Pressure/Vac in the fuel tank

[nick.parker]

Hello,
What systems are in place to limit the amount of pressure / vacuum that can build up in the fuel tank of modern EFI cars (or in my case AW11 MR2)?
If the carbon canister was left disconnected (the pipe that goes to the engines inlet system) would pressure / vacuum build up in the tank?
Regards, Nick

[JustCallMeOrlando]

Define disconnected? If nothing is connected to the pipe, it'll just be open to atmosphere.

[Unforgiven]

Yeh, i would imagine the pressure would drop to atmospheric pressure and then the whole EFI system would be useless. If you clamped the hose and then removed it then plugged it up, it would be a diferent story.

[CrUZida]

The above would not happen.

The tank is not pressurised in an EFI system, only the lines after the pump are.

If its disconnected, it would just be to atmosphere as stated two posts up.

[JustCallMeOrlando]

I doubt it would be useless. You'd have any excess fuel vapour going to atmosphere instead of the charcoal cannister.

[Unforgiven]

well it would probably still work but there is pressure built up in the tank, i guess not enough to make a difference.

[YLD-16L]

well it would probably still work

Not probably! It WILL still work perfectly fine.

My car never had a carbon cannister from the factory, it essentially just has a vent on the tank that has an elaborate path and then vents to atmosphere. It is EFI and works perfectly fine.

[nick.parker]

Thanks for the replies.
On my car the pipe from the cannister to the inlet system in pulled off (when I removed the solendoid since my car has a custom ECU). BUT I still get both pressure and vacuum in the tank under different conditions. Maybe my venting system is blocked? I will have to pull it all out and investigate.
Cheers, Nick

[SilverGhost]

I cant imgaine a bit of pressure in tank would be a bad thing...unless it puts back pressure on the return line, but I dont see that happening either....

[slide86]

the fuel cap is the thing that can control the vacumn in the tank. a lot of cars have a valve that is in teh cap to allow air to go into the tank to stop a vacumn from building up and sucking the tank in!

Yes, it happens. on commodore tanks, they suck in at the bottom and break the sender unit that has the pump in it. then the car doesnt go anymore! can be fixed woth a cap that only costs a few bucks!

[nick.parker]

Thanks Slide86,
I might try an aftermarket fuel cap. I admit I was wondering if the tank would get damaged/deformed slightly from experiencing excessive vac/pressure etc.
Cheers, Nick

[roadsailing]

if your tank is dented, pressure fluctuations can "oil can" it and cause cracks to form.

a mate had a similar problem, we drilled a hole in his fuel cap and then fuel sloshed out of it on left hand corners (duh)

a proper cap would probably work better.

My tank gets pressurised/vacuumised, apart from the funny "PSSHHHHH" noise at petrol stations it doesnt seem to do anything. think my charcol can is blocked, its pretty old.

and as others have said, having an "open" tank will not effect your EFI at all, the reason they are sealed is to stop the fuel boiling off, better to burn it in the engine as it makes you go further (car companies have to consider emissions as well)

[BeRad]

i had that problem my tank was pressurisin and fuel was pissing out of the carbon canister line, cause i had the carbon canister off the car, just replaced the fuel cap and was fine again.

edit :
its all been answered before haha

[Rex_Kelway]

Sorry (not sorry) to rehash this topic.

Rex has a problem. At the last Track-Day he was almost booted because the AW11 seems to have developed a nasty fuel leak at the filler.

Its a hard thing to diagnose because it only happens under track/very harsh driving conditions, fuel evaporates very fast and the sheer amount that appears to be coming out.

The rubber seal at the cap is 100% intact, so before He yanks half of the system out, If the cap has a one way valve in it, what are the chances of this failing and allowing it to leak?

AND/OR..

what else should He be looking at?

Cheers Lads,

Rex

[nick.parker]

Sorry Rex,
don't know the answer to that, but since this thread has been ressurected...

Define disconnected? If nothing is connected to the pipe, it'll just be open to atmosphere.

Well I found that you have to SUCK the vent hose on the cannister to make nasty fumes come out, when I suck there is soft clunk (a check valve I presume?? for when some dilberry's hose is left disconnected or perishes I suppose etc ) ...so I guess its not DIRECTLY open to atmosphere after all.

Cheers, Nick