Join Date: Feb 2012
Boat: Pearson Ariel, 26 feet
Defective Moeller fuel caps cause fuel starvation
I purchased a Low Permeation Above-Deck Fuel Tank (6.5 gallon), model 620040LP, approximately one year ago from West Marine.
I began experiencing mysterious engine failures. Each time, as part of the troubleshooting, I removed the fuel cap to visually verify the amount of fuel in the tank. Each time, after removing the cap, the engine started and ran normally. I never run the engine for long periods, using it only for docking on my sailboat.
During the most recent failure, I tried priming the engine with the primer bulb on the fuel line before removing the fuel cap. The bulb collapsed when I squeezed it and didn't re-expand. There is only one condition that will produce that effect: a vacuum in the fuel tank. I then removed the cap slowly and heard air rushing into the tank. The engine then started normally.
The vent valve on the cap was open. When I returned to the dock, I removed the cap, removed the tether from the bottom of the cap valve, and - with the vent open - tried to suck air through the cap from the inside (facing the gasoline) surface of the cap. No air would pass. The cap vent had failed. As a workaround, I drilled a very small vent hole in the cap until I could replace the cap.
This defect will cause otherwise-unexplainable engine failures due to fuel starvation. Instead of just having a vent hole, the cap has a two-way diaphragm valve. The vent valve on top of the cap doesn't actually vent the tank to ambient pressure. Instead, a significant under-pressure has to exist inside the tank before any air can enter the tank. In the case of my cap, that under-pressure was greater than the fuel pump could counter, and therefore, fuel flow was cut off to the engine.
I reported the problem to Moeller, and they admitted the defect and responded: "There were some initial issues with some caps contributing to the tank starving for fuel. In order to correct this issue the latest version of the cap has a lower vacuum release, or vent in, than the original cap. This should correct the problem that you are having."
The whole design is apparently an effort to idiot-proof the cap so the tank doesn't vent small amounts of gas fumes when the operator forgets to close the vent. I think that's a rather high price to pay in cost and reliability to prevent a maximum potential of 6 gallons of gas evaporation! And much worse things could happen as a consequence of a failed engine. At least on a sailboat, I have an alternative form of propulsion.
If you have a Moeller fuel tank and your engine experiences mysterious failures, try momentarily removing the fuel tank cap, and if the engine starts right up afterward, you may have one of the defective caps. I replaced mine with an old-style non-California-approved cap, and I haven't had any problems since. I reported the problem to the Coast Guard through their defect reporting system that's used to issue product recalls, but so far, I've seen no response.
Of course, Murphy's Law dictates that your engine will quit due to fuel starvation at the worst possible moment, so you might want to fix the problem before it occurs.