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The "World Engine" separates coolant to the cylinder heads and main engine block.

It keeps the Cylinder heads at a colder temperature in order to control cam pinging and provide better MPG by keeping incoming air cooler (bigger compression).

The secondary (hotter) thermostat keeps the main engine block hotter to help lubrication (hotter oil being slicker) and thus reducing friction/wear.

According to the dealer service manual:

The engine cooling thermostat is a wax pellet driven, reverse poppet choke type. The thermostat is designed to
provide the fastest warm up possible by preventing leakage through it and to guarantee a minimum engine oper-
ating temperature of 88 to 93°C (192 to 199°F).

The primary thermostat starts to open at 77° C (171° F) and is fully open at 89° C (193° F).
The secondary thermostat starts to open at 95° C (203°F) and is fully open at 108° C (226° F).
Subliminally sucking-in rep points (Twilight Zone sound clip here)
 

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If one fails do they both fail?
Do they fail in the open position?
No. They are completely independent.

As for their location:

The primary thermostat is located on the front of the water plenum in the thermostat housing/coolant inlet.
The thermostat has an air bleed vent located on its flange. The air bleed vent (1) and the locator dimple on thermostat seal provide for proper positioning of thermostat in inlet housing.

The secondary thermostat is located in the cylinder head under the water plenum.
In pictures:


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Oh. And yes, they COULD fail in open position:

The thermostat is operated by a wax filled chamber (pellet) which is sealed. When coolant reaches a predetermined temperature, the wax expands enough to overcome the closing spring and water pump pressure, which forces the thermostat to open. Coolant leakage into the wax pellet will cause a thermostat to fail in the open position. Do not attempt to free-up a stuck open thermostat.

If the thermostat is stuck open or allows coolant leakage through it, the engine will not operate at the proper temperature for obtaining engine fuel efficiency, performance and emissions levels. If this condition occurs, a diagnostic trouble code will be set and a MIL light will be turned on. Refer to the Powertrain Diagnostic Procedures manual for further information and diagnostics provided.

Thermostat diagnostics is included in powertrain control module’s (PCM) programing for on-board diagnosis. The malfunction indicator light (MIL) will illuminate and a diagnostic trouble code (DTC) will be set when an “open too soon” condition occurs. Do not change a thermostat for lack of heater performance or temperature gauge position, unless a DTC is present. For other probable causes, (Refer to 7 - COOLING/ENGINE - DIAGNOSIS AND TESTING). Thermostat failing shut is the normal long term mode of failure, and normally, only on high mileage vehicles. The temperature gauge will indicate this (Refer to 7 - COOLING/ENGINE - DIAGNOSIS AND TESTING).
 
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He has the service manual,that's where!!
Shhhh!

Actually, the technical info was in the dealer service manual but the exact reason why there are two, I had to google it up.

As a software developer, I spend half my time digging up information so I do have a knack for finding stuff.
 

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Shhhh!

Actually, the technical info was in the dealer service manual but the exact reason why there are two, I had to google it up.

As a software developer, I spend half my time digging up information so I do have a knack for finding stuff.
Well then! it's a good thing you are here. :worshippy: ;)
 

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Yes it is. He can take over my job some day.:b1:
That's some tough shoes to fill, This place just wouldn't be the same without you.;)
 

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We now have downloadable copies of the service manual in our "Manuals" section.
 
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