i replaced my oem fuel cap on my eg with a similar aftermarket one 4 days ago. i was complaining that when stopped, i can sometimes smell fuel vapors from my fuel door. i had the dealer take a look and he suggested that i replace it with a new one as the rubber o-ring on the cap seems to be loose already. today, after not driving it all day, fuel was dripping out of my fuel door through the cap! when i opened the cap, fuel rushed out as if there was pressure from somewhere that pushed it all the way up! fuel was right to the lip of the filler neck. never happened to me before and certainly had the dealer scratching his head too.

the dealer explained to me that it was probably the breather tube got clogged or the vapor return line from the fuel rail/intake manifold got clogged thereby making the pressure build up. however, another mechanic thought it was the new fuel cap that caused the problem. although after putting the old one on, it did seem to go back to normal.

so it seems that this approach fixed things, i just cannot quite understand how a fuel cap can suck(?) fuel up? i mean, i understand it might leak if its the wrong size/seal but actually suck the fuel up to the neck while just sitting in the garage? is there something in the design of these things that alters the pressure or whatever inside the fuel tank system perhaps? the thread looks the same although the design of the inside of the thread is different between the two but i am no hydrodanimicist or whatever so any enlightenment is most welcome. i hope i don't have to have my entire fuel system overhauled.

 

I think the first mechanic has it right in saying that the vapor control return line isn't working. You don't have the problem with the old cap because it does not seal the gas tank and permits gas vapor pressure release. The new cap probably functions properly and seals the gas tank, resulting in abnormally high fuel vapor pressure due to the fact that vapor control line is clogged. This is possibly caused by a faulty two-way valve:

 

Aftermarket cap? like what? A locking gas cap or what?

 

thanks ronj for the diagram. i actually just put the old one back on because that's the only "doable" thing for me last night. actually, when i did that, the fuel went back down and no leaking whatsoever and no smell of vapors. drove the car all day and everything seems alright. weary of the possible changes, when i returned home, i opened the cap and the usual hissing noise came out which is obviously vapor venting from the tank. isn't this an indication enough that the old cap still seals it well or maybe that there are minute leaks in the tank that seems to mask the real problem with the return line?

except for the first mechanic, i asked 3 more and they all blamed the new cap for that. although the designs of the innards of both vary, the old one was simple with a plastic cap with 4 little holes in it, the new one was more elaborate and more hollow on the inside. i don't know now if this is the possible issue? one of them had the same problem, although in a motorbike, he said he replaced a cap because his was venting fumes, when he put a new aftermarket cap on, same thing, fuel gushed out unprovoked.

yes, its the locking, clicking type. no keyhole though. honda oem cap for the '93 eg

also looking back, i refuelled to full tank (one click) 4 days before the event and i wonder why did it just manifest itself now that i haven't even driven the car all day yesterday.