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Fuel coming out filler point under pressure, why?


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Today I noticed a strong odour of fuel when walking past my car parked in the driveway. On closer inspection (about an hour after I first noticed the smell) I saw fuel was dribbling out of the filler point. As I removed the cap what seemed like a litre or two of fuel poured out under pressure!


It was a very hot day (37C or 100F), car parked in the sun on a slight crossfall fuel filler on the low side. The tank was full but not overfull (filled to the first click on the bowser) and I had driven about 40km home the night before so it should not have been overflowing. I did move the car in the morning to a different part of the driveway but the car was only running for about 1 minute, not enough to heat things up.


Any ideas on what happened? 




My guess is that the combination of full tank and parking on a cros fall caused a vapour pocket on the high side of the tank to force the fuel into the filler pipe, but I am not sure why so much fuel came out under pressure. The crossfall on its own was certainly not enough to cause fuel to come out filler point. 


Also the hard line vent in the filler neck appears blocked. Should this be a free flowing pipe or does it have a one way valve or some other restrictor in it? 


Is it normal for 02 fuel systems to pressurise?  I have noticed a few times when refuelling that there is a slight over pressure in the fuel tank when removing the fuel cap. I know this is normal for modern cars but on a 1971 2002? At least it suggests that my fuel lines are still air tight!


Annoyed about the damage to the paintwork but I guess there is nothing I can do about this. 



The car starts fine hot or cold and has a weber 32/36 with mechanical fuel pump. 


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There is obviously a pressure build up.There must be a blockage in the tank vent pipe,so with the heat pressure builds up in the tank from evaporation and forces the fuel out of the filler as there is no other place to vent.You should be able to blow from where the pipe connects to the filler tube onwards

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Do you still have the stock pressure bypass-fuel return system in use with your Weber?  It uses a fuel return line, which changes from steel to rubber down under the battery and sends excess pressure/fuel back to the tank.  


That piece of the fuel system was removed on my car (not needed with the Weber) and the return line had been plugged.  In an attempt to resolve a hot start flooding issue I installed a pressure bypass fuel filter and put the return line back in use.  Well, not long after that, I started noticing a strong fuel smell in the trunk and pressure when I took the filler cap off.  Apparently, when a previous owner removed the emissions equipment and plugged the return line, they also plugged the vent line, which had originally been run to the charcoal canister under the hood.  It is a small plastic line above the brake booster on my 76, I am not sure about a 71 though.  I pulled the screw out of that line and the pressure problems went away.  (FWIW, the hot start issues went away, but I don't believe they were related to excessive fuel pressure).


I never had fuel spilling problems, but felt fortunate that I did not have any leaks.  


I am sorry to see what it did to your paint.  100% suck, (as a friend from Hong Kong used to say).



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 first click on the bowser


Well, I can't help you with your clicking bowser, but you've answered your own question:



Also the hard line vent in the filler neck appears blocked.


Is it normal for 02 fuel systems to pressurise?


Nope, that hard line is supposed to be open, at least to the charcoal canister, and air should flow through it pretty freely

if you try to blow on it.  Something's blocked that shouldn't be, and the vent line's not venting.


On early cars, that pipe simply vents to atmosphere...



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