Q&A / Tech

Ask Away with Jeff Smith: Understanding Vapor Lock–and How You Can Fix It!

What exactly is vapor lock? My friends say I have a problem with vapor lock. Last summer, my small block 350 Camaro had problems in the heat of late summer where it would start running poorly and eventually quit running and it has already happened again this year. If I let it cool down for a couple of hours, it starts hard but eventually runs okay. This doesn’t happen all the time, but it seems like it happens more when I drive in traffic. Any thoughts?


Jeff Smith: Vapor lock is a term used to describe when fuel changes from a liquid to a vapor before it arrives at the carburetor. Most enthusiasts tend to focus on the pressure side of the fuel delivery system when the real problems are more likely located on the low pressure or inlet side of the pump. But first, it’s important to mention that fuel is designed to vaporize. Liquid fuel doesn’t really burn. It’s the fuel that has vaporized that supports combustion. Gasoline is a complex mix of light and heavy hydrocarbons that are designed so the light or volatile portion of the fuel vaporizes quickly, even at normal or low temperatures. This allows a cold engine to start quickly. So we’re dealing with a fuel that is designed to vaporize.

ASK-08-01Now let’s imagine we have a very warm day of 90 degrees. At this temperature, roughly 20 percent of the fuel has already converted to a vapor as measured at normal atmospheric pressure. Now let’s introduce a mechanical fuel pump into this application where the pump must “pull” fuel from the tank all the way to the engine where it can now push the fuel into the carburetor. Before we go any further, it’s important to note that pumps do not “suck” the fuel. The pump creates low pressure on its inlet side and higher atmospheric pressure pushing down on the fuel in the tank (because the tank is vented), is what pushes the fuel toward the pump. However, the fuel is still subject to lower atmospheric pressure. This reduced pressure also lowers the boiling point of any liquid and especially gasoline which means a higher percentage of fuel will vaporize.

Can you see where we’re headed?

So we have hot fuel (that has been percolating in the fuel tank exposed to heat coming off the pavement that might be 120 degrees F, and now we subject the fuel to lower pressure in the fuel line. Then, if there is any kind of restriction in the fuel line between the fuel in the tank and the pump at the front of the vehicle, this restriction will create even lower pressure in the line heading to the pump inlet. For example, an older car with an original and a fossilized sock stuck on the fuel inlet in the tank could cause such a restriction. Or perhaps the fuel line has been kinked or bent or the existing tank vent has become plugged or restricted. Any combination of these situations will force the pump to work harder and create lower pressure on the inlet side in an effort to continue to pump the fuel.

This creates a perfect scenario for vapor lock.

What really happens is that enough fuel turns to vapor that a pump designed to push liquid is now trying to pump vapor. That won’t work very well and the result is that fuel pressure drops very low as only a small portion of the fuel is still in liquid form and is not sufficient to feed the engine. So the engine stumbles and dies.

A common and successful remedy for inlet side vapor lock problems is to place an electric pump inline near the tank inlet. If you think about this, this is an excellent solution because it minimizes the distance the mechanical pump must work to move fuel under low pressure. With an electric pump, the fuel pump has now pressurized the fuel during a majority of the distance it must travel, greatly reducing its exposure to low pressure. This radically reduces the chances of the fuel vaporizing since higher pressure requires higher temperature to vaporize the fuel. This is a big reason why in-tank electric fuel pumps are used in all new car applications.

So far, we have mainly discussed the inlet side of the fuel system because this is where the majority of the problems occur. However, it is also possible that fuel can percolate in the fuel line between the pump and the carburetor–especially if the fuel line is exposed to high temperatures. We’ve seen a small block Chevy experience vapor lock when the car builder positioned the fuel line directly against a heater hose that literally pre-heated the fuel in the line. Relocating the fuel line solved that issue. Other remedies for fuel boiling in the carburetor include heat insulation plates (such as a phenolic spacer) between the carburetor and the intake manifold. A cold air induction system for the carburetor will also help since this cooler air will keep the carburetor cooler. This can be especially helpful when underhood temperatures can easily reach as much as 160 degrees F.

We’ve offered several solutions to the problem with the important information emphasizing the inlet side of the pump rather than the pressure side. This is not based on speculation, but rather on simple physics that point to the low-pressure side of the fuel delivery system.

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  1. with my 72 camaro bbc i have the old style performer intake where the holley carb bowls set directly close the fuel was boiling causing my ride to die set for a few minutes it would fire back up.The cure 1 inch adapter plate to get the carb off the intake problem fixed.

  2. Try putting wooden clothes pins on the gas line going into the carb. That’s what we did in the old days. It works!!!!

    • Donald moore says:

      I know exactly what you mean. I put it in the same category as an old wives tale only because wood is a lousy conductor. Living 50 years in the desert we find it better to block off the heat crossover or to use a phenolic spacer.

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  7. Wanda Kierstead says:

    What cause a vapor lock in the engine? I start my Ford F250 to let it warm up, i turn it off, but when i start it again it won’t start it locks up. What do you think?

    • OnAllCylinders Staff says:

      Hey Wanda, without re-hashing too much of what Jeff said above, vapor lock can be caused by a few issues.

      You didn’t mention how long you let your truck sit before re-starting but it’s possible that the built up heat in a stationary engine could cause vapor lock, but that problem would go away as the heat began to dissipate from the engine after, say, a handful of minutes. We assume the engine starts normally after being shut off for a night, or even a few hours.

