# Is it vapor lock?



## kensoldgoat (Apr 28, 2013)

On my 69 GTO with the base 400ci I'm running a rebuilt Quadrajet that appears to be from an mid 70's Buick. Prior to running the rebuilt carb the car ran a little rough but got me where I was going and back again. Now with the rebuilt I can go about 7 miles and the car just dies. The fuel line at the carb is so hot that I can't keep my hand on it for more than a few seconds. The fuel line is in its factory routing and the top radiator hose is also very close to the fuel line. I'm thinking that the problem is the fuel line is vapor locking and am looking for thoughts, suggestions and comments. Thanks


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## 67Twistytee (Feb 24, 2014)

Doesn't sound like it. Vapor lock usually occurs between the tank and fuel pump. Not common from the pump to the carb. Are you running a fuel return line? Constant flow of fuel from the tank through the pump and back helps to keep gas lines cooler. Any lines running too close to manifold or exhaust? Mine runs close to the radiator hose too but that doesn't get hot enough to vaporize fuel. 

Could also be a spark issue. Have you checked your coil?


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## kensoldgoat (Apr 28, 2013)

thanks. Yes, I have a new coil, new distributor and a fresh tune-up. All fuel lines are original and new fuel filter installed at the carb when the carb was installed less than 50 miles ago. Less than 50 miles on the completely rebuilt original 400 including the heads. Less than 20 miles on the rebuilt trans. Fuel tank cleaned and a new fuel pump installed. Old fuel lines. I haven't tried to blow the lines out. After reading earlier response thinking maybe the new pump picked up debris but the fuel filter at the carb is clean. After sitting for a while the fuel flows and the car starts. Frustrating and confusing. Thanks


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## 67Twistytee (Feb 24, 2014)

Did you check the fuel filter in the carb? Should be a second one in the front fuel line housing. If you're using a correct inline fuel filter with the return line, you can remove the one in the carb housing to eliminate additional restriction.

This sounds to me more like a fuel level problem in the carb. I would take the carb off the manifold, remove the air horn and check your float level. You might have an issue with the float not allowing enough fuel (or too much fuel) forcing it to run rough and then stall on you after a few miles. If it's flooding that might explain while it eventually starts back up after you let it sit. Opening it up will also allow you to check the needle and seat plus make sure your jets are clear. I was using a rebuilt Buick carb from a 68 and noticed it had the wrong filter and incorrect needle installed. Fixed them both and put in a new float and it cured my flow problems. Might be worth the effort to look inside. 

Did you replace the sender when you had the fuel tank cleaned? The stock sender accommodates the fuel return line and could cause a restriction along the return path. If your getting flow at the pump and carb, I doubt you have trash in the main line.


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## PontiacJim (Dec 29, 2012)

" Now with the rebuilt I can go about 7 miles and the car just dies."
"After sitting for a while the fuel flows and the car starts."

Just my guess at this point. Did you replace the gas cap? If you use a non-vented cap, you are creating a vacuum in the system and the gas flow will stop as you described. Let it sit, vacuum eases, and you can start it and go until the vacuum builds in the system once again.

Take the gas cap off and see if it'll run more than 7 miles, or a few minutes.

I would look into sort of happening where fuel flow is not being drawn to the carb rather than vapor lock. Not familiar with gas tanks as some have vents and others did not, but I would start here seeing you have done work to your tank.:thumbsup:


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## geeteeohguy (Feb 2, 2008)

I respectfully disagree with Twisty here. I have run into vapor lock after the pump with the new fuels that have ethanol added (like all of the gas in CA!). A fuel return line set-up will help. That said, verify that it IS running out of fuel. When it dies, pull off the air cleaner, and look down the carb while you pump the throttle. If there is no fuel, you have vapor lock, and it's either in the carb, or somewhere in the lines. The rubber fuel lines can develop cracks that don't leak fuel, but cause the pump to pull air. Check the lines at the tank and at the pump. Again, verify that it is indeed running out of fuel. Your car should have a vented tank. Verify that the tank vent is clear, as well. I used to have a fuel boiling/vapor lock with my '67 until I re-installed the steel insulator plate under the carb, but your car doesn't have one. Sometimes, a fiber or cardboard insulator plate under the carb will cure this issue, if it's boiling in the float bowl. Keep us posted.


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## 67Twistytee (Feb 24, 2014)

Geetee is correct about modern fuels having higher vapor pressure. And I agree you can get vapor lock carb side, but it typically occurs in the fuel bowl after sitting and the fuel stops flowing - like when you turn the engine off on a hot day and you can't restart because the residual fuel boils in the bowl. Using a metal heat shield between the intake gasket and the carb helps prevent this condition as does using a fuel return line close to the carb like one that comes off the inline filter. 

My earlier response was directed at hot conditions causing vapor lock on the suction side of the pump. Vaporized fuel in this location can cause the pressure at the pump diaphragm to drop significantly and stall the engine. Your fuel lines will have higher pressure between the pump and carb and this will raise the boiling point along this section, which is another reason vapor lock typically occurs suction side of the pump. You can cut 5/8" rubber line and try to wrap this section for additional insulation if you feel the line is picking up too much heat. 

Jim makes an interesting comment about the gas cap because vacuum can cause similar conditions.


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## geeteeohguy (Feb 2, 2008)

Agreed. Fuel is most likely to boil in the carb after shut down, when the engine heat soaks. With today's fuels, they can and will boil anywhere in the fuel system where excessive heat is present. A lot of the vapor lock issues happening these days were unheard of with the old leaded gas of days gone by!


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## kensoldgoat (Apr 28, 2013)

problem solved and I say that with a great deal of embarrassment and anger. I tried all of the solutions provided and thank you all very much. I took the car in for service before the problem started. When I took it in I had a full tank of fuel and a working fuel gauge. When I picked it up several items were wrong but the fuel gauge still showed full. After trying everything above, my loving wife of 50 years said maybe it's out of gas. I said I just filled it before I took it in, she said it's probably out of gas. After a long discussion she convinced me to put the lawn mower gas in the tank. I did, it started. While at the shop the gas had been siphoned and the gas gauge no longer worked. So sorry, but again thank you all for the help. Now, to fix the gas gauge.


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## 67Twistytee (Feb 24, 2014)

Glad you got it sorted Ken. No need to be embarrassed. That sounds exactly like several conversations I've had with my wife. Frustrating when going through it, but I always end up learning something new during the trouble shooting.


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## PontiacJim (Dec 29, 2012)

Here's my experience on that. I did some work on the side on cars and a guy had his car towed in to me to work on. Said it would not run. Put a little gas down the carb, it would fire off, but quit. Ok, so fuel delivery problem. 

He tells me he just put gas in it the night before and his gauge reads it has gas in it. Quick thought is its the fuel pump -its older and they do fail. So I replaced it. Still would not run. I pulled the hose off the line coming from the tank to the fuel pump, put it in a little gas can, it draws up the gas and runs. I tried to draw gas from the tank line and nothing, but it was not blocked either because I could draw through it. Hmmmmm. Connected the line up, put a gallon of gas in it, and it fired right up.

Someone had siphoned the gas out of his tank that night and the gauge was still showing plenty of gas in the tank. Needless to say, I just charged him for the new fuel pump and nothing more. Who knew?:smile2:

PS - Funny how your wife knew to do this. That my friend is called a woman's intuition.:blush2: You owe her a dinner.:thumbsup:


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