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<x-charset iso-8859-1>Hello- > > 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. Well, that is the normal way to run out of gas with a high-pressure, high-volume, Bosch CIS pump (i.e. not like the little D-Jet pump). Instead of the D-Jet fuel system, I use a fuel pump, filter, and accumulator from a Mercedes CIS car. True, my car isn't CIS, but the Bosch CIS pumps are an excellent buy, as when run below the spec'd ~75psi for CIS, they put out lots of flow. And, since they're not made from a one-piece billet or anything fancy like that, they're a heck of a lot cheaper than the ultra-cool (and expensive) aftermarket FI pumps. The only unnormal part is it happening at 9.6gal :-). > I'm assuming you have already cleaned the sock. When the symptoms occur, I pull of the gas sender and look down in there. The sock isn't clogged. There isn't *that* much debris down there. > The normal FI pump will pull the tank completely dry; it is not set higher at > all. If I pull off the gas level sender, I can peek down inside there and see that the line for the pump inlet is ~1/4" higher than the bottom of the tank. Is this the abnormality? > 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. How big is this cup? Would I be able to see/notice it from the gas level sender hole? Or, would I only see the cup? > 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. Aye, this happens :-). > 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. That's possible. > 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. Why not? I may have tossed the T3 engine and the D-Jet aside, but my T4 is FI'ed, too :-). > 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. I've pulled off the gas sender when I've "run out of gas" with some still left, and can see that the fuel level is right at the top of the output nipple, the one that's 1/4" or so above the bottom of the tank/cup/thing. > 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 first time it happened, I stopped and inspected it while the pump was just staring to get loud. My first guess was that I'd kinked the suction like to the pump, but after inspection that wasn't true. I pulled off the gas level sender, and I inspected it with the car running. The pump was loud - air was getting in there. And the gas level was just *barely* above the top of the nipple, i.e. so little that it looked like the walls of the tube saw fuel and the middle didn't. > 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. I might have to figure out something like this. <sigh> Thanks! Take care, Shad Laws LN Engineering - Aircooled Precision Performance http://www.LNengineering.com ------------------------------------------------ To unsubscribe, E-mail to: <type3-off@vwtype3.org> For more help, see http://vwtype3.org/list/ </x-charset>