What's this tank for?

[mnez]

Can someone tell me what the second plastic tank is for that is above the main windshield and headlight washer tank? The lines disappear into the fender somewhere under the fuse box area. The car is a 1986 that also has the 'special' windshield cleaner reservoir in the trunk.
Thanks

[D911SC]

Fuel overflow (expansion) tank

[HarryD]

Vapor Tank (Part #2) in the diagrams below.

Originally Posted by Grady Clay

tref,
In the third diagram I posted above (labeled “05” is an
exploded parts diagram) the hoses from the rear of the
tunnel to the fan shroud and air filter assembly are
#22 and #23. In the trunk and to the front of the
tunnel (by the steering rack) are hoses #20 and #21.
Not labeled with a number are two plastic pipes in the
tunnel. Hoses #20 & #23 are 5 and hoses #21
& #22 are 6, including the un-labeled plastic pipes
in the tunnel.


Philippe, here are the later versions, starting in ’74.

Is the plastic vent pipe original in your RS?


1974 to 1977.
"
"
© Dr. Ing. h.c. F. Porsche A.G.


1 – Fuel tank
2 – Expansion chamber
3 – Activated charcoal filter
4 – Cooling fan upper shroud with hose connector
5 – Hose from cooling fan to activated charcoal filter
6 – Hose from activated charcoal filter to engine air filter
7 – Engine air filter
8 – Return hose connecting fuel tank with expansion chamber



1978 and later.
"
"
© Dr. Ing. h.c. F. Porsche A.G.


Some features to note:

The Expansion chamber hose to the fuel tank is at the
bottom of the chamber. This returns any liquid fuel to
the fuel tank during cool-off. Ideally this hose should
be continously down hill to the fuel tank. The vapor
only hose to the Activated charcoal filter is at the top
of the Expansion chamber with baffles.

While the ’78-> system is slightly simpler, the ’74-’77
system won’t give sudden surges of vapor to the
engine intake.

The ’69-’73 system had plastic hoses in the trunk. Over
time, temperature and fuel vapor, they shrink. I think
they all should be replaced with the braided fabric over
rubber hoses. These were not originally equipped with
hose clamps. Every connection should have the Norma-
Schellen (NS) clamps. Fuel vapors in the trunk can lead
to disaster. The little expansion tank under the cowel (1
in the early diagrams) can split a seal from age. That
should be inspected also.

Even race cars should have a proper vent system that
includes an expansion tank.

Best,
Grady

[daepp]

Why use a charcoal filter if you're just gonna reburn the vapors?

[mnez]

Thanks guys. That would explain the occasional gas smell. Now to find the leak.

[shadowjack1]

In your picture the small hose is disconnected. It's attached right next the the large line from the expansion tank. Mine was the same way only had the gas smell when I filled up.

[Neel]

Originally Posted by DAEpperson

Why use a charcoal filter if you're just gonna reburn the vapors?

The charcoal filter actually stores(absorbs) gasoline vapors when the engine is off. They are then burned when the engine is running..

[RWebb]

Originally Posted by DAEpperson

Why use a charcoal filter if you're just gonna reburn the vapors?

yes, the canister serves as a reservoir until you burn them