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I've run into a bit of a problem following the installation of the Evaporative Emmision System. Most of the time the system works as intended. The engine runs well and life is good.

However, if I really get the tank full of fuel, so that I can see it in the filler neck, then the problem starts. The engine will lose power,sometimes just a hesitation, other times it is like a "hard cut" as if the fuel flow has been stopped. I've found with experience that the car will rev to 3000 rpm with no problem. as long as I do not exceed this, the car runs normally. As fuel is used from the tank, the problem will eventually go away, although i may have to drive more than 80 miles before the system behaves again.

If I don't fill the tank completely the problem does not occur. The problem is knowing when to stop pumping fuel.

The fuel tank vent is 1/4" OD tubing going slightly up hill to a check valve. It connects to the tank through a bulkhead fitting in the top of the tank. Photos of the installation are shown below.

I've found that there is a device called an overfill limiter, but apparently they never fail, since they don't seem to be available from after market parts suppliers. It's also not completely clear from the diagrams I've seen where they should be mounted in the system.

What I'm thinking of trying is to replace the section of 1/4" tubing coming from the tank with a piece of 3/8" and then reduce that down to 1/4" so it will attach to the roll over valve. My thinking is that if fuel has splashed into the vent line, it may be partly occluding it. A larger tube might allow fuel to run back into the tank, and not block the vent line.

I'm certainly interested in hearing what others might think about this situation. Especially if it's possible to come up with some sort of overfill prevention valve.

Thanks in advance.

Bill

Put a little float switch near the top of your tank and have an LED near the tank come on when it's full. They sell sealed, fuel safe float switches.

Or put a slight glass on the side that extends into the neck, bracketing the "problem" fuel level.

For what it's worth, I THINK I might have a solution to the over-fill problem. I installed a vent line from the tpo of the tank up to the filler neck. This was done by installing an AN8 bulkhead fitting into the plate on the top of the tank, and running a 1/2" aluminum line up to the top of the filler neck, just below the cap.

The filler neck on my tank extends about 1 1/4" below the to of the tank and so does the bulkhead fitting. The 'theory' is that air being displaced as fuel is being pumped in will vent through the 1/2" line until the level rises enough to cover the opening in the tank. As fuel continues to be pumped in, the air will vent through the filler neck, causing the fuel to 'bubble up' and shut off the fuel nozzle.

I haven't actually tested this at the gas pump yet, but I have poured gas from a 5 gallon can into the tank and it flowed in without a problem. Maybe I've got this chronic problem of getting fuel in fixed.

Bill

What is lambda when this happens? That'll tell you if it's getting lean or rich.

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I really have no idea. I don't have a real time display of AFR. Since I get a sudden cut, I suspect it is actually fuel starvation. What I think happens is that if I got fuel into the vent line, it partially occluded it until it likely evaporated. The vent lone was 1/4" dia. While this was big enough, it also had a relatively long horizontal section. I fuel got in there, it would have no tendency to return to the tank.

I have also redone the vent line. It is now 3/8" and connects to the tank through a 45 deg. bulkhead fitting, rather than a 90 deg. one. This will hopefully help fuel drain back into the tank and if it doesn't, still allow enough air to pas.

I haven't filled the tank yet, so I have yet to test the theory.

Bill

By George, I think I got it!. I just got back from a short blat and needed to fill the tank. I bravely shoved the nozzle into the filler neck and squeezed the handle. The fuel ran in just like it does on the 'real cars'! And, after pumping in just under 6 gallons, the fuel bubbled up in the filler neck and shut off the valve, just like it was supposed to.

And, as an added bonus, the engine didn't cut out after filling the tank.

Life is good.

Bill