Hi Guys

Question on fuel tank pressure. sometimes I open the cap and there is significant pressure in there, as in the cap nearly flies out of your hand. Other times there is no pressure at all.

It doesnt seem to be linked to hot days, low fuel high fuel etc. 

I am assuming that on the times when there is no pressure maybe the cap is not on as tight as it could be.

Question: is pressure in the tank part of the fuel injection system, do you need the pressure there to help the car get the fuel through, is it part of how things work.

Just after some info.

Thanks in advance Guys.

High pressure isn't part of fuel injection. The tank breather will be blocked. The down sides to this is, long term, the tank may split(as pressure goes up and down, the tank slightly expands and then back to normal. But over years). Also with extream high pressure in the system, may run slightly richer...but not to common.
     General run down of the system and set up is,
          The tank has two breathers, the large one is usually near the filler neck of the tank and goes to the fuel cap. This one is to let the air out when filling up.
           The second, and usually the smaller one, goes to a carbon canister(or sometimes, depending on car, to a fuel/vapour seporater first, then carbon canister). The carbon canister filters the fuel vapour from the fuel tank, and then through a small solenoid valve, dumps the vapour into the inlet manifold to be burnt in the engine. This valve opens(depending on manufacturer) when eg, brake is applied, or on start up, ect.
     This set up is an ADR requirment for all fuel injected cars in order to meet emission related ADR's
           Also, if this system is working right...you shouldn't get any fuel smells in your car...

There are several fuel system versions (for the GTV6 and 75) in the manual on the website "Craigs place":

http://www.users.on.net/~craigf/

Look in group_04_fuel_system.zip

Page 4-86/2 "Exhaust Emission Control System (Australia only)” shows a line going from the oil vapour separator on the engine to a "vapour separating tank "on the top of the fuel tank. There is a "fuel vapour venting valve" in this line near the tank. This is a 1985 manual, so it may not be what you have in an earlier car.

Page 4-80/1 shows a system as described by greengtv8, with a filler venting hose and a venting hose from the vapour tank, with a "tank venting inertia valve (except Australia version)".

- Neither diagram show where the vents exit the car. But it looks like the "fuel vapour venting valve" is on the driver’s side at the rear of the car. The two hoses on page 4-80/1 both look like the exit near the fuel cap, under the car.

I had the smell of fuel in the car on Sunday (a bit warm), so I'll have a crack at finding the vents on my '84 GTV6.

I was going to start looking there.

It may very well vent into the boot, which would explain the smell in the car. Something to look at.

I have read somewhere that the return line from the oil-vapour seperator is a hard plastic tube that is run inside the cabin, under the carpets.

AL.