Gas Cap/Vapor Lock Issue

[John Simion]

I took my car to a car show on Sunday and encountered an odd problem.  The situation is that it’s Las Vegas, it’s 105 outside, and it’s a 20 mile ride, almost all stoplight-to-stoplight.  We drove about 3/4 of the way and then the engine started surging and lost power.  I pulled over into a side street and did absolutely nothing, but the problem just went away.  Maybe dirt passing through the carburetor, I thought.  Anyway, no further problem the rest of the way to the car show.  Stayed at the show for perhaps an hour, then got in the car to leave.  Started great, pulled out of the parking lot and it stalled and would not start.  I don’t know why I thought of this, but I opened the hood and opened the gas cap and there was a sound of pressure, whether going “in” or “out” I don’t know.  Regardless, thereafter it started and ran perfectly the rest of the way home.  Nothing actually even happened but it scared my wife, so naturally I had to listen to her repeat over and over on the whole trip about why she only likes “new” vehicles, yadda, yadda, yadda.

Being an internet expert mechanic, it seems like I had a vapor lock issue, especially after I checked my fuel line and part of it is lying on the heat riser for the carb.  However, when I spoke to my new expert VW mechanic, he thinks the cap is sealed too tight and creating a vacuum as the carb draws fuel, so eventually the fuel pump can’t pull fuel down the line.  He said there is supposed to be a nipple on the filler neck to release pressure, but there isn’t on mine.  I asked him how I could solve this, and his suggestion was to drill the tiniest hole possible in the top of the VW cap (obviously not while on the tank).

I’m not entirely crazy about this idea.  I don’t want to smell gas in my garage, and this is an original VW cap that is one of the 70 mm caps with the prongs around the outside edge.  I can’t get a new replacement that will fit under the hood — there are replacements available but they are taller and won’t fit under the hood.

Wondering what would be the wisdom of the forum on this.  Drill the hole in the cap or not?  Or do I just buy some fireproof heat insulation and put it on my fuel line to keep it away from the heat?  Or both?  Or neither — I guess I could just release the pressure in the tank every so often IF that is what’s actually causing this.

 

[Al Greig]

There was some discussion of gas tank venting a while back.  Don’t know why the pictures no longer show up.

https://tdreplica.com/forums/topic/new-gas-tank-install-problem/page/2/

[John Simion]

The discussion Al mentions does not really help me.  I have no vent at all right now.  That’s really odd, but that’s the way the guy built it.  When I got the car, the gas cap was poorly sealed and the garage smelled like gas all the time, and a tank would evaporate in short order.  However, I’m sure he never had my problem of fuel starvation.  Since I can’t find a new gas cap to fit, I re-used the old one with a new seal.  Now that my tank is fully sealed, apparently I’ve traded gas smell and evaporation for gas starvation on the road.

Right now everything is serviceable except for this one issue.  I am not about to change the tank because this tank doesn’t look like anything in any of the various VW parts vendors and I’m simply not going to go out and buy a new one and then try to fit it into my car.  I’ve had more than enough re-fitting stuff.

By the same token, “fixing” the tank as it now stands isn’t very palatable.  I hate the idea of emptying the full gas tank, taking the tank out of the car, and fabricating something (that I don’t even understand) in order to solve this one little problem — assuming that the venting is even the problem and not just vapor lock based on the fuel lines in the engine compartment.  As the car sits in the garage, there is no issue, no gas smell, no problem.  I hate to create a new problem where one exists only in extreme heat.  I just don’t know about drilling a hole in the gas cap; I just got rid of the fumes in my garage and that would put them back again … and under the hood of the car as well.  I’m thinking I’m probably better off just to relieve the *&^% pressure from time to time and ignore the problem.  May not be as much of a problem when it cools down here, too.

[edward ericson]

drill a small hole in the side of the filler neck just under the cap. Make the hole about the diameter of this. Which you buy. JB weld it in. (Or use this stuff, fuel-safe). Wait a day or so for the the epoxy to set up.

Run the tube up a little above the cap’s level, then down in front of the tank, to just above the little hole in the floor pan next to the tunnel up front on the passenger side. For style points, put a crush ball on the line, get a tiny bit of rubber hose, and hang one of these off the end of it, arrow pointing up. You will have a vented fuel tank then as God and Dr. Porsche intended, but you should not have any significant gas smell (assuming your carb’s float bowls aren’t too pungent).

Enjoy the ride.

[John Simion]

Ed, that is incredibly helpful!  I’ll get on it.  Can I ask two more questions, though?  First, would it be safe to drill that tiny hole into the filler neck even though there’s gas in the tank?  Probably should be, as long as the cap is still in place while I’m drilling (?).  I don’t want to set my car and house on fire.  Second, what is a “crush ball”?  I tried googling it but I’m not getting anywhere.

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