I've given this thread the same title as a previous thread on the topic so that anyone searching it will come up with both threads and have an opportunity to hear an alternate view.
As some will already know, I am not in agreement with the frequent diagnosis of heat soak and percolation as the cause of dripping carbs, hard hot starts and rough idle. I've outlined a few of my reasons below.
The proponents of this theory have previously posted a fuel distillation chart along with reported temp readings of the fuel system parts as support of their argument.
My skepticism was not based on the science outlined in the chart thread, but rather on almost 40 years as a Journeyman in the trade and having worked on these cars back in the day. Fuel perc was real, but it's symptoms were very different than what is being described here on the Board. But as I looked at things and remembered my high school physics flaws in the scienc became apparent.
Further, why do the fuel producers tell us that today's fuels are less likely to have vapour lock and perc issues than they did in years gone by?
In previous postings it is reported that infrared temp of 150 to 180* were being read on fuel lines and carb bowls. That appears to have come as a surprise to some people, but I know that to be perfectly normal. Always has been. You wouldn't expect anything any different with all the air that comes into the engine compartment having gone through the rad, then is further warmed by the areas of the engine that run above cooling system temps. And gasoline has always had boiling points well below those recorded temp readings. So why doesn't everyone's fuel boil all day long? And why was perc historically only a hot day (not hot engine) event?
The original thread with the distillation chart surmises that with 40% of the fuel having boiling points below the temp read on the bowl and lines, that fuel must be violently boiling in their respective containers.
There are two fundamental flaws with that.
The first is the assumption that the temp of the fuel is the same as the temp measured on the outside of the vessel containing the fuel. All of these theories are based on infrared temp readings of the lines and carb bowl, not of the fuel itself. Given that the source of heat for the fuel is the ambient air and engine components surrounding the containers, that would be like taking the temp of the pot you are boiling water in at a point adjacent to the flame or element. You can put 500* of heat to a pot of water and the water will never exceed 212*. Assuming the temp of the fuel to be the same as it's heating container is more than a stretch, it is illogical.
The second is that if you understand the reasons the water won't exceed the temp at which it boils, you will know that the fuel can't reach a particular temp until it has boiled off all the elements with boiling points lower than that temp. That's how distillation works and how the curves are established, monitoring the temp as you monitor the decrease in volume. So in order to hit these mid range boiling points you would have had to boil off about 40% of the volume of the fuel you started with. The original thread would seem to suggest that the chart is showing 40% of the fuel is boiling rapidly at the temp in question.
That's not at all what happens. The way that chart is made is by watching the decrease in the volume of the container as the temp rises. Up to 100*, no change in volume, nothing boiling off yet. As the temp reaches the boiling point of the lowest components it stalls there and does not rise any further no matter how much heat you apply, until it has completely boiled off that element. It is noted how much the volume has decreased at the point that the temp starts to rise again. It levels off again as it reaches the boiling point of the next component. How long it sits at a given temp is determined by how much that particular component makes up of the total.
So not only is 40% of the fuel not boiling at the same time with a bowl reading of 150*, it can not reach 150* until it has boiled off all the elements with lower boiling points, about 40% of it's total volume.
So then, when does fuel perc and vapour lock happen. And what is it?
Historically, those events happened on very hot days. Days when the fuel in the tank approached the lowest of the fuel's boiling points, closing in on 100*. Stuff boils when it's own vapor pressure exceeds the pressure of the elements pushing down on it. This is often atmospheric pressure, but we can raise that boiling point by raising the pressure in the container such as happens in the cooling system under pressure. Similarly, you can lower the boiling point by reducing the pressure above it. This is what happens on the suction side of the fuel pump and why that's where vapour lock occurs. The fuel leaving the tank is almost at the lowest, or beginning, of it's boiling points. The lower pressure on the fuel created by the suction of the fuel pump causes those elements to boil, or vaporize. The pump wants to pull fuel, but because of the low pressure it's creating and the temp of the fuel, all it's getting is the vapour coming off the fuel. It is now vapour locked.
If the hot fuel makes it past the fuel pump it will continue to pick up heat. It is unlikely to boil in the line from the pump to the carb because it is under pressure, which raises it's boiling point. Once in the bowl it is at atmospheric pressure, or close to it, and it may start to boil, again starting with the components of the fuel with the lowest boiling point. As it boils those off the temp will gradually move up. But as the charts tell us, it has to have lost 40% of it's volume before it could ever hit 150*.
Which is precisely why it has always been that percing fuel is identified by whispy white vapour coming out the vent, not liquid fuel dropping down the throat when the engine has been shut off and the temp of the engine rises. The liquid fuel level is actually dropping, exiting the bowl via the vent as vapour.
This is why the fuel industry claims there to be far less of a problem with perc and vapour lock than ever before. Because it's not the mid range of the chart that's important, it's the very beginning that matters. It's the lowest temp at which any of the elements boil that matters. And that starting point is actually significantly higher in the ethanol fuel.
distillation curves.jpg
And that is why for over 80 years the automotive repair industry has identified perc by it's white vapour from the vent and attributed dripping liquid fuel down the throat to a high fuel level in the bowl caused by a mechanical malfunction or misadjustment.
That's my take on it. Due respect to those with differing opinions.
