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On 7 Oct 2002, at 2:38, Shad Laws wrote: > Yes, all the spec books say 10.6 gallons. But I'm having a problem, and was > wondering if it was typical or atypical. By the time I get to a bit over 9.6gal > or so, the fuel pump starts cavitating (on flat road... inclined roads are > different, and there's a VERY interesting and entertaining story that follows > from this if anyone is interested... :-), i.e. the pump gets really loud, and > after awhile, you'll start misfiring. This is not normal. > Like a stock FI T3, my electric fuel pump is right below the tank. When I > was building the car, I did clean out the tank. And, I inspected it. It > hasn't been *noticibly* bent up to change the volume, and although it isn't > squeaky clean, there isn't anywhere even remotely near 1.0gal worth of rust crud > in there. > > I know that the FI fuel system was designed to purposefully have the intake > spout (the one with the "sock" on it) above the bottom of the tank to avoid > clogging with crud, but is it really by a whole gallon? I'm assuming you have already cleaned the sock. The normal FI pump will pull the tank completely dry; it is not set higher at all. I suspect there is another explanation. The FI tank has an insert, a kind of "cup" that sits on the bottom of the tank. It is open at the top to allow the gas gauge to stick down into this volume, too, but it has a special feature at the bottom. The normal FI pump circulates excess fuel around the engine and back to the tank. Where it returns to the tank, that inlet squirts the returning fuel past the double wall into the "cup" mentioned above. There is a small space between the cup and the bottom of the tank, so this returning fuel will draw gas from the main part of the tank and drag in along, into the "cup." When the tank is getting low, the fuel level in the cup will actually be higher than in the main part of the tank. You can actually see this if you look in the tank of a FI car while the engine is running and the tank is less than 1/4 full. (One of the effects of this is to make the gas gauge lie to us during this time.) This scavenges gas from the main part of the tank and keeps the pump primed until you're REALLY out of gas. Those of you with FI cars will also have noted that you can start these cars up on a nearly empty tank only to discover that the gauge reads nearly 1/4 full a few minutes later. This is simply because the jet pump has filled the cup while you were driving, pushing the gauge up. At this point it would still be a good idea to be looking for a gas station. If your tank was rusty it is likely that the passage from the main tank to the inside of the cup is at least partially blocked. In addition, I'm gonna guess that you don't have any fuel return to your tank, so you don't get the "jet pump" scavenging action. The result of all this would be that gas can only leak slowly from the main tank into the cup via leakage thru the rust driven by the differential head of the different fluid levels. This probably leads to starvation of your pump when there is still significant gas in the main portion of your tank. If you stopped and then looked down in the tank, it would probably have leveled out in the time it took you to get the tank open to inspect it, however. The space between the cup and the main tank wall will be very hard to clean. Chemical treatment MIGHT be effective, but you might be better off finding a tank in better condition. I have several. - Jim Adney jadney@vwtype3.org Madison, WI 53711-3054 USA ------------------------------------------------ To unsubscribe, E-mail to: <type3-off@vwtype3.org> For more help, see http://vwtype3.org/list/