gas pumps question!

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Hey! My sister and I own all older cars. We're poor, what can I say. But we keep good care of them. Well, this summer, I replaced the tank on my 1990 colony park. Guess what? It leaks fuel now. First, it was the donut-sized grommet that the filler neck went into. It's always partially submerged, it turns out, and after being once-disturbed, the still pliable but not quite pliable enough 20-yr-old grommet let fuel leak past it. Replaced that. Still can leak... turned out that it fills the tank to brimming before the pump shuts off, consistently. It's coming out of the evap hookup, which is a loose barbed plastic fit and not pressure/fluid-tight at all. Probably. Only after a complete fill up, it drips out of a location that does not suggest the pump and IS not the level sender, leaving only the evap line as a culprit. Also, it only leaks after a drive (and the fuel has expanded, a little), and only after a completely full tank coming from the gas station, suggesting, consistently after several checks, that it's the brandy new dorman fuel tank now overfilling to the brim where before it did not. It ceases to leak at all once I've burned a couple gallons (at a guess). Flash forward. This November I replaced the fuel tank on my sister's 1989 mercury tracer (mazda 323). Completely different car. This fuel tank was actually a mess-- on my colony park it was more for peace of mind. Rusted clean through in 2 spots, including the evap hookup. Seeping fuel out of the welded seam. We only found out, when we dropped it to get to the brake lines (also imminently about to rust through!). Also did the evap line, which was metal and rusted through. Fun fact. New $100 NAPA fuel pump lasted all of 1/2 a second, pumping just 6" of fuel out before failing --yes, we installed it correctly in ever detail-- had to drop the tank again and put the original back in. We did this to replace the tank, and the new pump was just because. Too bad we had to do the job twice frown Now, hey, it looks like the tank actually PRESSURIZES after a drive. Remove the gas cap, and it hisses air out at you. You can feel the puff and hear the noise of evacuating pressure. Tells me plainly that it's sealed where once it had all those holes-- if pressure build up is also a problem and a symptom of something wrong that I should know about and correct, like a stuck evap solenoid, PLEASE let me know. And furthermore, now, it overfills. And not just to the top of the tank like my colony park. It will go nearly all the way up the filler neck on this car! So she guesses at gas used and only puts in maybe 8-10 gallons instead of 12+. Btw, still pressurizes no matter how empty the tank is: not just when it's been overfilled. It's really annoying to have to guess at how many gallons to add! I liked it when the pumps just clicked off when they were supposed to. What is going ON here, with both cars I put new tanks on! I hooked up everything very carefully, firmly, properly, and highly doubt an install error. I looked up how the automatic shutoff valve works.. has to do with rising vapor pressure as the fuel tank gets more and more fuel. But these tanks are plain jane. Originals and replacements, both were just tanks with some holes in them. No internal baffles (as one old forum post tried to talk about), just steel cans to hold gas. Can't really imagine that dorman or spectra "built it wrong," in any way that would cause it to overflow-- it was identical to the original for both cars. We may just have to live with guessing at how much gas to add without overfilling/overflowing, but I'd like a little further depth in explanation and enlightenment as to how automatic shutoff works as the gas pump, and why both these cars may now have this concern. No mechanic I've spoken with yet, can provide a real explanation. And I'm an ase master tech myself (good with book knowledge and tests.. not yet quick with my hands). This never was covered in my textbooks or the tests; how pump gas works and what might now be wrong with our tanks. By the way, if it's relevant for the mercury tracer: there are two necks coming up from the tank to the fuel door. One's for the gas, as the main filler neck, and the other, about half the diameter, MUST have to do with that vapor pressure/ shutoff system. Yes, both are firmly installed with quality replacement hose clamps. The colony park just has the one filler tube going into that silly grommet I had to replace to stop leaking.
 
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You're not the only one to notice EVAP issues on older cars. They're the bane of high mileage OBD-II vehicles where the system has to pass a state inspection. I had a mazda 323 that leaked gas when filled. See I'm not the only one. AFAIK the 2nd filler hose is an "air comes out" one so the "gas goes in" one doesn't get airlocked/ start bubbling/ slowing things down. Has your sister tried holding the filler "gangsta style" (sideways) or just barely pushing it through the unleaded restrictor? Sometimes this helps. Maybe the colony park has a much wider hose than the unleaded restrictor so they designed it to pour straight into the tank and there wouldn't be airlock problems. There's a "rollover" check ball on top of a lot of tanks that IMO jams and leads to slow fillups. Vacuum diagrams for fuel systems I've pored over over the years have a couple-psi relief valve. They want it to go to the charcoal canister but that relief valve should blow off and maybe it is for all we know, and leaving your tank at 3 psi or so. As you know with old stuff you can get wasp nests in weird orifices and stuff just jams.
 
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It's pretty easy to damage one of those Ford filler necks if you try to remove it without first cutting the grommet. Unfortunately, you'd have to pull it back out to check it.
 
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