We get all kinds of questions about thermostats and flaps... maybe you are wondering:
What are thermostat flaps anyway?
The thermostat flaps fit up inside the fan shroud on either side and are connected by a linkage to each other and down to the thermostat which is located between the #1 & #2 cylinders. The flaps are nearly (but not completely) closed when the engine is cold. When you start the engine, heated air from around these cylinders causes the thermostat to expand once the air reaches the opening temperature. This causes the linkage to move and the flaps to open. As the engine temp rises, the air over the cylinders gets hotter, causing the thermostat to expand more. This is self-regulating, meaning that when the engine heats up, the flaps expand, and when the engine cools down, the
flaps close.
It does not get cold where I live, or I only drive in the summer. Do I really need to run thermostat
flaps?
Would you take the thermostat out of your Ford, your Toyota? no? then why take it out of your Volkswagen?
The point of the thermostat flaps is not to provide cooling. The flaps are there to get the engine heated up to proper operating temperature as fast as possible and then maintain
that temperature. This is the #1 misconception about the thermostat and flaps system. Everyone thinks they are meant to keep the engine cool, when it really is meant to heat it up quickly. But NO, you are not required to run the thermostat flaps. Many air cooled VW's are running all over the world, in all sorts of temperatures and conditions with the flaps missing. However, all of those engines are running too cold at startup. This means that all of those engines have lower fuel economy (less MPG), more engine wear, and resulting shorter engine life. So, if you are not
concerned about those things, you don't need thermostat flaps.
Can the flaps get stuck closed, causing the engine to overheat?
Nothing is impossible, so yes, they could get rusted closed or hung up in the closed position, which would result in an overheated engine. However, I have rebuilt 100's of thermostat flaps and
only seen three that were rusted shut. With a fresh blasting and powdercoat, you should be fine for many years to come.
Can the thermostat get stuck closed, causing the engine to overheat?
The German thermostats and ours are fail safe, meaning that if they fail, they SHOULD fail in the full open position, giving you maximum cooling. I have seen 100's of thermostats over the years, and I have only seen three used stats that were contracted that did not expand when heated. So it is possible, but it is not very likely.
The VW Mexico version of the thermostat is NOT fail safe, meaning that if the wax inside leaks out, it will stay fully closed. However, I have never actually seen a failed thermostat of this style,
nor have I heard of any of them failing, so it must not be very common.
How does the thermostat work anyway?
The German units have a small amount of liquid inside that expands when heated. The bellows are held closed by the partial vacuum inside the sealed unit. As the stat is heated, the liquid
vaporizes and the bellows expands. When the stat cools off, the vapor returns to liquid and the bellows contracts. If the liquid ever leaks out, the bellows will expand, causing the flaps to go full
open. This is why the German stat is considered fail safe.
The Mexican units have a small amount of wax inside that expands when heated. The unit is held closed due to a large high tension spring. As the stat heats up, the wax expands, extending the stat. When the stat cools off, the wax is compressed by the high tension spring. If the wax ever leaks out, the stat will stay in the closed position. This is why the Mexican stat is NOT considered fail safe.
At what temperature does the thermostat open?
It depends on which thermostat you have:
German stat for early 36hp type I carb engines, "stale air" 75-80C opening temp
German stat for type I carb engine, starts to open at 65-70C (149-158F) = MOST COMMON
German stat for type I FI engine, starts to open at 80-85C (176-185F)
German stat for type IV carb and FI engines, and 4 cylinder
Porsche 912 and 914 engines, starts to open at 85-90C (185-194F)
German stat for Porsche 356, these engines do not have flaps, the stat opens ductwork to run heated air to the carb intakes (23-34C)
new VW Mexico - this is a real mess, as it comes in both the 65-70C range and the 80-85C range, yet somehow has the same part number? So, what do you got? The only way to know is to test it.
My engine oil temperature is X, so should the flaps be open now?
Both the engine oil temperature and the temperature of the air coming off the cylinders are due to the heat generated by the engine running. But there is not a direct connection. Don't try and use one to determine the other.
I drove the car for five minutes and the flaps are not open yet. What is wrong?
The flaps are still closed because the engine has not heated up yet. The minimum temperature the thermostat has to see is 65C (149F) - it just has not gotten that hot yet.
Do I have to run all the lower cylinder tins (heater box sled, #2/#4 air deflectors, industrial shields)
or can I leave all that off?
You MUST run all of the lower engine tin when you are running a thermostat. If you do not, then cooler air from below the car will mix with the heated air from you engine. This will result in the
stat not opening the right amount, or even keep it from opening at all. This is super important! If you are not running heater boxes, then you need to run the industrial shields to fill in this open
area.
I have a VW type IV or Porsche 356, Porsche 912 or 914 engine. Can I run a bug thermostat on
it? They look the same.
They look the same, but the type IV opens at a higher temperature.
Can I run a Corvair thermostat on my bug engine? They look similar.
The Corvair stat opens at 85-90C, which is too hot for a bug engine, but would work for the type IV, Porsche 912 and 914 engines. But the fittings on both ends are wrong. You would have to make some sort of adapter.
Can I run a bug thermostat on my corvair engine? They look similar.
The Corvair stat opens at 85-90C, which is too hot for a bug engine, but would work for the type IV, Porsche 912 and 914 engines. But the fittings on both ends are wrong. You would have to make some sort of adapter.
Will a bug thermostat work on a Porsche 356?
No, the Porsche 356 thermostat is a much lower temperature. There are no flaps on the 356 engine. This stat opens ductwork for warm air to go to the carburetor intakes, a totally different
application.