It's late and I'm out of time. More later.
Steve
As some will already know, I am not in agreement with the frequent diagnosis of heat soak and percolation as the cause of dripping carbs, hard hot starts and rough idle. I've outlined a few of my reasons below.
The proponents of this theory have previously posted a fuel distillation chart along with reported temp readings of the fuel system parts as support of their argument.
My skepticism was not based on the science outlined in the chart thread, but rather on almost 40 years as a Journeyman in the trade and having worked on these cars back in the day. Fuel perc was real, but it's symptoms were very different than what is being described here on the Board. But as I looked at things and remembered my high school physics flaws in the scienc became apparent.
Further, why do the fuel producers tell us that today's fuels are less likely to have vapour lock and perc issues than they did in years gone by?
In previous postings it is reported that infrared temp of 150 to 180* were being read on fuel lines and carb bowls. That appears to have come as a surprise to some people, but I know that to be perfectly normal. Always has been. You wouldn't expect anything any different with all the air that comes into the engine compartment having gone through the rad, then is further warmed by the areas of the engine that run above cooling system temps. And gasoline has always had boiling points well below those recorded temp readings. So why doesn't everyone's fuel boil all day long? And why was perc historically only a hot day (not hot engine) event?
The original thread with the distillation chart surmises that with 40% of the fuel having boiling points below the temp read on the bowl and lines, that fuel must be violently boiling in their respective containers.
There are two fundamental flaws with that.
The first is the assumption that the temp of the fuel is the same as the temp measured on the outside of the vessel containing the fuel. All of these theories are based on infrared temp readings of the lines and carb bowl, not of the fuel itself. Given that the source of heat for the fuel is the ambient air and engine components surrounding the containers, that would be like taking the temp of the pot you are boiling water in at a point adjacent to the flame or element. You can put 500* of heat to a pot of water and the water will never exceed 212*. Assuming the temp of the fuel to be the same as it's heating container is more than a stretch, it is illogical.
The second is that if you understand the reasons the water won't exceed the temp at which it boils, you will know that the fuel can't reach a particular temp until it has boiled off all the elements with boiling points lower than that temp. That's how distillation works and how the curves are established, monitoring the temp as you monitor the decrease in volume. So in order to hit these mid range boiling points you would have had to boil off about 40% of the volume of the fuel you started with. The original thread would seem to suggest that the chart is showing 40% of the fuel is boiling rapidly at the temp in question.
That's not at all what happens. The way that chart is made is by watching the decrease in the volume of the container as the temp rises. Up to 100*, no change in volume, nothing boiling off yet. As the temp reaches the boiling point of the lowest components it stalls there and does not rise any further no matter how much heat you apply, until it has completely boiled off that element. It is noted how much the volume has decreased at the point that the temp starts to rise again. It levels off again as it reaches the boiling point of the next component. How long it sits at a given temp is determined by how much that particular component makes up of the total.
So not only is 40% of the fuel not boiling at the same time with a bowl reading of 150*, it can not reach 150* until it has boiled off all the elements with lower boiling points, about 40% of it's total volume.
So then, when does fuel perc and vapour lock happen. And what is it?
Historically, those events happened on very hot days. Days when the fuel in the tank approached the lowest of the fuel's boiling points, closing in on 100*. Stuff boils when it's own vapor pressure exceeds the pressure of the elements pushing down on it. This is often atmospheric pressure, but we can raise that boiling point by raising the pressure in the container such as happens in the cooling system under pressure. Similarly, you can lower the boiling point by reducing the pressure above it. This is what happens on the suction side of the fuel pump and why that's where vapour lock occurs. The fuel leaving the tank is almost at the lowest, or beginning, of it's boiling points. The lower pressure on the fuel created by the suction of the fuel pump causes those elements to boil, or vaporize. The pump wants to pull fuel, but because of the low pressure it's creating and the temp of the fuel, all it's getting is the vapour coming off the fuel. It is now vapour locked.
If the hot fuel makes it past the fuel pump it will continue to pick up heat. It is unlikely to boil in the line from the pump to the carb because it is under pressure, which raises it's boiling point. Once in the bowl it is at atmospheric pressure, or close to it, and it may start to boil, again starting with the components of the fuel with the lowest boiling point. As it boils those off the temp will gradually move up. But as the charts tell us, it has to have lost 40% of it's volume before it could ever hit 150*.
Which is precisely why it has always been that percing fuel is identified by whispy white vapour coming out the vent, not liquid fuel dropping down the throat when the engine has been shut off and the temp of the engine rises. The liquid fuel level is actually dropping, exiting the bowl via the vent as vapour.
This is why the fuel industry claims there to be far less of a problem with perc and vapour lock than ever before. Because it's not the mid range of the chart that's important, it's the very beginning that matters. It's the lowest temp at which any of the elements boil that matters. And that starting point is actually significantly higher in the ethanol fuel.
distillation curves.jpg
And that is why for over 80 years the automotive repair industry has identified perc by it's white vapour from the vent and attributed dripping liquid fuel down the throat to a high fuel level in the bowl caused by a mechanical malfunction or misadjustment.
That's my take on it. Due respect to those with differing opinions.
It's late and I'm out of time. More later.
Steve
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