Thermostat

[Mike9000Aero]

Am I right in thinking 89 dgrees is right?
Thanks
Mike

[phoenix]

Yes, that is the norm for the UK. The other option is 82  for very warm climates.

[danilo]

NO!! Buy and use the 80C thermostat.
Higher temp ones will only wear out your engine quicker.

[phoenix]

What's the reasoning behind the theory of a higher temp thermostat cause more wear?

I did some tests a while ago when investigating heat management for my old 9000 when it used to get very hot on the track. Although the 82 deg C opens earlier, as the temperature rises the difference decreases until as they approach  max temp both are equally open.

Using a lower temp one does however increase fuel consumption, partly because more heat will inevitably be dissipated but also because the ECU monitors coolant temp and will think the engine is not fully up to temp and so add more fuel.

[danilo]

What's the reasoning behind the theory of a higher temp thermostat cause more wear?

It's not actually a theory it was 'proven' decades ago . Google is our friend . 
Somehow, like silly and perennial  "oil' arguements.... everyone has their own pet beliefs :-)
Oddly I've recently been informed that the IDEAL operating temp for My Yamaha is actually 50 C (race track/dyno proven) rather than the oem 80C Doesn't that seem even more counterintuitive  ? :-)
ECU has a reasonable range of operational temps in it's maps,  a non issue.

[phoenix]

It's not a pet belief at all, it's just applying what seems to me at least to be a logical thought process  . That process is:

It is known that most wear to engines is caused during the warm up period. A lower temperature thermostat  will extend that period, and thus wear.

I tried Googling  "higher temperature thermostat causes more wear" and the answers on the first couple of pages concur with my reasoning,  so if you could provide an explanation either directly or by link, it would be appreciated.

[chengny]

Yep, that's right.

From the Saab shop manual;
 
OPENING TEMPERATURE 192 ±4°F (89 ±2°C)  

Note: this is only the opening temperature.

Under normal circumstances, the thermostat does not control coolant temperature. The thermostat is not like an on/off switch, it allows for wide variations in flow rates. Upon start up, it is basically intended to help the engine come up to optimum temp quickly.  After that it stays open and the coolant sensors control the temperature of the circulating coolant by means of the radiator fan.

T-stat opens @ 89 but the normal coolant temp is maintained at about 100-110 C.
 

[Toe]

I had 82 degree in mine when bought and even with the ECU controlled temp gauge it was hanging on the blue when driving and because of this I didn't have much heat either so in this country at least 89 degree is a better bet

[danilo]

T-stat opens @ 89 but the normal coolant temp is maintained at about 100-110 C.

Ermm.. sort of accurate    the Fan is a failsafe  to control MAX temperature so the thing does not overtemp ..Only.
 
IF you were not getting enough heat with a 82 c Thermostat  and you don't reside above the arctic circle..then there are genuine issues with your car 

[chengny]

­Any liquid-cooled car engine has a small device called the thermostat that sits between the engine and the radiator. The thermostat in most cars is about 2 inches (5 cm) in diameter. Its job is to block the flow of coolant to the radiator until the engine has warmed up. When the engine is cold, no coolant flows through the engine. Once the engine reaches its operating temperature (generally about 200 degrees F, 95 degrees C), the thermostat opens. By letting the engine warm up as quickly as possible, the thermostat reduces engine wear, deposits and emissions.

[chengny]

Mike the radiator man says this about t-stat function:

Thermostat

The thermostat is much maligned, often misunderstood, and seldom credited for its mostly reliable service.

Thermostats in proper working order, control only the minimum operating temperature at which engine coolant is allowed to heat. A properly operating thermostat has absolutely nothing to do with how hot the engine becomes.

