Problem with fuel vapor system

[d9103365]

I have a 75 Fury (318) which has the fuel vapor recovery system. The gas cap is non-vented, and fuel vapors are sent from the gas tank to a charcoal canister in the engine bay. Most of you know what I'm talking about. Anyway, my system is stopped up and I'm wondering if its worth fixing it back stock or if I should just get a vented gas cap and solve the problem that way.

What's happening is that somehow the system is clogged somewhere, and I'm getting tremendous pressure buildup in the gas tank. When I open the tank, there is huge pressure release. The other day I filled my tank higher than I usually do (cause gas is cheap now!!) and I forgot to "burp" the tank like I usually do. After sitting for about a day or two, gas started leaking out around the fuel pump. Once I "burped" the tank, the gas leak stopped. I should also mention that my fuel sending unit doesn't work...and I have to guess how much gas to put in the tank. In any case, here are my questions:

1. What are the common causes for this problem and how difficult are they to fix?
2. Would I be better off just getting a vented gas cap and not bother fixing the elaborate fuel vapor recovery system?

At this point, I'm worried there may be some damage....I haven't bothered running it since I burped it. My guess is that the car may need a complete fuel system overhaul including new fuel pump, lines, filter, sending unit, and maybe even a new tank. I dunno, but it could get expensive. Any advice?

 

[mopar 3 B]

There should be a maybe 5/16" fuel line going into the vapar canister. The fuel line connects to a metal line that goes to the fuel tank. Some where in the metal line between the vapor canister and the fuel tank there is a roll over valve. Make sure the metal vent line is not collapsed and the roll over valve is open.

 

[383man]

You can check your stock vent lines and make sure they are ok. The fuel tank has a roll over valve in the top of it that lets the fuel vapors go out from the tank and through the vapor line to get stored in the charcoal canister. But in case of a crash where the car would roll over that valve will close so gas cant run up the vent line and come out of the tank and leak on the ground. You want to check that line and sometimes that line can have a pressure relief valve in it also. Now it is normal to have some vacum/pressure in the fuel tank when the cap is released on the closed vapor recovery system as I saw it many times when I worked in a gas station as that system keeps all the vapors sealed from the tank up into the charcoal canister which then gets purged into the eng after the eng is running. The gas cap has a pressure/vacum release and they can hold some vacum in them while running since its a sealed tank but its a small amount that the gas cap will release before to much vacum can build in the gas tank to damage it or hinder the fuel flow. But until you cheack it out and fix it you can throw a vented cap on it to be sure its venting until you check your vent lines and roll over valve. And some years have a vapor seperator in front of the gas tank with about 4 vent lines from the tank going to it and then a vent line from the vapor seperator to the charcoal canister. Ron

 

[YY1]

I have the same problem on my 73.

The pressure was so bad it leaked through the pinch weld between the gas tank halves on my brand new tank,

I examined the charcoal can and the lines to the carb and valve. All OK.

The only thing I've found that might be amiss is that I hooked up my vacuum pump at the line into the can (from the tank), and it was not at all free flowing. I was able to get 20lbs pumped up in just a few pumps, and it slowly bled off. Not what I would think would happen.

I'd like to know for sure where any potentially stuck check valve may be, other wise I'm suspecting that difficult to service 4 way vapor manifold in front of the tank.

That project is on hold due to the engine being removed for other service, but it will be back at the top of the list as soon as it's road ready.

 

[mopar 3 B]

The way these systems function are unique to the vehicle and year an answer to a 73 system will not work for a 74 and so on in fact I have 2 - 72 RR big block cars with entirely different systems. They all serve the same purpose but have different equipment and function differently. The answer I gave for the 75 system was year spacific and may not solve the problem for any other year system.