Vapor lock

[semperfi48]

Was thinking about putting a 6 volt fuel pump before the mechanical pump to help prevent vapor lock in summer heat thoughts? or should I by pass mechanical for good and go 100% electric? recommendations on pumps available? 48 P15. Thanks everybody.

[Chester Brzostowski]

As far as electric or mechanical fuel pump it makes no difference to me. As far as vapor lock is concerned I believe the issue is more urban legend then fact. If you vaporize gas in your fuel line it will be under pressure. As soon as the carburetor float falls to allow more gas to flow in.., the pressure would instead vent through the overflow tube in the carburetor because it will certainly not raise the float and trap the gas in the line. If you are having issues with starting while hot look for reasons that your intake manifold is being flooded with gas not vapor lock.

My 2 cents, Chet…

[TodFitch]

As far as electric or mechanical fuel pump it makes no difference to me. As far as vapor lock is concerned I believe the issue is more urban legend then fact. If you vaporize gas in your fuel line it will be under pressure. As soon as the carburetor float falls to allow more gas to flow in.., the pressure would instead vent through the overflow tube in the carburetor because it will certainly not raise the float and trap the gas in the line. If you are having issues with starting while hot look for reasons that your intake manifold is being flooded with gas not vapor lock.

My 2 cents, Chet…

I have had vapor lock occur while idling after long bouts of high speed driving in 95 to 100F temperatures. Temporary cure was to cool the fuel pump down by pour a bit of water on it. I've never had this happen on cooler days even when the engine temperature was about the same, so I think this is partly due to the gas in the tank being at 95 to 100F and so it doesn't take long for it to heat up to a point where some components can vaporize inside the fuel pump.

I agree that hot restart problems are often caused by overflow into the manifold as the carburetor heats up due to under-hood heat and lack of airflow when the engine is stopped. And those are often erroneously referred to as vapor lock.

But if the fuel pump is too hot then gas in it can vaporize on the suction stroke and the pump will not work well.

Was thinking about putting a 6 volt fuel pump before the mechanical pump to help prevent vapor lock in summer heat thoughts? or should I by pass mechanical for good and go 100% electric? recommendations on pumps available? 48 P15. Thanks everybody.

If you put an electric pump into the system, I suggest you put it close to the gas tank away from heat in the engine compartment. You don't want the suction side of the pump to be hot as that is where the issue rises. If the pump is in a cool location and the line from the pump into the engine compartment is under a couple of psi pressure then you shouldn't have an issue for the reasons Chet gives: Any bubbles will be released out via the float valve and vent on the carburetor bowl.

[suntennis]

I went with adding an electric pump near the gas tank and using the mechanical one for all of my driving. The electric is used to prime the carb and when I have a vapor lock issue. It does keep the engine running when the vapor lock happens. Most 6 volt pumps now days have some form of impeller to move the fuel and the makers say not to pull fuel thru it. To do such would require a bypass setup with a check valve. I did not install a bypass with the install and so far it works good. With the electric pump used to prime it makes cold and hot starts much better.

[greg g]

most electric pumps are designe to push rather than pull fuel, so the hearer the tank the better. If you run it in line the check valves in the mechanical pump will allow fuel to pass through with no adverse effects. Conversely the mechanical pump can pull fuel through the electrical pump so having them in series it the conventional wisdom. Most folks use the electric to prime the system on start up then allow the mechanical to take over its regular duty. Just make sure your electric pump is in line with the recommended pressure and flow rate.

Most folks usually equate perculation with vapor lock. Perculation is a over rich or flooded conditio caused by the fuel i the float bowl acting as a heat sink collection heat, expanding and overflowing the fuel (and todays fuel is much more prone to expansion than the old stuff) puddling it on the closed throttle plate or into the intake manifold.

When you try to start it you should treat it as flooded engine, cranking the stater witht he throttel wide open to clear out the raw fuel. A simple fix it to lower the float level slightly to allow more space for expansion before overflow occurs.

[Don Coatney]

Did you ever put the dog house back on that truck?

[Chester Brzostowski]

I have had vapor lock occur while idling after long bouts of high speed driving in 95 to 100F temperatures. Temporary cure was to cool the fuel pump down by pour a bit of water on it.

