Vapor Line Question

[Travis1960]

I have a '73 GMC I am piecing back together. The vapor canister is gone. The vapor line ends near the steering box and is just open (see image). Can I just put some type of breathable filter at the end of the line? Or should I try to find a canister and gather up all the parts to put it back like it originally was? There is no smog test here in rural Texas, so that is no problem. Has anyone used a filter on the end of the line? If so, what type of filter? Would I also need a vented fuel cap?

[ray_mcavoy]

You won't need a vented gas cap as long as you use some sort of vented cap on that line.

I'd recommend using a cap like this --> http://www.classicparts.com/1947-91-...ctinfo/93-245/

Those type of caps were commonly used on transmissions, transfer cases, and axles. In fact, that's the type of vent cap GM used on the gas tanks of HD emissions trucks that didn't have the canisters.

[Travis1960]

Thanks for the reply. Now that is a very good idea. Could I just use a short piece of rubber hose to connect it?

[ray_mcavoy]

Originally Posted by Travis1960

Could I just use a short piece of rubber hose to connect it?

Yes. I'd recommend using fuel/vapor hose so it won't deteriorate from the gas fumes. If I remember correctly, 5/16" ID hose should be just about right for that application.

You might also want to consider running the hose up to the radiator support and clamping the end with the vent cap in place there ... similar to what GM did with the front axle vent on the 4x4 trucks.

[Travis1960]

Ahhh.....good idea, keep it up high to keep water out when driving in rain. Thanks for the help.

[Travis1960]

One more question on this. This vapor line has been open to the elements. Should I blow some compressed air through it to make sure it is open? Maybe take the fuel cap off and have someone listen near the filler neck to see if you can hear air coming into the tank. Since the engine was out when I bought it, both of the fuel pump lines are open too, should I blow air back through those also to make sure they are clear?

[ray_mcavoy]

Yes, since the lines have been left open for a while, blowing some air through them would be a good idea.

However, I'd recommend blowing the air in through the tank ends of the lines so it forces any dirt/debris out through the open ends instead of back into the tank. Ideally, you'd want to disconnect the lines at the sending unit to do this. But if the lines aren't too dirty, you might be able to get away with blowing air in through the filler neck ... use a rag to somewhat "seal" the large filler neck opening around your compressed air line. And if the tank still has some fuel in it, be prepared with a catch can on the open end of the feed line that goes to the fuel pump.

[piecesparts]

I put an in-line fuel filter on mine and routed it out under the radiator core support.

[Travis1960]

Thanks for the replies. I will try blowing from the filler neck on the tank, if that isn't successful, I will drop the tank and blow from the sending unit end.