      You also didn’t mention what year your truck was, but in the story above, Jeff recommended a few basic troubleshooting steps: look around the engine and make sure none of your fuel lines are making unnecessary contact with the engine, headers, or anything else that generates heat. You’ll also want to ensure none of your fuel lines or hoses have any odd kinks or bends that could restrict flow.

      Also, inspect the condition/health of your fuel pump, and any fuel filters you have installed, as Jeff said above, blockages can contribute to vapor lock.

      • Thank you Paul, my husband found this truck. Him and his friends worked on this truck for months and it seems that the truck was modified with a 74 engine in a 98 Ford ( i think thats the year he said it was) glass pack exhausts. He made sure none of the line could heat up or kink up. He even checked the gas cap. All the parts replaced are new. But anyhow thanks for your reply.

  8. bill lum says:

    I have tried almost everything guys have suggested on how to eliminate gas percolation in a carburetor. My latest experiment is using using holley quick change jet bowls, taping the plug holes on the passenger side to 1/8 npt on both primary and secondary bowls and jury rigging fittings and hoses to a return line. I thought by circulating the fuel in the bowl that it would remain cool and the bowls would drain after the engine is turned off. End of percolating. Used a 45 degree fitting at the bowl, then a straight fitting, 1/8 npt to 5/16 barb on the front and rear bowls that went to a T with 1/4 npt on each side with 5/16 barbs From there to a return line.

    • Gas still percolates even with the change I made to circulate the fuel in the bowls. Shut dow the engine, bowls drained but could see fuel coming from vent. Think whatever gas was left in the metering block, was boiling. Think the problem is that the engine compartment is too hot. Another thing to work on.

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  10. I have a 1976 Chev. Truck with a 454 and a quadra-jet carb. My vapor lock issue is getting worse. I have an electric pump by the tank. The fuel filters and the mechanical pump is new. I had the fuel line from the pump to the carb insulated. I drove it 30 minutes and stopped for 45 minutes. Only 83 degrees. After it sat another 45 minutes; I finally got it started. Ran rough at first, then smooth as can be. Anyone have a idea of what to do, short of replacing carb with EFI?

    • Jeff Smith says:

      Your problem may not be related to heat soak. The Q-jet was famous for leaking all the fuel out of the bowl through two small drilled passages at the base of the float bowl that were plugged with lead balls. These balls will eventually leak. Since I don’t know if your carburetor has ever be rebuilt – if so, it’s possible the rebuilder did not cover these plugs with epoxy.That is a standard rebuild technique. If not, then sitting for awhile will drain the fuel and make the engine act just like it has vapor locked. This is made worse with higher engine temperature. I’ve seen Q-jets that would only leak after they warmed up! Check this out before looking into other heat-related issues.

      • Jeff: Thanks for the response. If the carb was rebuilt; it was a long, long time ago. If fuel leaked out the bowl, wouldn’t it refill once you tried to start the engine? When cold or if you stop for a short time, it starts quickly. I have an clear aux fuel filter before gas gets to the pump and it had fuel in it. Once the temp gauge dropped to 160-170 degrees, it started. Very frustrating and appears to be getting worse.

        • Jeff Smith says:

          The fuel would refill in the bowl, but with all that fuel in the intake manifold, it would not want to start right away and would be loaded up – much like you described. This may not be teh problem – best thing to do is to pull the carburetor and look for these protrusions below the fuel float bowl well. There’s also a large round plug that might leak. Look for epoxy covering these areas. If there’s no epoxy – I would start by applying it on these plugs. If you look it up online, there are photos that will show where to place the epoxy. Or- take it to a carb specialist and let him do it.

        • Jeff Smith says:

          Jim: I asked my buddy Sean Murphy about your problem and he said that the biggest problem he sees with Q-jets is that they are extremely sensitive to fuel pressure. The float assembly lever is really short so any fuel pressure above 5 pis tends to overpower the float and push additional fuel into the bowl. This allows fuel to immediately start to drip through the boosters and into the engine. So check your fuel pressure with the truck running. Best way is to tap into the fuel line and run a low-pressure gauge (0 to 10 psi) and tape the gauge to the windshield and drive around. You can also check after driving by pulling the air cleaner and looking to see if fuel is dripping from the boosters. This can also occur because those old nitrophyl (plastic) floats will eventually go bad. Murphy says he’s seen bad gas even melt the plastic – but that’s rare. Check it out and let me know if any of this works out.

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  12. I put a vapor fuel filter on, I have a low pressure electric fuel pump for carbs, now it’s got a 1/4 “ return line, my fuel gauge now reads 1lb instead of 5lbs , how do I fix this ?

  13. I have a 1996 Ford bronco I smell fuel but no leaks in the morning it starts fine but during the day it does not like to start. What can I do to fix it

  14. Hi Jeff
    I have a stock 69 z28 302. After driving my car and when I shut it off for a little while, it wont fire up until I hold the throttle wide open. I heard these carbs are known for this. Is this a vapor lock?


  15. Richard L. Jones says:

    Hi Jeff I have a 1980 Chevy truck that has a vapor lock issue
    and what I have been using to eliminate it is about 4 wooden
    cloths pins on the fuel line from pump to carb. Its cheezie I know but affective. I was wondering if there is something on the market I can use to insulate the line ? maybe a spray on or hose of some sort I slip over the line any help would be appreciated.

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