Interesting fuel smell problem w/snorkel

I've had a hard to diagnose fuel smell problem for quite a while and finally figured it out. First I had a leaky gasket on the fuel tank, but that didn't make the problem go away. It happens when it gets hot, or pulling hard on the highway, and was worse at a half tank and below.

As it turns out it had at least a small part to do with having a snorkel. I am running a 4.7L stroker. When I installed it, I pulled the venturi from the air box. As it turns out, that venturi is not there only to reduce intake noise. It creates a small vacuum that is used to purge the fuel vapor canister. It probably isn't a big deal on a normal intake, but with the snorkel, going down the road at speed, a positive pressure is generated in the air box instead of a small vacuum. That leads to forcing gas out the vapor canister instead of sucking the fumes back in and burning them.

I am going to try and put the venturi back in and see if that helps. If not, I will need to tap a small amount of vacuum to purge the canister. You can't just run straight manifold vacuum because it is too strong and might collapse the fuel tank.

Has anyone else seen a similar problem?

The fumes are pulled from a hose into the intake right? Would there be any effect if you move the hose closer to the throttle body?

-Chris

The problem is that the whole intake is pressurized with the snorkel.

How do they purge the vapor canister with a turbo system? Do they just put the purge infront of the turbo?

What about some kind of valve and dump into the intake manifold? Like somethign used on air tools. But you would have to know what kind of pressure it needs.. i'm guessing anything low would work (1-3 psi).

-Chris

If the whole manifold is pressurized then your brake booster would not work either. So, I think that just the intake leading up to the throttle body is pressurized and that maybe you need to tap into 1 of the vaccum ports on the manifold.

Like I said before, the vacuum from the manifold is too high, it will collapse the tank. I may have to make some type of orifice and bleed setup to generate just a bit of vacuum. The problem with an orifice is that they get easily plugged. Maybe I can put a fuel filter in series with the orifice if the venturi doesn't work.

You could create your own venturi effect by sheilding just the front half of the spot where the line enters the intake hose. Maybe push in a pipe that fits into the line with 15-20 mm sticking out into the intake hose with the end cut off at a 45 degree angle. You just want to disturb the airflow enough to cause a eddie effect at the entrance to remove the pressure from the canistor line.
Otherwise picking up manifold vacuume with something restricting the end of the hose to say 1/16"-1/8"? You just need to push something into the end of the pipe to do this. This will reduce the vacuum too.

Gojeep has it, this is exactly what i did on my snorkel. Not b/c of the vacuum problem (did not realize at first that I had that problem) I did it b/c I had collapsed the port at the air box (the 2") after removing the venturi. Used an old shop vac pipe, perfect fit 10-15mm into the box top and cut at about 60deg RTV to seal it up.

I am going to glue an eliptical plate on the side of the airbox tube over the port to try and get some venturi effect. Hope it works. The wife really hates the smell. I've been working on vehicles for so many years, I guess I'm just used to it.

Is the positive pressure in the air box caused by the air being forced into the intake at the top of the snorkel? Could you turn the snorkel intake so it faces backwards or would people point and laugh? I think the engine would still be able to pull in all the air it needs.

SJ