drooman

I have a USA 78 VIN 664 that has been fooled around with by a previous owner in the charcoal canister and fuel tank areas I can't make sense of it.

Consider the first diagram, the charcoal canister area:





There *may* be missing parts in the engine compartment... Namely items 18 21 19 and 27
There is no Y piece (24) present, instead that heads directly to the hardline (14) and back to the roll over valve near the filter then onto #21 in the second diagram:





It has a new Porsche gas tank in it, but they *may* have deleted the rear tank vent that is supposed to hang down near the rear bumper.. instead having hose #16 from the canister diagram going into the vapor tank (13)

My questions:
How do these 2 diagrams even go together? What is the connection between the vapor tank and the charcoal canister? Are there 2 different systems represented here?
Does anyone have a picture of the installed locations of #18 19 21 and 27 from the canister diagram, and what they connect to?

This is a nice original car in general, I'd like to get it correct.

GregBBRD

It sounds like they are passively venting to the charcoal canister (no vacuum to purge the system.)
A large percentage of the fuel tank failures are caused by valve #18 (first drawing) failing open, which if the "Y" (#24 in the first drawing) isn't broken (common to find broken) will suck the tank "flat", when the engine is running. Cracked tanks are the result.
Valve #18 rarely sticks open from a vacuum problem. They almost always are stuck open from small carbon pellets migrating to the valve, from a failed charcoal canister. (We replace quite a few "failed" charcoal canisters.)

In the later cars (I believe 1983), Porsche added a second "control valve" (besides #18) in order to attempt a double safety system so vacuum would not reach the tank. The problem was that the added "second safety valves" all failed "open" long before #18 valve failed....so this second valve did virtually nothing.

I can't tell, from your description, how the fuel tank vents, currently.

Worth mentioning is that the original charcoal canisters are no longer supplied. The replacement canister (993 part number) has three ports, instead of two. This eliminates the problematic "Y" (#24 in the first diagram.) This also requires some thinking, in order to get hoses doing what they should do. This is a much better way to do this task, BTW.

Rob Edwards

18, 19, 21, 27 didn't get used at that point in '78 (if at all, in '78.)

Here's the three hoses that come off the carbon canister in #107:




And their destinations in the engine compartment. Forward one is open to the air, middle one to the airbox, rear one to the hardline back underneath the car.

Darklands

My Euro S 16 V cars don‘ t have this stuff, maybe the Euro 86.5 32 V.

Btw, nice looking fuel lines.

8cyl

No Euro models had any emissions controls equipment, thankfully. Some Japanese and Australian delivered cars for certain years had lower compression, though.

drooman

Thanks Greg and Rob.. that clarifies a lot but a few more points worth discussing

- The system in USA vin 664 is identical to the one in rob's picture of USA Vin 107
- The blue line clarifies how the canister connects to the rear vapor tank in the 1978 USA system
- It appears that the currently available 993 canister is like the original 1978 canister in that it also has three ports. My guess is a new supplier of 2 port canisters got the contract at some point then the infamously fragile "Y fitting" (24) system was adapted to use with canisters with only 2 ports.
- Greg's observation that it looks like a passive venting system with no vacuum action is correct...that's exactly what's going on.
- In this type of early USA system, without vacuum component , there should no imploded fuel tanks
- would be nice to know when the developments occurred; the addition of the vacuum in the original 3 port canister systems, and if at a different point, the introduction of the 2 port tank and associated Y fitting.

I welcome a reality check, corrections, and additional discussion here.

Rob Edwards

I know that US '78 VIN 1117 ( 23 VINs from the end of US production) did not have the vacuum system, while it's present on the earliest '79 on BAT that I can find (#470), so I'd venture that the vacuum diaphragm (#8) is '79 only. The Y-pipe is listed in PET as having started in 1980.

GregBBRD

So, the '78 cars purged the charcoal canister through the intake vacuum created below the air filter. The open hose, under the coolant overflow tank, would have had to be the vent.
Very cool.
And the '79's apparently used the 3 port charcoal canister, but purged it with intake vacuum. This probably switched the hoses, on the charcoal canister, as the hose to the air filter would then become the vent.

I can see that both of these systems could allow gasoline vapors to escape, when the car was not running.

Apparently, Porsche, in 1980, then changed to the 2 port charcoal canister, with the "Y" hose, so there was no hose open to air?

Learn something new, every day!

drooman

What goes on inside of a 2 port canister versus a 3 port canister ? how do they function?