Sorry Tod,

I don't doubt that you have some type of issue with moving gas from your tank to the fuel pump but gas vaporized in the line is in my opinion an incorrect theory. The electric fuel pump helps because you either have a design deficiency in the fuel delivery system or a problem created by the fuel line size, number of bends, or the height of the line in relationship to the pump. I would believe gravity working against you before I would consider vapor. Obviously the electric fuel pump appears to fix your vapor lock problem because it pushes gas to the mechanical fuel pump.

Sorry Tod, Chet…

[captden29]

i re-routed the fuel line so it does not go near the exhaust manifold. problem solved. capt den

[TodFitch]

Sorry Tod,

I don't doubt that you have some type of issue with moving gas from your tank to the fuel pump but gas vaporized in the line is in my opinion an incorrect theory. The electric fuel pump helps because you either have a design deficiency in the fuel delivery system or a problem created by the fuel line size, number of bends, or the height of the line in relationship to the pump. I would believe gravity working against you before I would consider vapor. Obviously the electric fuel pump appears to fix your vapor lock problem because it pushes gas to the mechanical fuel pump.

Sorry Tod, Chet…

Actually, I don't have an electrical fuel pump. And the "vapor lock" issue has only occurred a couple of times, both in very hot weather that is unusual where I live. So unusual, in fact, that both occasions I had a problem were when I was driving in other areas of the state that get much warmer than here.

Since it is a rare occurrence for me and I know how to deal with it when it happens I don't have plans for altering the fuel system away from stock. But I can see where other people living in other climates could have an issue.

[semperfi48]

Wow love to ask questions with you guys out there. Don't know what happen this summer. BUT it was bloody hot 97-98 and the engine ran good for 10 miles maybe once it burped then I made a stop idled for 3 minutes in the bloody sun shut it down came out 5 minutes later started right up but then only got 500 yards and she died and would not start. Left it came back 3 hours later still not start fuel down the carb and boom she started? Anyone recommend a 6 volt fuel pump???? and what pressure am I looking for? Thanks! Scott 48 P-15 and 41 Dodge pickup.

[garbagestate 44]

I don't know what presssure is good but they do turn up on Ebay all the time.

[greg g]

I believe the manual calls for 5 to 7 psi.

[semperfi48]

Thanks guys, I think I will adjust the float level and use a electric pump to keep pressure up....love over kill. Think I will try the pump first for kicks, Also will drive it now that it is cold and see if it does it at all before putting the pump in. Thanks very much for the talk.

[JohnTeee]

Just out of curiosity . . . are you driving a period correct car/engine?. I have not seen any mention of two things in this thread yet:

Do you have the heat shield installed installed above your fuel pump? You can see it in this picture Don Coatney posted of his engine, between the exhaust manifold and the fuel pump.

Lots of discussion about this in the forums.

What condition is your heat riser in? A heat riser stuck in the 'cold' position, called 'Open' or 'Closed' depending on what you read, could certainly cause problems when the engine is at operating temperatures. You can see it right in the middle of the exhaust manifold in Bob Riding's 'engine porn'. It should rotate 90 degrees forward/clockwise when hot.

And this internal picture from Dodgeb4ya:

Lots of discussion on that too!

Also a nice article on heat risers up front on the website, in the maintenance section.

Cheers!

John

[Don Coatney]

Do you have the heat shield installed installed above your fuel pump? You can see it in this picture Don Coatney posted of his engine, between the exhaust manifold and the fuel pump.

John

Two things here.

1- not my engine, my engine is not silver, I have dual carburetors and dual exhaust

2- I have never installed a heat shield on my runing engine.

Here is my engine.

[Young Ed]

Thats my engine. It had the heat shield so I put it back. My 46 pickup doesn't have one but it might get one next year.....

[JohnTeee]

My error. Still begs the question . . .

Cheers!

John

[claybill]

my dad told me 50 years ago that when it wont start when hot.." there is too much gas in the intake and not enough air. " solution. "...slowly depress the gas pedal (so as not to actuate the pump) and HOLD IT FIRMLY TO THE FLOOR WHILE YOU press down ON THE STarter, being careful not to pump it, or activate the acc. pump. it will start after a FEW SECONDS, LIKE 10.!

bill

[semperfi48]

Yes good info, I do have the heat shield on and my heat riser functions good. I picked up the airtex 6 volt pump from rock auto....But i will drive it this week while it is cold and see if I have any issues. I will adjust the float and install the pump for the hell of it. yes the car P-15 48 is all stock original.

