How in the F do our thermostats work???

[SnowDrifter]

This has been bothering me the more I think about it

So our rigs have a reverse flow system on it. The radiator is pressurized by the waterpump, but return to the engine is blocked by the thermostat

So then the return to the thermostat is going to be receiving cold coolant

Likewise, the return for the heatercore is on the front side of the thermostat, and will be receiving cold coolant


So HOW does the darn thing open? I can't wrap my head around it

 

[swathdiver]

This has been bothering me the more I think about it

So our rigs have a reverse flow system on it. The radiator is pressurized by the waterpump, but return to the engine is blocked by the thermostat

So then the return to the thermostat is going to be receiving cold coolant

Likewise, the return for the heatercore is on the front side of the thermostat, and will be receiving cold coolant


So HOW does the darn thing open? I can't wrap my head around it

View attachment 360183

It just opens when it gets up to temp. Then coolant from the radiator can get in.

 

[Mudsport96]

I think it gets warmed by coolant returning from the heater core side. It warms the general area up on the return and the stat pops. Edumacated guess from the picture posted.

 

[Bill 1960]

I don’t know if that diagram is precisely accurate or just a general representation. Other diagrams show a bypass behind the stat which is circulating water from the block. And in any case, the block side of the stat is exposed to the hot water the pump is pushing through the block and heater. That’s the side the thermostatic element is on.

 

[Donal]

Coolant is drawn from the radiator outlet and into the water pump inlet by the water pump. Coolant will then be pumped through the water pump outlet and into the engine block. In the engine block, the coolant circulates through the water pump outlet and into the engine block. In the engine block, the coolant circulates through the water jackets surrounding the cylinders, where it absorbs heat. Some coolant is also pumped from the water pump to the heater core, then back to the water pump. This provides the passenger compartment with heat and defrost. The coolant is then forced through the cylinder head gasket openings and into the cylinder heads. In the cylinder heads, the coolant flows through the water jackets surrounding the combustion chambers and valve seats, where it absorbs additional heat. Coolant is also directed to the throttle body. There it circulates through passages in the casting. During initial start up, the coolant assists in warming the throttle body. During normal operating temperatures, the coolant assists in regulating the throttle body temperature.
The surge tank is a plastic tank with a threaded pressure cap. The tank is mounted at a point higher than all other coolant passages. The surge tank provides an air space in the cooling system that allows the coolant to expand and contract. The surge tank provides a coolant fill point and a central air bleed location. During vehicle use, the coolant heats and expands.

The increased coolant volume flows into the surge tank. As the coolant circulates, any air is allowed to bubble out. Coolant without air bubbles absorbs heat much better than coolant with bubbles.
The water pump is a centrifugal vane impeller type pump. The pump consists of a housing with coolant inlet and outlet passages and an impeller. The impeller is mounted on the pump shaft and consists of a series of flat or curved blades or vanes on a flat plate. When the impeller rotates, the coolant between the vanes is thrown outward by centrifugal force.

The thermostat is a coolant flow control component. It's purpose is to help regulate the operating temperature of the engine. It utilizes a temperature sensitive wax-pellet element. The element connects to a valve through a small piston. When the element is heated, it expands and exerts pressure against the small piston. This pressure forces the valve to open. As the element is cooled, it contracts. This contraction allows a spring to push the valve closed. When the coolant temperature is below the rated thermostat opening temperature, the thermostat valve remains closed. This prevents circulation of the coolant to the radiator and allows the engine to warm up. After the coolant temperature reaches the rated thermostat opening temperature, the thermostat valve will open. The coolant is then allowed to circulate through the thermostat to the radiator where the engine heat is dissipated to the atmosphere. The thermostat also provides a restriction in the cooling system, after it has opened. This restriction creates a pressure difference which prevents cavitation at the water pump and forces coolant to circulate through the engine block.
sourced from Service and repair manual beginning on page 3536 of 9979 pages.

 

[SnowDrifter]

I think it gets warmed by coolant returning from the heater core side. It warms the general area up on the return and the stat pops. Edumacated guess from the picture posted.

But if that were the case, then it would be feasible that having the heat on would cause the thing to never open because the return temp would be below the activation temp

But we know from experience that isn't the case

Likewise, when it does open, it would be bathed in cold fluid returning from the radiator and would snap shut again, but we also know that isn't the case


So like... What's going on?

 

[Doubeleive]

But if that were the case, then it would be feasible that having the heat on would cause the thing to never open because the return temp would be below the activation temp

But we know from experience that isn't the case

Likewise, when it does open, it would be bathed in cold fluid returning from the radiator and would snap shut again, but we also know that isn't the case


So like... What's going on?

the thermostat does not snap shut even if dropped in cold water, it closes slowly as the coolant passes thru pushed by the water pump pressure, initially the coolant is hot (at running temp) as that passes thru it slowly gets cooler and the thermostat then slowly closes at the same time.
the little return line from the radiator to the reservoir tank is what keeps it from feeding back in from the backside., it is restricted to 1/8" so only a small amount gets fed back.

 

[rockola1971]

But if that were the case, then it would be feasible that having the heat on would cause the thing to never open because the return temp would be below the activation temp

But we know from experience that isn't the case

Likewise, when it does open, it would be bathed in cold fluid returning from the radiator and would snap shut again, but we also know that isn't the case


So like... What's going on?

Remember the whole system should have no air in it so the coolant around the cylinders gets warmed up first and it ultimately warms the fluid toward the sensing wax bead in the thermostat. The coolant coming from the radiator is not COLD, it is just substantially cooler than the coolant that is in direct contact with the cylinders and all the way to the inlet of the radiator. After the engine is warmed up, go underneath and feel the lower radiator hose. It will be substantially warm but wont burn your fingers off. Now go feel the upper radiator hose. It will be substantially warmer than the temp of the lower radiator hose and will be hard to keep your hands on it for long. There will be a temperature differential between the 2 but they both are far from "cold". Coolant can hold only so much heat and we take advantage of that characteristic and rid as much heat as we can with the fan and radiator before it gets sent back to the engine in the loop. The thermostat does regulate the flow dependent on heat load. Thermostat is likely wide open most of the time in a very hot summer day while driving. But when it is 5deg outside the thermostat will be more toward the closed side than it is fully open. The actual position of the thermostat is dependent on alot of variables like outside temp, cleanliness of radiator, engine load, speed of vehicle (airflow through radiator) and even altitude of vehicle.

 

[tom3]

Wow. I know a lot less than I did before seeing all this. Can't imagine regulating coolant flow on the suction side of the pump due to cavitation. And as the OP asks, how does it get a good temperature reading on the cold side of the system. Just amazes me how these automakers can complicate the simplest functions. The early 265 cuin small blocks didn't even have an oil filter.

 

[Mudsport96]

Just amazes me how these automakers can complicate the simplest functions. The early 265 cuin small blocks didn't even have an oil filter.

Off topic sorry...
I had one of those old no filter 265s years ago, got it at an estate sale for 50 bucks. Ended up selling it to a guy doing a numbers matching restore for 500 bucks.
Ok rant over