The V8 Owners Forum -

Odd one here, I got home this evening, parked the car and started chatting to my next door neighbour 5 mins later I went back to the car and could hear a hissing, at first I thought it was a puncture, turned out to be the fuel filler cap hissing, not a vacuum but the tank was pressurised. opened the cap to a haze of petrol, reversed the car back into the garage parked up and the hissing was back, the car could not have run for more than a minuite max, Ive never noticed it before, suggestions???? as far as I can remember ther is no vent, unless its built into the cap.

Warm air temp ??

Possibly, but I would have thought taking off the cap, refitting the cap and running the engine for 1min would not cause the fuel to expand that quickly.

ian is there a vent in the cap small vent hole?
my tank has a small vent pipe from the top via a small type of valve.

I have asked elsewhere about the cap to see if there is a vented version, but why has the tank got a possative pressure, and where can the pressure come from as its in theory a closed loop circulating fuel, possibly its got a knackered non return valve in the pump so its pumping pressure into the tank. the pump has been making some funny noises recently.
Im thinking about drilling a hole in the cap just to see what it does. just as well I have a spare, but I cant see why its just started happening after 6 years.

strange,
the capri does it sometimes, mainly when its real low on fuel and its a hot day,
when you crack the cap it hisses loudly at you,

The return fuel temperature will be pretty high if the engine is hot, so it will definitely increase tank pressure. There should be a vent somewhere.

Chris.

Do you have fuel coolers in your system Ian? I'd say this was purely to do with ambient tempretures. Even when you vented the tank by removing the cap I reckon any air inside would heat up again pretty quickly if the tank was already hot. The noisy pump should be looked at too of course!

I've had a god few filler cap vents block up over the years. There is normally a pinhole that bypasses the rubber seal somewhere.

My old Chevy Nova used to do the same thing. I thought it should be a vacuum, but it were t'other way round.

I have managed to blag a brand new pump, and its a freebee so tomorrow im off to get the pump, so we will see what comes of this,

What car ?

All tanks should have a vent of some description.

Many modern cars vent the tank, thru a charcoal canister into teh intake manifold, controlled by a solenoid/ecu.

Older cars tend to just vent to atmos.

my tank does it all the time , in fact when i release the pressure you can here the tank ping !
i thought about drilling a hole through the cap but never got round to it yet
the trouble is with ciculating fule systems is that they draw only fuel from the bottom but return frothed fuel hence air/ fuel to the air space at the top of the tank , nothing to do with heat as mine does it even when it's freezing cold outside

Hi All,

Well forgive me on throwing a spanner into the chafing dish but....

Has anyone maybe missed that the pressure build up could be from a small air ingress into the return feed drawn in by a sort of venturi effect on the return line.

It is easy to confuse vacuum hiss on opening the filler cap with overpressure hiss, but the cloud of petrol is a pretty good indicator. Thank goodness you were not having a fag at the time or you might have looked like the inside of a charcoal filter.

Just thought that it is important not to overlook the very rare but possible things.

I'll go and hiss off myself now lol.

Doc

Well now

I believe there is a purge valve circuit on these systems (I know there is on on my 1990 hotwire system which I have not connected as I have other vents built in)

I believe it works on the principle that 1) engine must be at running temp and 2) revs must be above a set RPM at which time the signal opens a solenoid that vents the tank / charcoal caniister etc into the inlet tract.


This would be to stop hydrocarbons exiting to atmosphere

Ian