What are thermostat flaps anyway?
The thermostat flaps fit up inside the fan shroud on either side and are connected by a linkage to each other and down to the thermostat which is located between the #1 & #2 cylinders. The flaps are nearly (but not completely) closed when the engine is cold. When you start the engine, heated air from around these cylinders causes the thermostat to expand once the air reaches the opening temperature. This causes the linkage to move and the flaps to open. As the engine temp rises, the air over the cylinders gets hotter, causing the thermostat to expand more. This is self-regulating, meaning that when the engine heats up, the flaps expand, and when the engine cools down, the
flaps close.
It does not get cold where I live, or I only drive in the summer. Do I really need to run thermostat
flaps?
Would you take the thermostat out of your Ford, your Toyota? no? then why take it out of your Volkswagen?
The point of the thermostat flaps is not to provide cooling. The flaps are there to get the engine heated up to proper operating temperature as fast as possible and then maintain
that temperature. This is the #1 misconception about the thermostat and flaps system. Everyone thinks they are meant to keep the engine cool, when it really is meant to heat it up quickly. But NO, you are not required to run the thermostat flaps. Many air cooled VW's are running all over the world, in all sorts of temperatures and conditions with the flaps missing. However, all of those engines are running too cold at startup. This means that all of those engines have lower fuel economy (less MPG), more engine wear, and resulting shorter engine life. So, if you are not
concerned about those things, you don't need thermostat flaps.
Can the flaps get stuck closed, causing the engine to overheat?
Nothing is impossible, so yes, they could get rusted closed or hung up in the closed position, which would result in an overheated engine. However, I have rebuilt 100's of thermostat flaps and
only seen three that were rusted shut. With a fresh blasting and powdercoat, you should be fine for many years to come.
Can the thermostat get stuck closed, causing the engine to overheat?
The German thermostats and ours are fail safe, meaning that if they fail, they SHOULD fail in the full open position, giving you maximum cooling. I have seen 100's of thermostats over the years, and I have only seen three used stats that were contracted that did not expand when heated. So it is possible, but it is not very likely.
The VW Mexico version of the thermostat is NOT fail safe, meaning that if the wax inside leaks out, it will stay fully closed. However, I have never actually seen a failed thermostat of this style,
nor have I heard of any of them failing, so it must not be very common.
How does the thermostat work anyway?
The German units have a small amount of liquid inside that expands when heated. The bellows are held closed by the partial vacuum inside the sealed unit. As the stat is heated, the liquid
vaporizes and the bellows expands. When the stat cools off, the vapor returns to liquid and the bellows contracts. If the liquid ever leaks out, the bellows will expand, causing the flaps to go full
open. This is why the German stat is considered fail safe.
The Mexican units have a small amount of wax inside that expands when heated. The unit is held closed due to a large high tension spring. As the stat heats up, the wax expands, extending the stat. When the stat cools off, the wax is compressed by the high tension spring. If the wax ever leaks out, the stat will stay in the closed position. This is why the Mexican stat is NOT considered fail safe.
At what temperature does the thermostat open?
It depends on which thermostat you have:
German stat for early 36hp type I carb engines, "stale air" 75-80C opening temp
German stat for type I carb engine, starts to open at 65-70C (149-158F) = MOST COMMON
German stat for type I FI engine, starts to open at 80-85C (176-185F)
German stat for type IV carb and FI engines, and 4 cylinder
Porsche 912 and 914 engines, starts to open at 85-90C (185-194F)
German stat for Porsche 356, these engines do not have flaps, the stat opens ductwork to run heated air to the carb intakes (23-34C)
new VW Mexico - this is a real mess, as it comes in both the 65-70C range and the 80-85C range, yet somehow has the same part number? So, what do you got? The only way to know is to test it.
My engine oil temperature is X, so should the flaps be open now?
Both the engine oil temperature and the temperature of the air coming off the cylinders are due to the heat generated by the engine running. But there is not a direct connection. Don't try and use one to determine the other.
I drove the car for five minutes and the flaps are not open yet. What is wrong?
The flaps are still closed because the engine has not heated up yet. The minimum temperature the thermostat has to see is 65C (149F) - it just has not gotten that hot yet.
Do I have to run all the lower cylinder tins (heater box sled, #2/#4 air deflectors, industrial shields)
or can I leave all that off?
You MUST run all of the lower engine tin when you are running a thermostat. If you do not, then cooler air from below the car will mix with the heated air from you engine. This will result in the
stat not opening the right amount, or even keep it from opening at all. This is super important! If you are not running heater boxes, then you need to run the industrial shields to fill in this open
area.
I have a VW type IV or Porsche 356, Porsche 912 or 914 engine. Can I run a bug thermostat on
it? They look the same.
They look the same, but the type IV opens at a higher temperature.
Can I run a Corvair thermostat on my bug engine? They look similar.
The Corvair stat opens at 85-90C, which is too hot for a bug engine, but would work for the type IV, Porsche 912 and 914 engines. But the fittings on both ends are wrong. You would have to make some sort of adapter.
Can I run a bug thermostat on my corvair engine? They look similar.
The Corvair stat opens at 85-90C, which is too hot for a bug engine, but would work for the type IV, Porsche 912 and 914 engines. But the fittings on both ends are wrong. You would have to make some sort of adapter.
Will a bug thermostat work on a Porsche 356?
No, the Porsche 356 thermostat is a much lower temperature. There are no flaps on the 356 engine. This stat opens ductwork for warm air to go to the carburetor intakes, a totally different
application.