[semperfi48]

No issues at all in cold weather. Will check float level and put electric pump on close to tank with an on off switch and wait for the HOT summer see what happens. Did have a great weekend of driving but at the end of the week master cylinder leaked all fluid over night? Bump I hate bleeding and pulling that damn thing?:mad:

[austinsailor]

Vapor lock has been talked about for some time. Here's a snippet from the Gus Stores from the Model Garage (Popular Science) June 1932 edition:

"Score's even again," Gus laughed. "You're right, Jeff. Clogged carburetors certainly aren't the most frequent cause of road stops, especially in cars less then two or three years old. Last year we had more trouble with 'vapor lock' than we did with clogged carburetors."

"I never had that. What is it?"

"You can blame last year's troubles with vapor lock on good gas," Gus explained. "A few years ago the gas was so poor that automobile had other heating gadgets to get the gasoline to vaporize at all. Then came overproduction in gas and refiners faced such stiff competition that they improved the quality of their products. Automobile manufacturers have been making better motors, too, and last year the more efficient and hotter-running motors, combined with gasoline that turns into gas at a lower temperature than the old stuff, brought a new trouble we'd never bumped into before.

"You'd be bearing along as at a good smart pace on a warm summer day and all of a sudden the motor would begin to spit and blow back just as it does when water gets into the gas line. Sometimes the motor would stop dead. Then after you'd spent a few minutes trying to find the trouble, you'd step on the starter and she'd tick over as though nothing had happened. As first they blamed it on water or dirt in the gas line, but when it kept on happening, the engineers got busy and found that when the motor got just so hot, the gasoline in the pipe line near the carburetor started to boil. Sometimes it would boil in the carburetor bowl itself. Then nothing but gas vapor went into the carburetor with the air and the mixture got too thin to burn."

"By golly!" said Tim excitedly, "Maybe all that clogged carburetor trouble I've been having is vapor lock. I put in a new gasoline pipe last year and I remember I made it shorter than the old one by running it alongside the exhaust pipe. I'll move it over to the other side of the at the frame today."

"Don't bother," Gus advised. "Just move it away a couple of inches and wrap it with asbestos. That'll keep the heat out of it. That's the way the new cars are fixed to get rid of vapor lock - a little heat insulation where it's most needed. Sometimes a few sheets of asbestos slipped in around the carburetor bowl will be worth while if it's real close to part of the exhaust manifold.

[greg g]

Asbestos???? I think it may be hard to come by these days.

I still lean toward perculation and flooding more than a bubble of gas vapopr blocking fluid under pressure from going to a vented receptical as the cause for hot start problems. Lower the float and follow the hot start procedure mentioned above.

Now several folks have mentioned that the evaporation that takes place in the fuel pump rather than in the carburetor as being more of a trouble spot than the carb, as under those conditions the pump will have a problem trying to pull gas from the tank as they are designed to pump a fluid not air.

[TodFitch]

...Now several folks have mentioned that the evaporation that takes place in the fuel pump rather than in the carburetor as being more of a trouble spot than the carb, as under those conditions the pump will have a problem trying to pull gas from the tank as they are designed to pump a fluid not air.

That is certainly the problem my car seems to have developed in the last couple of years when the weather is hot (higher than 95°F) and the car has been driven long and hard enough to be fully warmed up. A little drinking water poured on the fuel pump gets me going again. Maybe one of these days I'll fit one of those fuel pump heat shields that the came with after they built my car.

[Don Coatney]

the pump will have a problem trying to pull gas from the tank as they are designed to pump a fluid not air.

I you run out of gas and air enters the fuel line how can the pump once again pump fuel after you add gas to the tank?

I may be incorrect but I believe that as long as there are no air leaks in the fuel line and the pick up tube is sumbmerged a healthy mechanical fuel pump will pull a vacuum and pump liquid from the tank even if there is air in the line.

[james curl]

I agree