The Stovebolt Site Forums The Shop Area Engine Shop Fuel tank pressure

I replaced the old loose cap on my 66 C-10 & I cannot believe the amount of pressure that the tank builds. It will actually try & blow out of your hand as you twist it loose. It is a vented cap but I guess I dont know what they mean by vented..vented to be means there could be no pressure or vacuum built. I took the cap off & I cannot blow through the valve but I can suck through it. I tried to change my fuel filter all the way up by the carb & when I released the clamp it was like putting you thumb over a garden hose. Is this just how they are or am I over thinking this?

you may be overthinking it - the vented cap is so as the gas level goes down there's no vacuum created which would make the fuel pump work overtime as the level decreased, until the vacuum created in the tank caused it to starve - the pressure buildup is prolly from temp expansion of the fuel - try opening it at the coldest part of the day [night] and see if it still does it - I'd think they'd not make it vent outwards to avoid letting the fuel vapors into the air, tho' my caps don't hold pressure like that, and you may have gotten a cap designed for a newer setup

if the fuel pump is good and there's no leaks in the system, it'll take a long time [if ever] for that line to the carb to lose enuff pressure that it doesn't spray when you undo it - it's the job of the needle n seat in the carb to hold that pressure back until the float says otherwise

Bill

Sounds good to me...all is functioning fine but it seem odd to me. The pressure at the fuel filter was not the average pressure, this didnt go away. I finally had to release the cap after loosing close to a pint of fuel waiting for it to slow but it never did. I have a friend at work with a 69 & we switched caps & his cap didnt build up nearly as much pressure, he was shocked at how much there was with my cap. Yes, this is during the heat of the day & it isnt as bad in the evening. Oh well, all is functioning good..thanks for the info Bill.

Drill a little hole in the cap. The caps we buy now are not designed for the older systems. That pressure can cause problems & is dangerous.

Bought an aftermarket vented cap for my 54, and kept smelling gas..couldn't find any leaks, and saw a post awhile back about drilling a small hole in the cap..Did mine about two weeks ago, and don't have anymore smell..

Where should I drill the hole? Somewhere inside the seal area I'm sure & I imagine the smallest hole I can make would be best.

Thanks

Don't know if I did it right, but it seems to work..I've got a stainless cap, and all I did was find the center, drilled a real small hole right in the top of the cap..I remember the old days, and all caps had a hole in them..Still looks good..Whatever works,Steve