Thread: gas tank venting

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I'm still working on improving my gas tank vent system. I found a couple of sources of decent info on how a modern evap system works.

EVAP Evaporative Emission Control System

https://www.youtube.com/watch?v=AlfOSMmisPg

I don't really want all the complication of an evap system, but I do want to keep the stink out of the car.

I've got a high-mounted marine flip top gas cap that isn't vacuum tight like a modern auto gas cap, but it's only vapor leak source is the small clearance area between a large spring loaded washer (with rubber sealing gasket) and the 5/16" shaft that it slides up and down on. I'm sure that the area is no more than that of a 1/16" diameter hole and not enough to vent the tank under normal operating conditions.

I ordered a remote mount roller-over vent valve that will be mounted about as high as the filler cap, but even with a vent hose routed outside the car, it will still stink up the garage, with the 1/4" hose open all the time.

I got a 1/4" solenoid purge valve with the LS motor that I bought, so I'm thinking of using it as a vent valve, to allow air into the tank only when the engine is running. Since the gas cap is not totally sealed, I won't have a problem with temperature changes causing the tank to swell or contract.

It should be an interesting experiment.

I suspect that as long as the cap is vented you will have a stinky garage.

Dave, I have been making a charcoal cannister for years for streetrods, never have I smelled gas in my garage and have done multiple cars, very simple also after your rollover valve, run the line from the valve up higher than the gas tank and make a loop (either out of hose or tubing) from there I pass it through a bulkhead to the outside of the car and I make a small 2" diameter x 7" long aluminum tube and put caps on each end then put a hose to the tube, I drill and thread the end cap and use an fittings, on the other end cap I drill a series of small holes so air can pass through the tube. Then go to Walmart in the fish section and get activated charcoal, fill a section of nylon panty hose tie it in a knot and insert it in the aluminum tube, your good to go with never a smell and the tank is well vented. I have made one for a VW with just PVC pipe and had equal success, you could do this and put it in the trunk and just put a hose on each end and run the hose on outside. Somebody is making one just like I mentioned (billet) and marketing it for a few hundered, but I made my first in 1998!
The whole thing looks like a large inline fuel filter just with charcoal in it

I've seen many versions of the home made charcoal canister. Some use an aluminum water bottle, others a length of PVC pipe and one person even made one from a pop-up lawn sprinkler case that already had a 1/2" NPT in the bottom. The canister may be the best answer. Some people question why the charcoal doesn't get saturated with fumes, when the cars sit idle for a long time and are often driven few miles. Driving would help purge the canister, as some air flows through it. From what I've read, modern cars have a vent valve off the canister that stays open when the car is not running. It's only close during the purge cycle. The purge valve allows vacuum to suck the fumes from the canister, but the vent valve must be closed, or it would create a big vacuum leak at the engine.

Hot rods by Dean sells the $250 canister. No sense in that, when it's hidden.

Dave, Why doesn't the charcoal get saturated? on my application my best guess is the loop in the line before the cannister, I think it probably prevents the fumes from just pouring out and still allowing expansion and suction to occur, Like I mentioned I have done it on at least 6 vehicles and never a hint of fumes in the garage, yes Deans the one selling the cannister for a kings ransom, my where turned and polished out of thick wall polished aluminum with 1/4 plate aluminum ends that could be tapped for fittings, it has served me well and looks nice but I kept them visible from the underside. From what I found out activated charcoal is activated charcoal wether its for a few bucks in the aquarium section or used in medical supplies costing way more. I also put a small wire screen on the intake even though the nylon stocking kept it enclosed just incase os some unforseen failure.

Just curious, could you possibly have a fuel leak, not huge weeping kind of deal, cracked weld leaky flexible joint?

My experience, my tank lives behind the seats. When I first got the car had gas smell in the cabin. Found the seal of the tank to the body mounted filler cap was the culprit. Thinking back 10 years I believe the hose clamp was skewed just enough for fumes to leak and fowl the cabin. Once that was fixed the problem was solved.

My point is this, I suggest that you look at the tank fill system and try to determine if there is a way fumes are leaking into the cabin, flexible hose connections from body mounted cap too fuel fill tube, or flexible hose from fill tube to tank.. Not knowing the details of how your fuel system is mounted this could be useless information, in that case never mind.. :-)

I have two cars both have fuel cells with vented caps, carb induction, never have any fume smells, in the car or the garage.

I don't have a specific problem at the present, but want to avoid one, once my car is complete. A roll-over vent is nothing but an open hose and it will stink, unless a carbon canister is used or the end of hose valved-off when the car is not in use.

I once had a fill-through gas cap from Tanks, combined with a short filler neck and it did not work at all. The fill-through hinged opening was wet with splash, after every ride. The folks at tanks denied that these gas caps had a problem, but offered to sell me another with no freight. I tried that and the second one worked just like the first. That's when I changed to an external filler cap mounted high, on the deck ahead of the trunk lid. Fill-through caps, if built right work great. I have two relatively new cars that came with them from the factory. Those are totally sealed and must be vacuum tight to work with a EVAP system.

Originally Posted by pepi

I have two cars both have fuel cells with vented caps, carb induction, never have any fume smells, in the car or the garage.

My experience is much the same as Pepi's. I have a rollover vent valve, mounted in a pipe nipple above the top of tank about 4" with the vent hose routed high, coiled and then turning down to a bulkhead fitting on the trunk floor. I chased fuel fumes for months, but found that it was from running rich, leaving raw fuel in the induction runners, exiting the short lake style headers; and also once from a ruptured fuel regulator diaphragm dumping masses of raw gas into the tunnel ram. Once those issues were addressed my garage is virtually fume free.
The only time you'll see fumes emitted from the tank vent is in a significant increase in temperature of the tank volume. If the ambient temps are decreasing or running relatively constant there's no emission (in-flow if cooling), and in a normal temperature increase the fume volume emitted is quite low, likely not even enough to smell, IMO. As Pepi suggested, I'd say if you're smelling raw gas you have a leak liquid.