GTi filler cap pressure

Jaba · Post by **Jaba** » Fri Mar 25, 2011 10:06 pm

My first GTi always seemed to have pressure in the tank as demonstrated by a hissing release of pressure sound when opening the filler cap.
My current GTi, in my hands for 12 years now has never done this.

I had a bit of a look into this last summer while changing the rusted through tank filler neck and found after cleaning that the pressure release valve in the rear wheel arch was working ok and I was able to raise the pressure in the tank by blowing into a removed pipe having blocked up the other open end. This pressure stayed until I released it.
So it seems that there is no pressure leakage in the tank, breather and fuel system but I never get anything but atmospheric pressure in the tank.

Now I reckon that the tank should be pressurized as the injection pump is pumping fuel at 3 bar and the return is being pumped too at near to 3 bar too but I do not get a fart when I open the filler.

This car has always had uneven poor running at low throttle openings and I am wondering if the two are related.

Thoughts anyone ?

Philip Chidlow · Post by **Philip Chidlow** » Fri Mar 25, 2011 10:51 pm

Sounds akin to an issue I am having. Air getting in slowly, but enough (in my case) to affect fuel pressure at warm start up/idle. A perished return pipe or a fubared diaphragm /non return in the pump whatever; both pump and pipe are being replaced soon. It's unlikely to be the pressure regulator on the fuel rail I am told. I never get hissing pressure release either.

sdelasal · Post by **sdelasal** » Fri Mar 25, 2011 11:23 pm

Early systems simply vented the tank to atmosphere. At some point, manufacturers started to use charcoal filled canisters to store fuel vapor from the tank - the vapor would be fed into the intake manifold thru a controlled valve when conditions were right. I'm not sure if/when Citroen adopted this system but most others did in the late 80s' - this link describes the concepts http://www.autoshop101.com/forms/h62.pdf When the various rubber pipes perish with age, you end up with petrol smells. I think there should generally be a small amount of pressure in the tank, but it will depend on temperature. It will not affect engine running - unless the valve is permanently open - in which case the symptoms will be of an air leak or running slight rich - depending on whether fresh air or fuel vapor is venting from the tank into the intake manifold. Obvious thing to do is disconnect the vapor line into the manifold, plug it and see if it makes any difference.

The most likely cause of poor/uneven running at low throttle is air leaks. Check all of the rubber tubing, crank breather & PCV tubing etc that is connected to the inlet manifold.

Post by **Tim Leech** » Sat Mar 26, 2011 8:06 am

My TZi has a charcoal filter but I think it was only BX's with catalysts that had these?

Jaba · Post by **Jaba** » Sat Mar 26, 2011 11:45 am

On the subject of charcoal canisters my GTis were not fitted with them. Venting is to the outside atmosphere via the venting valve in the rear wing. Diesels have this same venting system too.

Phil, poor hot starting is a prob I have suffered from. I had to spin the engine for 10s or so with full throttle to get it to start after being left parked for 10 mins or more. Changed all the bits (p/r, pump, injectors) thinking it was a 3 bar pressure leak somewhere. I eventually cured it by replacing the inlet manifold and throttle housing although I could find no leaks in the original removed parts. So it was not a pressure loss in the injection sytem that was the issue. I plan to refit the throttle housing one day to see if the problems returns to prove which part was playing me up.

Interesting that you get no hissing as well from the filler.

Anyone else ?
.