Vapor Lock

[celentanogj]

Have an interesting issue with vapor lock, when I have a full tank of gas car runs great with no issues, as it gets down to just less than a 1/2 a tank I start having vapor lock issues. The problem isn't continuous driving and running the tank down and than having issues, I'll have the car out than put it away take it out again and when after several weekend run and it gets down to the 1/2 tank level the vapor lock issues start. I just installed an insulator gasket and insulated the fuel line and the dual feed lines, it has helped but now the problem starts when it's down to a 1/4 tank. The car has an external electric fuel pump, 1/2 inch braided lines, and a fuel regulator (no return line), would changing the fuel regulator to one with a return line solve the problem?

[SwiftDB4]

Return line solved vapor lock on my GT40. Also extended fuel pump life.

[Tommy]

A better description of the symptoms would help. For example, when the fuel level gets to 1/4 tank, is the problem there even on a cold start, or does it have to be run for a while before the symptom appears? Is the problem there only on startup or does it occur while the engine is running? And is your fuel pump lower than the bottom of your tank?

[celentanogj]

The fuel pump is higher than the tank and it happens when the car runs not on a cold start. A good example-while driving over the past weekend (85 degrees) the car was on and off the hwy and when in light traffic it stalled while stopped at a light, luckily I was only 50 yards from the gas station, the gas gauge was showing just over a 1/4 tank when it stalled, I was able to restart the car and got it to the gas station when it stalled again as I got to the pump. Gas the car up drove it home (another 5 miles) no issues.

[Tommy]

Gasoline (and other liquids) boil when the combination of temperature and pressure is right. Higher pressures and lower temperatures inhibit boiling. That is why vapor lock is not an issue downstream of a functioning fuel pump under normal underhood temperatures. Fuel may boil in a carburetor float bowl, but that is not "vapor lock" and fuel in the float bowl is not pressurized.

Vapor lock occurs at the fuel pump when the combination of pressure and temperature of the incoming fuel is right. There are two reasons why the temperature of the fuel may be high enough to boil. One is a very high ambient temperature in the fuel tank or line to the pump. Ethanol makes that more possible, but it is still very unlikely. The second reason is in the design of the pump's internal pressure control system. On systems without an external pressure control and fuel return line, the pump has a spring controlled valve on the output side of the pump that closes the flow of fuel to the carburetor when it reaches the designed pressure (e.g., 8 PSI). To keep from stalling the rotating pump vanes, the fuel is diverted through the pump housing back to the intake side. That means that the same fuel will be pumped through the vanes repeatedly until the output valve opens. As electric fuel pumps get hot while operating, that fuel will get hotter with each pass. When the temperature gets hot enough it will boil and the pump will become vapor locked as the vanes cannot pump vapor effectively. This phenomenon is why an external pressure relief valve and return line helps. It keeps fuel from getting cycled repeatedly inside the pump housing.

The pressure of fuel flowing to the fuel pump can also cause vapor lock. As the pump's vanes grab incoming fuel and push it out toward the carburetor, it creates a low pressure area at the intake. The combination of ambient air pressure inside the tank and gravity from the weight of the fuel in the tank and above the pump forces fuel to fill that low pressure space. If the fuel pump is mounted too high relative to the fuel level in the tank, the pressure from gravity will be reduced. This reduced pressure means the fuel can boil at a lower temperature. So with a combination of high ambient temperature and low pressure at the intake of the pump, vapor lock can occur.

The observation that your problem becomes more pronounced as the fuel level in the tank comes down is what lead me to suspect that your fuel pump is mounted too high. Either that or it is failing and unable to pump efficiently without help from the weight of a full tank of fuel.

[Unique427]

Originally Posted by Tommy

Gasoline (and other liquids) boil when the combination of temperature and pressure is right. Higher pressures and lower temperatures inhibit boiling. That is why vapor lock is not an issue downstream of a functioning fuel pump under normal underhood temperatures. Fuel may boil in a carburetor float bowl, but that is not "vapor lock" and fuel in the float bowl is not pressurized.

