Petrol cap and roof fluid

Evening all. Can anybody advise me on what fluid I need for my electric roof and tell me if my fuel tank should be pressurised I.e.hissing when I open the cap to fill up. Was told this was the case with mine but the previous owner had bought a cheapo replacement which fits skewiff and it virtually falls out.
Car is 1990 GTI 1800.

Cheers.

Yes there is pressure in the tank cold days not so much as hot days.

I  would think that Hydraulic Jack Fluid would work, but I imagine there is a specific type that you should buy.

Thanks for that Briano. Do you know what the consequences are of having a leaky petrol cap as I'm guessing it not a big problem!
Cheers.

Hydraulic fluid for the roof can be bought over the counter at your local VW dealer!

I'm moble at the moment otherwise I send you the part number and a picture of the bootle. But it's a no brained because they still use it to this day.  

When searching for pressure in the fuel tank I found plenty of posts but the answers were conflicting. Mine was bulding up serious pressure and sometimes it would blow fuel out of the trumpet breather under the car, fuel would spit out of the filler when I removed the cap and I could hear something creak underneath the car!
I replaced the gravity valve and now I get a very slight whoosh from the fuel cap. Hopefully the problem is sorted for me but despite the amount of people that say the tank should be under pressure, I don't think it should be under any serious pressure. I would check that the breathers are clear and that the gravity valve is not blocked. You should be able to blow though it with slight effort and it should close if tilted but clear again when upright. Mine would allow air through but if shaken or tilted it would block but not clear until the pressure was released so the tank slowly built up to high pressure.
I hope this helps.

Thanks for that Saulty. So would you say my non sealing wonky cap was nothing to worry about - apart from an aesthetic point of view obviously.
Cheers

I don't see why the tank should be under pressure, in fact as fuel is being removed from it I would expect it to be under vacuum, and if I remember correctly when I ran a standard gti, a good few years ago, removal of the fuel cap was followed by a rush of air into the tank.

marcsa3 said

Thanks for that Saulty. So would you say my non sealing wonky cap was nothing to worry about - apart from an aesthetic point of view obviously.
Cheers

If your cap is not sealing then you probably won't get any running issues and I'm surprised you are getting a hissing when you remove the cap. I would say that your cap is sealing. If it's not I'd be more concerned about water getting into the fuel and causing other problems with the fuel system.

abfmk1 said

I don't see why the tank should be under pressure, in fact as fuel is being removed from it I would expect it to be under vacuum, and if I remember correctly when I ran a standard gti, a good few years ago, removal of the fuel cap was followed by a rush of air into the tank.

The fuel tank is vented, it pulls air in through the gravity valve as fuel is sucked out through the pump but excess fuel is returned to the tank. The fuel returned is warm and expands causing slight pressure. If the tank wasn't vented and you ended up with a vacuum then you wouldn't be able to remove the filler cap as the vacuum would be trying to suck it into the tank. I imagine you probably would also be experiencing fuel shortage if this were the case as the lifter pump wouldn't be able to pull any fuel out of the tank. Mine was definitely under pressure as it would push the cap away from the filler spit fuel when you undid it.

I'm no expert but I did a bit of research and asked a few people (one of them tunes VW's and is quite well known on the VW scene). The general consensus was, from the people I spoke to, that the tank should be under slight pressure, you should get a slight puff of air but not a big one and it shouldn't splash fuel out and it definitely shouldn't be a vacuum.

Sorry buddy. Confusion on my part. It doesn't fit right and therefore I'm getting no hissing when I undo it so should I be worried about this or make sure I get one of a better quality and make sure it is sealing.
Cheers.

Power hood fluid level must be checked with the roof down. Fluid should sit between min and max lines,
Correct VW Hydraulic Fluid is part Number is:
 G004 000 M2 however it may of been superseded, but they will be able to give you correct stuff from that number.

marcsa3 said

Sorry buddy. Confusion on my part. It doesn't fit right and therefore I'm getting no hissing when I undo it so should I be worried about this or make sure I get one of a better quality and make sure it is sealing.
Cheers.

No worries, if it's not sealing then you're unlikely to get any hissing as the tank will be venting through the badly sealed cap. If I were you, I would get another cap if only to stop rain getting into the fuel. If you get excessive hissing/whooshing then something else is wrong (probably gravity valve).
Hope you get it sorted.

Thanks for that chaps. Have a bottle of that Vw/Audi power steering fluid in the cupboard so that's saved me a few pennies. Good call on putting the roof down whilst topping up as I wouldn't of thought of that and have just got another cap on order so hopefully job done.
Thanks again all.