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Thruxton900 · Posted: Sun Sep 07, 2008 9:09 pm Post subject: Fuel cap pressure

From the Welcome Noob thread...

Ozzie · Posted: Sun Sep 07, 2008 9:28 pm Post subject:

I would not say major but I'd fix it.
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Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

Thruxton900 · Posted: Sun Sep 07, 2008 9:34 pm Post subject:

yeah i suppose, as it probably also lets water in when in the rain or when its being washed...!

Anyone recommend where I could get an original lockable replacement cap?
Ive also seen some aftermarket STANT caps here
http://www.autopartswarehouse.com/details/QQPorscheQQ924QQStantQQFuel_Tank_CapQQ19771979QQST10585.html

any suggestions?

thanks
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1977 NA 924 sold in 2008.
1967 Porsche 912

RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

I don`t think they`re pressurised, at least the 77s aren`t. Stock, the vent line runs to the cannister (LH fender) and to the intake tract. Vapours exit to the cannister and air must fill the volume displaced as fuel is consumed. Otherwise the pump will be trying to draw from a vacuum, losing efficiency and volume/pressure. Eventually the engine will stop.

As well as water entering the tank you also run the risk of having fuel run out the filler, especially when cornering left. with a faulty or missing seal. I found a NOS cap on ebay but they may also be available elsewhere (pelican, AZ) since they are a VW part. FYI a replacement locking cap can be got at supercheap for about $20. Doesn`t look as good as the star cap but they seal OK.

JackRabbit · Posted: Mon Sep 08, 2008 12:40 am Post subject:

the '79's are definately pressurized... at least it says so in the owners manual.
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'79 Porsche 924
'05 Honda Civic
'07 BMW K1200R
'78 Honda Express
'52 Willy's
'58 Wheel Horse

Druman · Joined: 19 Mar 2007 Posts: 419 Location: Middleport, Ohio

So you would probably be loosing pressure if you had cracks in the hose that comes from the cap to the tank too wouldn't you?
Just a thought

7osh
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87' 924S non-sunroof (Red) 45K miles
85' 944 "early" NA (Red) 130K miles

PORSCHE pisses excellence...

Smoothie · Posted: Mon Sep 08, 2008 6:47 am Post subject:

The US Haynes 924 manual has the diagram on pg.102 showing the EEC system. Item 14 there is a "safety valve". I know this valve has to allow flow toward the left as shown in the diagram so the charcoal canister can absorb and hold fumes from the fuel tanks' expanding vapours. -And I assume it would have to allow flow toward the right as well to limit vacuum in the tank as the fuel is used. -So I'm pretty sure it's a two-way valve that opens to release pressure over a certain limit, and also opens when a certain vacuum level is reached to allow flow back the other way. This is the part that determines how much pressure or vacuum the fuel tank holds.

Item 13 there is the EEC control valve. This one closes to direct flow from the tank to the charcoal canister at rest, idle, and full throttle. -And at part throttle it opens to allow flow from the charcoal canister into the intake.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox