Vapor lock

[Mike V. Florida]

Originally Posted by Bruce Compton, Canada

How is it possible to get " vapor lock" in a gravity feed system???? Please someone explain this as I can't imagine how it can occur. Thanks : Bruce

Please do a search and you find several threads on this subject including a video i believe showing bubbling.

[Bluebell]

Bruce, Tom. This is simplistic, as I have no knowledge of what the details of a Model A are.
I believe "vapor lock" is a relatively rare occurrence, when compared to the number of times the term is used. I think it occurs when a portion of the petrol between the fuel pump and the needle valve changes from the liquid to a gaseous state.
A negative pressure in the tank, cause by a blocked vent is not vapor lock.
A bubble of gas caught between a carb and a gravity feed tank is not vapor lock, its an air lock caused by the run of pipe not having a suitably consistent gradient. I say that because the bubble would run straight up into the tank, and be replaced by more petrol, if the pipe was correct.

[TerryH]

Maybe folks that have this condition should just refer to it as "vapor lock type symptoms", and not really label it as such. That would tend to not aggravate those folks who say it never happens...it could be one of several conditions causing it.

[Dave in Boise]

Interesting thread.. I am pretty hesitant to use 'vapor lock' as the cause for cars to stop running in warmer weather.. That is until our club took a 400 mile tour up and over mountains at 8700 ft in temperatures up to 110+ degrees ambient air at 5K ft. The top of the passes ran about 90-95 degrees at the top of 8-10 miles of 6+ % grade.. definitely a thorough workout for any model a, carrying people, spare parts and other touring items, up mountain passes in record-breaking heat.

I had seen cars that have never had an issue begin to fail at these temps and altitudes. Some were due to the excessive heat baking substandard or tired ignition components and having timing not properly set overheat motors and components to the point of failure. (abt. 60% in my estimation) One had a severe cooling failure as it boiled out nearly all of the coolant, and the driver did not pull over until the smoke started rolling off the engine due to the paint burning off and the oil smoking through the filler tube. Once cooled off, we fired it up and slowly added water to the radiator, checked ignition components and then retourqued the headgasket. the motor ran fine until it was shut down, and no vapor lock was noticed..

Most rolled along after setting timing and replacing ignition components with new parts. My personal car had issues with the coil (overheated so hot that I could not touch it and it swelled to the point that I had to take the clamp completely off the firewall to get it out of the clamp).

I did, though, actually encounter some vapor lock issues which I remedied by cracking the drain on my sidebowl and listening to the pressure hiss out until I got fuel flowing. Once flowing, the car started and performed well. I think the experiences we had were likely extreme, as we did hit 118 at one point, and we slowed our group down to 35mph and took our time in 3rd.

about 60% of our issues were ignition, 30% could be attributed to fuel boiling in the bowl of heat soaked carburetors (you could hear it and see the air rushing out of the line when cracked at the carb inlet) and some drivability issues due to carb jetting and other minor items making up for the last 10%

It was quite an experience.. We are still gathering up pictures from the tour and I will put out a thread on it soon fi anyone has interest in our collective insanity of taking these cars to the extremes we did.. 23 cars started, 23 finished (one on a trailer due to a suspected rear main failure we did not want to have the driver push his luck on)

- Dave in Boise

[steve s]

Originally Posted by Bluebell

... A bubble of gas caught between a carb and a gravity feed tank is not vapor lock, its an air lock caused by the run of pipe not having a suitably consistent gradient. I say that because the bubble would run straight up into the tank, and be replaced by more petrol, if the pipe was correct.

This crosses the line between semantics and chemical terminology. Air is not vapor, and gasoline vapor is not air.

[Bruce Compton, Canada]

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I was not saying that vapor lock does or does not exist, I was simply asking for someone to logically explain how it can happen in a gravity feed system. What I'm having trouble understanding is how the carb could become dry. The needle valve can not shut off air if the bowl is empty, full of air, or even a bit low due to the "crap" fuel boiling, so assuming it's open, it would'nt be long until fuel from the tank or line filled the bowl with fuel, or even a mixture of air and fuel. This flow can only be stopped by the needle valve when the bowl is full of fuel. I have driven flatheads for many many years and have been let down five times. All five were ignition problems (hot coils) and none were "vapor lock". Back in the day you would always hear about vapor lock and how it was fixed by putting a dime in the fuel pump arm, or wrapping the lines with cold rags etc.,but whatever the remedy, it usually took 15 to 20 min., which is just about the length or time required to let the coil cool down. Messing with the fuel pump could have been replaced by changing a tire or going for a stroll down the road and back, all would have had the same effect. A friend of mine with a Hershey winning '34 Phaeton just experienced "vapor lock" and almost killed the battery trying to restart it after a short stop. Fortunately it came to life after a very short push. What really happened was that the hot coil would not fire as the hot motor drew too much out of the battery. He now carries a spare coil and no cold rags or fuel pump saving dimes. Still waiting for that logical explanation. Cheers : Bruce

[Duffy1]

I think we should just call the occurence "IT" . So when your engine stops running and you pull off the road and some one stops and asked what is the problem just tell them "IT" happened again.

[Mitch//pa]

i had fuel lock once when my gas cap was not venting

[steve s]

Originally Posted by Bruce Compton, Canada

I was not saying that vapor lock does or does not exist, I was simply asking for someone to logically explain how it can happen in a gravity feed system. ...

It can happen due to surface tension combined with a narrow or convoluted pathway causing a bubble of gasoline vapor to effectively stick somewhere so as to block the gravity driven flow of liquid.

The science of it is no different than with "air lock," which is common in ordinary water plumbing. The best example I know is when a bath room sink sometimes acts as though the drain line is plugged, refusing to drain until you jiggle the stopper whereupon the trapped bubble of air burps out and flow resumes: This is a gravity fed system in which there is no blockage other than a bubble of gas--which SHOULD go up, but doesn't--preventing flow of a liquid that SHOULD go down, but doesn't.

Surface tension prevents the bubble from deforming far enough from its spherical ideal to slip through the "crack". Removing the stopper or enlarging the pathway would solve the problem.

Because fuel pumps used to be the common location of the problem, some folks now feel that vapor lock BY DEFINITION has to involve a fuel pump. We disagree, but that may be semantics.

Steve



Steve

[Tom Wesenberg]

What Steve said!
I was parked in the Ford factory parking lot for a few hours on a hot day. When I went to leave I turned the gas valve on and the engine fired right up, but died after a minute. I tried the choke, but no gas would flow until I unscrewed the fuel line at the carb. I then drove to another car show about 15 miles away and the same thing happened again after setting for a few hours. That was with 10% ethanol crap gas. So far I haven't had problems when using the good gas.

[Bluebell]

Terminology;
In a GRAVITY FEED SYSTEM ( with no pump) the fuel runs down the pipe to the needle valve. If the pipe has an up hill turn in it it can trap a bubble of vapourised fuel. Yes, you could call this vapour lock, because thats what it is. If however, the pipe is shaped correctly, it would not get trapped, but run straight back up into the tank as liquid fuel ran down to replace it.
A portion of the fuel would possibly under some conditions vaporize, however the driver would likely never know. This is not what we mean when we talk about vaporlock.

The term Vaporlock does apply here;
When the car has a fuel line that runs low in the car (along the chassis?) and up to the fuel pump, the fuel is drawn to the pump and pushed to the needle valve at the carb.
If heat conditions exist that cause the fuel to vaporize between the needle valve and the fuel pump, this is the result;
The fuel pump is delivering petrol at say 2.5 psi.
The fuel in the pipe changes from liquid to gas. This change of state causes an increase in pressure in the line, due to the volume increase caused by that change of state.
The pressure backs up to the pump, making the pump "idle".
(Delivery pressure is solely dictated(caused) by the spring under the diaphragm. When the pressure in the delivery pipe reaches the 2.5 psi, the pump stops working. The push rod is still pushing up and down against the pump arm, but the diaphragm has stayed down against the compressed spring.(the pump has an idle mechanism built into it.
Nothing can happen in the pump until the pressure drops in the delivery line.
You have vapor lock.
As a mechanic, this is the what the term really means.

[steve s]

Originally Posted by Bluebell

... Yes, you could call this vapour lock, because thats what it is. ...

Well, we agree on this much.

[Dave in Boise]

so, not to stir this up to much, would having a larger diameter fuel line allow the air or vapor bubble the opportunity to burp out more easily, and allow liquid fuel to flow to the carb in a vehicle with this issue?

[Bruce Compton, Canada]

Thanks Bluebell: I believe and understand everything you said about the fuel pump systems and vapor lock, but as there is no fuel pump in a Model A and the fuel line is basically vertical with two maybe 2-6" of almost horizontal runs, I still can't see where a section of boiled gasoline (vapor) in this line can stop the flow. Would the head pressure and weight of the fuel in the tank not push the "vapor" down and into the carb?? People are trying to convince us that the vapor or air in a vertical line can shut off the flow even if there was an open end at the bottom (which is the same thing as an open needle and seat assembly). Bruce

[Dave in Boise]

Originally Posted by Bruce Compton, Canada

Thanks Bluebell: I believe and understand everything you said about the fuel pump systems and vapor lock, but as there is no fuel pump in a Model A and the fuel line is basically vertical with two maybe 2-6" of almost horizontal runs, I still can't see where a section of boiled gasoline (vapor) in this line can stop the flow. Would the head pressure and weight of the fuel in the tank not push the "vapor" down and into the carb?? People are trying to convince us that the vapor or air in a vertical line can shut off the flow even if there was an open end at the bottom (which is the same thing as an open needle and seat assembly). Bruce

I kind of envision a percolator.. the pressure of the vaporizing fuel must be enough to offset the 1-2 psi of the gravity, or at least enough of it to slow the fuel to a point of starvation..

[Duffy1]

I had "IT". Turned out to be the gas cap was not venting enough.

[steve s]

Originally Posted by Dave in Boise

so, not to stir this up to much, would having a larger diameter fuel line allow the air or vapor bubble the opportunity to burp out more easily, and allow liquid fuel to flow to the carb in a vehicle with this issue?

IF the blockage is in the fuel line, a larger diameter should help. However, I suspect it is at some bottle neck or sharp bend, like the carb filter area.

I have watched with my own eyes as a clear plastic inline filter filled up with vapor and shut down the engine. I have also seen one of those slick little spring supported micron filters that go in the sediment bowl serve as a locus for vapor blockage. BUT, I have also watched vaporization proceed in the top of my sediment bowl and then the vapor bubble burp on down the line with barely a hiccup from the idling engine. Lots of variables.

Steve

[steve s]

Originally Posted by Dave in Boise

I kind of envision a percolator.. the pressure of the vaporizing fuel must be enough to offset the 1-2 psi of the gravity, or at least enough of it to slow the fuel to a point of starvation..

Bluebell's pressure increase theory sounds good if you read it fast, but it only makes sense if the system is sealed up to maintain a constant volume. That may be the case in the innards of a fuel pump (I don't know, but I doubt it), but it's definitely not the case in a Model A fuel line. The pressure in the vapor bubble will be atmospheric plus that due to the "hydrostatic" head of the column of gasoline above it, which, will actually be slightly decreased due to the height of the bubble. In other words, the depth of the fuel (and the pressure therefrom) in the fuel tank is not going to perceptibly increase when a vapor bubble forms somewhere in the fuel line. There is no magic, high-pressure bubble in there holding up the column of liquid; if there were, it would simply expand to equalize the pressures.

[steve s]

Originally Posted by Tom Wesenberg

What Steve said!
I was parked in the Ford factory parking lot for a few hours on a hot day. When I went to leave I turned the gas valve on and the engine fired right up, but died after a minute. I tried the choke, but no gas would flow until I unscrewed the fuel line at the carb. I then drove to another car show about 15 miles away and the same thing happened again after setting for a few hours. That was with 10% ethanol crap gas. So far I haven't had problems when using the good gas.

Originally Posted by Duffy1

I had "IT". Turned out to be the gas cap was not venting enough.

I had earlier concluded that my gas cap doesn't exist. I now believe that Tom's also doesn't exist.

[Bluebell]

[QUOTE=steve s; Bluebell's pressure increase theory sounds good if you read it fast, but it only makes sense if the system is sealed up to maintain a constant volume]

So Bruce, here we stand, back to back, against the mob, pondering the issues of life.

Steve S and others.
I have never put a spanner to a model A in my life, but I can tell you that there is something wrong with your car if a bubble of gas in your fuel line cannot make its way immediately, up into the tank.
The laws of nature / science dictate so. Try putting some fuel in a clear tube, leaving a bubble. Block both ends with your thumbs, and now tip it to vertical.
Does the bubble rise to the top?
Can you keep the bubble down?
Would you call the reason for the bubble being there "vapor lock"?
Would you trust a spirit level?
Maybe you should read my post slowly.

I have not tried to detail all facets of the operating process of a petrol pump, rather I have sought to clarify the reasoning behind the term "vapor lock" for those who do not know.
All the other airy fairy fuel blockage, blocked cap vent issues are all just patterns in the clouds.
I am here on this forum, firstly (and selfishly) to learn, and secondly (the payback) to help people with some of the stuff I have learnt from 43 years of mechanicing for a living.