Vapor Lock

[jamesv]

Can someone explain the basics of vapor lock to me, and include the typical pressures and temperatures involved? eg. What temps does this occur at in the typical fuel line at normal fuel delivery pressures (30-40psi?). Does it almost always occur in the fuel line under the hood?
Secondly, what pressure would it take to return the vapor to liquid? I'm wondering if by pinching off the fuel return hose to increase pressure, a case of vapor lock could be cured.

 

[Metalguy]

Some pumps have difficulty with gases vs. liquids. They can pump air, but when gasoline gets hot it makes a lot of vapor, and as the pump lowers the suction-line pressure it just makes more vapor. Either cool off the pump or add a booster back by the tank to keep the line pressurized and therefore full of liquid gas.

Different blends of gas have widely diff. vapor pressures, so it's hard to say at what temp. there will be a problem. You'd need the RVP (Reid vapor pressure).

 

[franzh]

The primary reason for the return line is to allow the vaporized gasoline to return to the tank.

Metalguy is correct in his description, plus the RVP of gasoline blends is critical, especially during seasonal temperature changes.

Instead of trying to run an end pass around the vapor locking problem, try to identify what is causing it. One of the most common problems is a restricted fuel tank filter (the submerged one). If it is restricted, the fuel pump will try to pull a greater vacuum, effectively lowering the pressure, and causing the gasoline to vaporize due to low pressure instead of high temperatures.

Increasing fuel pump pressure (or just replacing the fuel pump with a new one) will often cure the problem, but if the submerged filter is restricted, the new pump will exhibit the same problem.

Franz

 

[Chumley]

Jamesv wonders:

"I'm wondering if by pinching off the fuel return hose to increase pressure, a case of vapor lock could be cured."

Without knowing your specific application and assuming that
you have fuel injection and a roller cell type fuel pump,
the reason for the liquid fuel return line to the tank,
when present, is to help stabilize the fuel temperature
and pressure by circulating fuel out of the hot engine
compartment and back to the fuel tank where it can cool
off. By restricting the return line, you may cause
pressure spikes that fool the fuel injection metering
system, will most likely cause your fuel temperature
to go up rather than down, and will put additional load
on your fuel pump causing premature wear.

Most modern Bosch fuel injection systems circulate (and
filter) about 20 to 30 gallons of fuel per hour between the
tank and the engine. Driving at 60 mph and getting 20
miles per gallon would use only 3 gallons per hour.

The best plan as Franzh says is to find your specific
problem, and fix it! It could be either heat or pressure
related. A plugged fuel tank vent could cause the fuel
pump to cavitate and create bubbles in the line. Does
removing, and reinstalling your fuel cap solve the
problem? If you have a newer car (96 and above) don't take
the fuel cap off with the engine running. There should
usually be a slight pressure in the fuel tank.

Chumley