Posted by Alan Bowes on July 16, 1999 at 09:35:53:
I originally posted this on the M37 forum in response to someone's vapor lock question, but a couple of people suggested posting it here as well:
Some points to note about vapor lock:
1- If you install an electric fuel pump to try to avoid vapor lock, you should bypass the old mechanical fuel pump, since you will be adding the pressures together, which will affect the fuel level in the carb by
delaying the float valve shutoff until the level is high enough to shut the valve against the higher pressure. In more extreme cases, the float valve might not be able to shut off. If you
want, you could add a manual valve so that you could switch pumps.
2- If you install an electric fuel pump, do not mount it too high. The higher you are above the fuel in the tank, the more negative pressure you'll draw on the suction side, and the more
prone it will be to vapor lock.
3- Insulating the fuel lines is always a good idea, ESPECIALLY near exhaust components, which are much hotter than the engine block. Don't forget to insulate the suction side of the
fuel line. In fact, this may be even more critical than the upper part of the fuel line.
4- Make sure the heat shield is in place over the fuel pump.
5- Make sure you have a thick insulating gasket between the carburetor and the intake manifold.
6- Make sure the intake manifold heat valve is closed during warm weather operation.
7- Don't use higher-volatility gasoline (winter gas) during the summer.
Isolated vapor bubbles in the fuel pump or line are not really a big deal, since the stock diaphragm-type pump can certainly push it on through very quickly. However, if a situation
develops that CONTINUALLY produces vapor bubbles, then you have a "vapor lock" condition.
Understanding what vapor lock is may help you cure or avoid it. Here's a quick explanation:
If the fuel pump (or a fuel line) gets hot enough, the fuel passing through it will flash into vapor, which occupies MUCH more volume than the liquid fuel that produced it. The pressure
produced by the conversion to vapor pushes any remaining liquid fuel in the line(s) along the path of least resistance, usually back into the fuel tank. If the pump or line remains hot,
every time the pump draws (or forces) liquid fuel into the hot area, it just flashes into vapor again, repeating the cycle.
Here are three common vapor-lock scenarios:
1) Hot lower fuel line (suction side - between the pump and the fuel tank):
As the pump draws liquid fuel into the hot area of the line (usually near the engine or exhaust system), the fuel flashes into vapor and forces the remaining liquid fuel back into the fuel
tank. Every time the pump draws liquid fuel into the hot area, it just keeps flashing into vapor and getting forced back to the tank in a vicious cycle until the hot area of the line is
allowed to cool down. This is the most common vapor lock scenario, since fuel flashes into vapor MUCH more easily on the suction side than on the pressure side of the pump.
By the way, as already mentioned, if you have the vent on your fuel tank capped, it can build up a vacuum inside the fuel tank. If the seal is perfect, it can stop fuel flow, and even if it
isn't a perfect seal any additional vacuum will further drop the temperature required to convert liquid fuel to vapor and bring on vapor lock much more quickly.
2) Hot upper fuel line (pressure side - between the pump and the carburetor).
This is a less common scenario, partly because this side of the system is under pressure, which raises the temperature necessary to convert the liquid fuel to vapor, and also
because any vapor produced tends to be expelled rather quickly through the carb and its vented bowl, allowing liquid fuel to quickly take the place of vapor in the line.
However, if you have a combination of hot fuel that is already near its vaporization temperature AND a hot upper fuel line, you can get a vapor lock situation. So, don't just insulate the
upper part of the fuel line; insulate ALL fuel lines near hot engine or exhaust components.
Carburetor temperature is another factor, since a cooler carburetor will promote some condensation of fuel vapor back into liquid fuel. A thick insulating gasket between the
carburetor and intake manifold will help, as will shutting off the intake manifold heater valve for warm weather operation.
A very hot carburetor can also cause percolation of the fuel in the carb bowl. While this may not be a classic "vapor lock" situation, the results are the same: your engine stops.
3) Hot fuel pump.
As fuel is drawn into a hot pump, it flashes into vapor. Same basic situation as number 2.
Have fun,
Alan