kissov356

Could someone out there explain the parts and function of the fuel tank venting "system" on an early 911. What is the box behind the dash? What is the device in the driver wheel well? How should it work. How restrictive is it? When I blow on the pipe connecting the "system" to the tank it seems blocked and passes very little air. The connection to the gas filler seems fine, but with the gas cap on I'm worried somthing is not right. Thank you.

r911

there are two containers for vapor - the cowl (dash) box is one. the other is in the driver wheel well - check and see if it isn't all cracked up by now - no idea why there are two or the difference between them

there is a carbon canister to trap vapors (and return some liquid or vapor fuel to the engine) - probably helps mileage somewhat and defintiely helps emissions

if you search on randywebb + jim + sims + fuel + vapor on Pelican BBS you should find several threads and a couple of diagrams

Note: if the 'clear' hoses are no longer clear and are hard - replace them with good fuel vapor hose - don't use PVC tubing

Peter Zimmermann

This system was an early attempt to control emissions from the fuel system. The box under the dash is an expansion tank, while the tank under the fender (with the rounded top) is a vent chamber. Vapors that escape from the gas tank go to the expansion tank (which had to be above the top of the gas tank to avoid the transfer of raw fuel), then on to the vent chamber, which could be lower in the car. After the vapors reach the vent chamber they pass directly to a charcoal cannister (the part that you have to loosen, by removing a rubber strap, so you can access the car's left side battery). Hydrocarbons are trapped by the charcoal, and, when the engine is running, the engine cooling fan purges the charcoal cannister which sends the bad stuff to the car's air filter housing, where the bad stuff is mixed with fresh air and sucked into the engine. The downfall of the system is that the Tygon hoses deteriorate and the expansion chamber cracks, which produces a mysterious gas smell in some cars. Randy is right, be careful of the hose that you buy. Home Depot has clear hose that is quite good, but if I recall correctly one size needed was not available. Porsche started carrying the hose in bulk, but didn't always have it when I needed it.
Pete

r911

Peter - do you know why two chambers instead of just one were used?


BTW - you can mail order good hose or get it an industrial hose place -- it's cheap. IIRC, it is a special Tygon - not just regualr Tygon. I think Jim Sims posted that on Pelican....

Peter Zimmermann

Hi Randy: I've heard explanations, but nothing from the horse's mouth. I think that the small expansion box was used simply to get the first chamber as high in the car as possible, probably to prevent raw fuel from escaping at the top of the tank. The vent chamber probably had to be put where it is to control odor, and create a short, simple path to the charcoal cannister. I'm sure that the design of the system was fairly involved because of internal gas tank pressures. In the past I looked for reasonable explanations of the system, but never really found anything that was totally satisfactory.
Best, Pete

r911

Thx. My guess would be that they needed a certain min. volume -- and aslo needed it to be up high as you say. They couldn't get that vol. up under the cowl, so they added a 2nd lower chamber.

Or they could have given the design to a summer intern or something... you never know.

Peter Zimmermann

LOL! I know one thing for certain, if a crack occurs in the little box evil fuel smells invade the car! Darn interns!
Pete