Nascar.com's site says this:

The thermostat is a valve that either restricts or allows the flow of coolant from the engine to the radiator. When the engine is cold the thermostat stays shut confining the coolant to the engine only. The purpose of this is to help the coolant to warm up. As the coolant warms, the valve in the thermostat begins to open, allowing the coolant to now flow to the radiator and back to the engine. During normal conditions with a fully warmed engine, the thermostat will allow an unrestricted flow of coolant to the radiator.

Saab shop manual on main cooling fan:

Regardless of the position of the ignition switch, current is supplied to relay 155 via fuse 4 and to relay 81 via max fuse 2.

The radiator fan is controlled by the EDU control module.

The engine coolant temperature is measured by the EDU control module, which controls relay 155.

Single-Speed Radiator Fan
When the temperature reaches about 100 °C (212 °F) , relay 155 operates and radiator fan 37 starts. When the temperature drops to about 96 °C (205 °F) , the fan stops.

2-Speed Radiator Fan 366
In some markets, cars equipped with A/C have a 2-speed radiator fan for more efficient cooling.

The first stage (speed 1) operates as described above. The radiator fan starts at low speed

When the coolant temperature reaches 111 °C (232 °F) , relay 81 is grounded via the EDU control module. The radiator fan runs at full speed. When the coolant temperature drops to 107 °C (225 °F) , fan speed returns to speed 1 (low speed).

I was taught that the purpose of the thermostat in a liquid cooled engine is primarily to accelerate the warm-up process of the lube oil and metal mass.

It is generally agreed that keeping the time period from start-up to operating temp as short as possible is critical. This greatly reduces the degree of wear, friction, stress  and exposure to acidic vapor on the internal moving parts.

Once the designed minimum coolant temp is achieved the thermostat opens and allows flow to to the primary coolant heat exchanger.

Whether that is an air cooled, cross flow radiator, a shell and tube  heat exchanger, or even heat demanding auxiliaries; from that point on, the temperature of the primary engine coolant has nothing to do with the thermostat. It is taken over by some other type of control system.

When all is said and done, both components are needed to keep coolant temp within normal range.

[phoenix]

­ When the engine is cold, no coolant flows through the engine.

I think you mean it doesn't flow through the radiator.

During warm up, coolant flows round the engine and heater matrix until the 'stat opens, at which point it also flows through the radiator.

Saab's design adds a further stage to this- if things become very hot, the flow to the heater matrix is blocked off thus diverting the coolant to the biggest heat exchanger- the radiator.

[Mark B]

When things get very hot, it's the bypass flow around the engine that the thermostat blocks (i.e. the flow path used before the stat opens), leaving flow to the radiator and heater.  I have had to test the heater functionality under conditions of great warmth!

[phoenix]

Interesting. I'd always thought it was just the heater matrix but closing the engine bypass circuit would make a lot of sense.

T'other half once had a problem stuck in traffic in the freezing cold with her C900, the upshot of which was that she lost flow to the heater matrix as things heated up under the bonnet. Now, to be fair there was also a problem with the matrix bypass valve but it illustrates the principle that flow to the matrix reduces as engine temp increases.

[phoenix]

From the official manual:

Thermostat closed

With the thermostat closed the coolant passes from the coolant pump - via a passage in the cylinder block - up through the cylinder head - via the throttle body (preheating) to the
thermostat housing - through the lower outlet of the thermostat housing - through a hose to the heat exchanger - through the return hose from the heat exchanger - and back to
the coolant pump. All the coolant goes directly to the pump from the heat exchanger.

Thermostat normally open

With the thermostat normally open the coolant passes from the coolant pump - via a passage in the cylinder block - up through the cylinder head - via the throttle body (preheating) to the thermostat housing - and through the thermostat housing's two outlets. The coolant passing out through the lower outlet goes back to the coolant pump via the heat exchanger. The coolant passing through the thermostat's upper outlet goes directly to the radiator and from there to the coolant pump.

Thermostat fully open

The thermostat closes the lower outlet so that all the coolant passes through the radiator. No coolant flows through the heat exchanger.