Question please advise how to prevent vapor lock on a 740 packard with an electric fuel pump mounted two feet forward of the gas tank. this is the first occurence in ten years with no change in fuel line routing.
AnswerUnfortunately this is becomming more and more common, as the new reformulated gasoline has a much higher vapor pressure, and boils much more easily than the gasoline of even a few years ago.
There isn't a lot you can do - but make VERY sure that there are no restrictions in the fuel line between the tank and the fuel pump - no bends, no filter, no crimps in the line, and also make sure that any rubber or other fittings are absolutly air tight. Remember that this area of the fuel system is actually under a vacuum when the pump is running, so a very minor leak, too small to observe fluid leaking when the car isn't running, will still admit some air to the suction side of the pump, bringing about vapor lock at any provocation (like a long hill climb or parking on a hot surface, for instance).
Mount the fuel pump as close to the tank as you can get it, with very short connection lines, and keep all the fuel system as far away from the exhaust system as you can arrange. Add a heat shield if neccesary to keep the lines and the pump cool.
As a last resort, (and totally inauthentic) take a page from modern cars and make a vapor return line from a tee in the carburetor inlet, which returns excess fuel back to the tank - thus keeping more fuel moving through the pump and lines, so the cooler gas in the tank will keep things cool enough to avoid vapor lock. You'll have to experiment with various restrictive orifices in the return line attachment fitting, so that you don't reduce the fuel pressure at the carburetor inlet below about 3 PSI. The fuel pump can provide much more gas than the car will need as long as the fuel is kept cool, so you should be able to find a return line quantity that keeps enough gas circulating that vapor lock is avoided. This design is why modern cars don't have vapor lock problems - the pump can provide about 4 times as much as as will ever be needed, the excess is simply drained back into the tank continuously, thus keeping the lines cool with fresh gas from the tank.
I'm just back from a 1500 mile trip in my Hudson - and experienced that problem for the first time in the 22 years I've owned the car - so I know how frustrating it can be. I plan to try this same fix on my car before the next trip.
Good luck,
Dick
