for your advice guys.

[ainarssems]

Have you changed fuel filter? toluene is more lubricating then petrol so it should not have negative effect on pump. But it is possible that it lifted gunk from tank an blocked filter.

[adjuster]

Yep, sorry I left that part out.

I've changed the fuel filter out, and that had no effect, but there was some "jello" like crap in there, and it was very dark with crud. (I cut it open and inspected the paper media, and very little large sediment was in there, but the paper was dark with fine material that had been trapped. It's either the worn brush material, or metal from the communtator?)

I have no idea what the jello like crud was, it competely evaporated away when I dumped it out onto my concrete driveway.

So, I then pulled the fuel injectors, and had them cleaned and checked. They all passed 100%, no resistance problems, and only one picked up 1cc of flow improvement, so they were not dirty at all. (The shop owner told me there was none of the jello crud there at all, and no debris in the filters at the injectors, so the stock fuel filter had done it's job.)

That just leaves the pump starting to fail, or not producing enough volume to supply the engine under certain loads/RPM from what I can tell.

It does not run out of gas, even when it's down to nearly completely empty. When I tried to pull the pump, and the bolt just turns, I had it down to about 1 gallon. I put 21 gallons back into the tank when I filled up after realizing that this pump would not be coming out without surgery anytime soon.

I drive it nearly every day, around town it's fine, powerful, and runs perfect. But put it on the highway at speed, or race someone at the light, and it will flash the CEL light. (And on the highway, it drops out injectors, and makes the engine run rough until I shut it off, and start it again, then it's smooth as usual.)

[Conan_the_Librarian]

Just another thought; is the fuel pressure regulator lifting to early so not enough pressure is being delivered to the injector rail?

Adding a high pressure pump won't help if the regulator is not allowing pressure to build up in the rail.

[adjuster]

Good question? I don't have the fitting(s) needed to hook up into the fuel line, and check the pressure at the rail.

If the pressure regulator is not working, that would release too much pressure back to the fuel tank, and create a lean condition under load. (Good call to rule that out before I do anything else.)

Guess It's time I have the pressure checked, or buy the fittings/gauge, and check it myself.

Is it pretty common that the fuel pressure regulator fails on this vehicle? I've never seen one fail on any vehicle, they are pretty simple devices, just a diaphram and spring that regulates the pressure. (I've even seen ways to raise fuel pressure, by crushing the stock Toyota units, on turbo cars. **Using a vice, and socket, you crush the metal housing, compressing the spring more, and raising the base fuel pressure.**)

There are no leaks at the rail, or pressure regulator. And all the vac lines are hooked up at the regulator too, but I'll check the hoses again.

[ainarssems]

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

[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.