cooling system theory - where coolant flows

[coffee-3000]

I just read in the Toyota manual that when the coolant is cold ( below 180 degrees ), the thermostat doesn't allow it to flow through the engine. The engine is suppose to get hot quickly so the cold coolant would prohibit that. But how will the coolant get hot if it doesn't flow through the engine ? It sounds like a catch-22.

Am I not reading this correctly ?

I thought cold coolant didn't flow through the radiator.
If it's not hot, why cool it.

[rusty9]

the thermostat opens to allow coolant to flow through the radiator.

[coffee-3000]

Thanks rusty9. Here is what the manual says. Must be a mis-print.

"The thermostat has a wax type bypass valve and is mounted in the water inlet housing. The
thermostat includes a type of automatic valve operated by fluctuations in the coolant tempera–ture. This valve closes when the coolant temperature drops, preventing the circulation of coolant

through the engine and thus permitting the engine to warm up rapidly."

[slavie]

Everything is correct.

When coolant circulates, it cools down the engine, right?
So when you want the engine warm, you let whatever coolant you have in the engine to warm up WITH the engine. When the engine reaches optimal operating temperature, the valve opens, allowing the cooling system to keep the temperature stable.

There's no way to "drain" the coolant out of the engine while the engine is warming up.

[Dave's]

There is coolant in the engine at all times. There's coolant in the radiator and heater core as well. But when the engine is cold the thermostat is closed which prevents the coolant in the engine to flow through the radiator. As it heats up while running the engine the thermostat begins to open. (It will also begin to close if the temperature drops too far.) Cool air flows through the radiator fins and past the tubes and removes some of the heat from the coolant which is flowing when the thermostat is open to any degree. This colder coolant cycles (courtesy of the water pump) through the engine 'absorbing' more heat while the heated coolant goes through the radiator. It's a constant cycle which includes the heater core unless there's a control valve closed in those lines. The amount of flow depends upon how far the t-stat is open. T-stats can become weak and open too soon or even stay open. This will cause the heater to not work well during colder months. It can also affect engine performance if the engine temp is too cold for the best efficiency. T-stats also can stick closed causing the engine to overheat. If there's a problem with the engine or system (not enough oil, clogged passages in the engine and/or radiator etc.) the t-stat can only open so far and the engine may still overheat even if the t-stat is OK.
Hope that's essentially correct and all you ever wanted to know about cooling system operation.

[coffee-3000]

Thanks to both slavie and Dave's. This makes a lot of sense. It seems that a lot of problems that posters have been having are concerning overheating and t-stats. It nice to understand how it all works.

[coffee-3000]

One more question about the coolant flow. Is the water pump always pumping coolant regardless of whether the thermostat is open or closed ? I thought that I heard that the thermostat had some sort of bypass valve that directed the coolant into the water pump when it wasn't directing the coolant to the radiator. But where does the water pump push the coolant.

Or does the entire flow of coolant stop when the thermostat is close.
In this case, the water pump must not be pumping.

[houra]

http://auto.howstuffworks.com/cooling-system2.htm

[JohnGD]

Yes, the coolant is always circulating within the engine. It's when the temp reaches the 82C range that the thermostat opens and let cooler coolant in from the radiator. The thermostat just helps to change the path.

Note that this (and many other aluminum engines) use a "Reverse Flow" cooling system design. So the thermostat is actually located on the INLET side instead of the outlet side in older applications. The advantages are less thermoshock for aluminum engine components and steadier system temperature.

When replacing the thermostat always use OEM (Kuzeh brand) or Stant's OE-Exact with a jiggle valve. See the little jiggle valve by the brass shaft:

http://www.rockauto.com/catalog/moreinfo.php?pk=1145694

And what you don't want:

http://www.rockauto.com/catalog/moreinfo.php?pk=366573

Originally Posted by coffee-3000

One more question about the coolant flow. Is the water pump always pumping coolant regardless of whether the thermostat is open or closed ? I though I heard that the thermostat had some sort of bypass valve that directed the coolant into the water pump when it wasn't directing the coolant to the radiator. But where does the water pump push the coolant.

Or does the entire flow of coolant stop when the thermostat is close.
In this case, the water pump must not be pumping.

[coffee-3000]

Thanks JohnGD and houra. I understand what your saying. The Toyota Manual was probably trying to say this but didn't say it well. The manual said that "This valve ( meaning the thermostat valve) closes when the coolant temperature drops, preventing the circulation of coolant
through the engine and thus permitting the engine to warm up rapidly".