Vapor lock occurs at the fuel pump when the combination of pressure and temperature of the incoming fuel is right. There are two reasons why the temperature of the fuel may be high enough to boil. One is a very high ambient temperature in the fuel tank or line to the pump. Ethanol makes that more possible, but it is still very unlikely. The second reason is in the design of the pump's internal pressure control system. On systems without an external pressure control and fuel return line, the pump has a spring controlled valve on the output side of the pump that closes the flow of fuel to the carburetor when it reaches the designed pressure (e.g., 8 PSI). To keep from stalling the rotating pump vanes, the fuel is diverted through the pump housing back to the intake side. That means that the same fuel will be pumped through the vanes repeatedly until the output valve opens. As electric fuel pumps get hot while operating, that fuel will get hotter with each pass. When the temperature gets hot enough it will boil and the pump will become vapor locked as the vanes cannot pump vapor effectively. This phenomenon is why an external pressure relief valve and return line helps. It keeps fuel from getting cycled repeatedly inside the pump housing.

The pressure of fuel flowing to the fuel pump can also cause vapor lock. As the pump's vanes grab incoming fuel and push it out toward the carburetor, it creates a low pressure area at the intake. The combination of ambient air pressure inside the tank and gravity from the weight of the fuel in the tank and above the pump forces fuel to fill that low pressure space. If the fuel pump is mounted too high relative to the fuel level in the tank, the pressure from gravity will be reduced. This reduced pressure means the fuel can boil at a lower temperature. So with a combination of high ambient temperature and low pressure at the intake of the pump, vapor lock can occur.

The observation that your problem becomes more pronounced as the fuel level in the tank comes down is what lead me to suspect that your fuel pump is mounted too high. Either that or it is failing and unable to pump efficiently without help from the weight of a full tank of fuel.

This sounds and feels like the correct diagnosis. I might add that it may
not be a single dimension problem. Don't overlook the possibility that
the fuel pump is mounted too high AND it's weak.

Also, here's a link to Sunoco's write-up about Reid Vapor Pressure (RVP)
and how race gas can help mitigate vapor lock.

https://www.sunocoracefuels.com/tech...pressure-vapor

[celentanogj]

Thanks Tommy, for the info. I'll have to check the pump. If the pump checks out than the answer is going to be a return line to keep the fuel cycling and reducing the stress on the pump. Considering that the car was built in 1997 electric parts "ARE" due to fail. Hopefully the simple solution will solve the problem.

[Tommy]

If this car has worked well for years and only recently developed this problem, then replacing the pump may be all it takes. Adding a return line can't hurt and may help. But lowering the pump would be my first effort unless the configuration of the car makes that a huge task. .... Remember to disconnect the battery and safely drain fuel sources early in the process.

[celentanogj]

The issue of the vapor lock continues but at least I found the problem, the issue is the pump itself, it's overheating. I finally got underneath the car and the fuel pump is a Barry Grant type 280, researching the pump it's not a street (continuous use) pump the BG is a racing pump used mostly for the drag strip, the pump is great for short trips but not for the long hauls. Question, what pump can I use to replace the BG for regular use? BTW this BG that I have has a built in bypass there is no provision for a return line.

[bkozlow]

How is your tank vented??

[ERA174]

Replacement fuel pump topic has recently been discussed in this thread, worth a look. The Holley 12-125 or 12-150 may work for you.
Holley Fuel Pump 12-125

Holley Fuel Pump Chart link, https://documents.holley.com/holleyf...ems_chart1.pdf

[Tommy]

My first step in choosing a replacement pump would be finding one with the same fittings and method of mounting. I like the Summit Racing site where I can input those kinds of parameters and see what will work. Pay attention to the output pressure so its compatible with your carb/FI.

[SwiftDB4]

Originally Posted by ERA174

Replacement fuel pump topic has recently been discussed in this thread, worth a look. The Holley 12-125 or 12-150 may work for you.
Holley Fuel Pump 12-125

Holley Fuel Pump Chart link, https://documents.holley.com/holleyf...ems_chart1.pdf

I would avoid the Holley gerotor fuel pumps. Went thru two of them in short order due to o ring seal failure on the shaft. Can only be repaired by sending them back to Holley.
Have had much better luck with Carter P4600 fuel pump.

[Tommy]

Originally Posted by SwiftDB4

I would avoid the Holley gerotor fuel pumps. Went thru two of them in short order due to o ring seal failure on the shaft. Can only be repaired by sending them back to Holley.
Have had much better luck with Carter P4600 fuel pump.

It would be wise to check out a wide range of reviews. My experience was just the opposite of the above. I just replaced a Carter P4600HP after 20 hours of use due to a failed internal pressure regulator. A small number of anecdotes don't tell you much.

[celentanogj]

Update on the vapor lock issue-did the return line on the fuel system, car runs flawless, no vapor lock problems.
Sometimes the simplest solution works.