What's that Thermostat really doing while I'm drving around?

[flash968]

sounds right - that's likely what i have - lol - the fan switch i am running is from a 90 vanagon - WAY cheaper than the porsche part, but the same part

[HondaDustR]

Originally Posted by Mask of Destiny

The thermostat opens at a cooler temperature and is located in a position that is likely to see higher coolant temperatures than the thermofan switch so if the conditions cause the fans to kick on it's unlikely that the thermostat will be closing.

.

The thermostat is in the suction side of the water pump which would probably be somewhat cooler than the top driver's side corner of the radiator where the thermofan switch is, and the hose carrying coolant from the cylinder head connects to the radiator being immediately above the switch.

Originally Posted by Mask of Destiny

I don't think the 968's have an oil thermostat, but I'm no expert. Doesn't look like Pelican sells it if they do, but I'm too lazy to go digging in the PET. Air cooled 911s do, but that's because the oil serves the role that coolant does in our cars in addition to being a lubricant.

The 968 does have a thermostat for the external oil cooler.

[AaronM]

Guys, I think that most of us here are over-thinking the question.

The thermostat is a simple device that relies on thermal expansion to perform. Because of this, it will necessarily have an infinitely-variable phase between the maximum "fully closed" temperature and the minimum "fully-opened" temperature. Typically this range is about 20 degrees Fahrenheit (e.g. a "180 degree" thermostat begins to open at 180 degrees and is usually fully open by 200 degrees).

Aaron

[Dean924s]

What I really want to know is did you win the argument?

[flash968]

i'd like to know where the oil thermostat is - that is something i would like to play with

[DaveBoettcher]

Originally Posted by Dean924s

What I really want to know is did you win the argument?

Hi Dean,

Well I was winning until I got a couple of posts like

"It's just a 2 position switch, it hits a certain degree and pops open, cools down and pops closed. simple. Maybe I'm not getting the question here.
Are we assuming there's a smart thermostat that knows my driving conditions and rpm's?"

which didn't help. So I have now found a manufacturer of thermostats and appealed directly to them for some authoritative information. So long as there are some people who think "It's just a 2 position switch" then I don't feel I am getting anywhere, despite it being perfectly clear that it cannot possibly work like that.

Thanks for the concern anyway - I haven't given up yet, just gathering more ammo!

[DaveBoettcher]

OK, I got a quick reply from Derek Lees, Technical Training & Services Manager at Quinton Hazell Automotive Ltd, who supply a lot of UK manufacturers. Note that the temperatures he quotes are all in degrees Celsius.

"Dear Mr. Boettcher,

In reply to your inquiry, basically you are correct in your understanding of the operation of an automobile thermostat in as much that the thermostat will only reach a fully open condition in "full load" engine conditions when maximum cooling is required.

In normal "light load" conditions the thermostat will vary between closed, to a partially open condition, to maintain the engine temperature at the optimum operating condition.

The temperature rating of the thermostat normally stamped on the body eg. 82, 88, 92 etc. is the temperature that the thermostat valve initially cracks open at (There is normally a manufacturing tolerance of +/- 2 degrees allowed).

As the temperature increases the valve will open further depending on the coolant temperature until fully open. The fully open temperature varies slightly across different part numbers, but is nominally 12-15 degrees above the opening temperature, therefore a 88 degree thermostat would not be fully open until 100-103 degrees, likewise a 92 degree thermostat would not be fully open until 104-107 degrees.

When testing the higher temperature thermostats >88 degrees, many people make the mistake of putting the thermostat into boiling water and expect the valve to open fully. This will not happen when immersing in plain water alone due to the boiling point of water being 100 degrees at sea level. To test these thermostats you need to do so in a water/ anti-freeze mixture, which will raise the boiling point of the coolant, or use cooking oil as an alternative.

Also when testing the thermostat, the temperature of the coolant should be raised slowly to simulate the heat sink conditions found in a vehicle. Boiling a kettle of water raises the temperature too quickly above the initial opening temperature, this is why people mistakenly think that the thermostat snaps to fully open in one step.

Regards,
Derek Lees
Technical Training & Services Manager
Quinton Hazell Automotive Ltd.
Logix Park,
A5 Watling Street,
Hinckley,
LE10 3BQ
Great Britain

Well I don't think there can be any arguing with that: he clearly states that the thermostat varies its opening in response to engine load to maintain the engine temperature at the optimum operating condition, and that it only reaches fully open when maximum cooling is required.

(I expect that there is actually a margin built in so that it never actually reaches full open unless under really extreme conditions, because when it is fully open there is no more cooling available and the engine is on the very edge of overheating.)

He also explains why some people who have tested a thermostat in a pan of water think that it snaps from open to shut - this is basically because the thermostat is designed to keep the engine temperature in a fairly narrow range which it is easy to zoom through when heating a pan on the stove; so you do see it open gradually, but so quickly that it looks like it snaps open.

Well at last I have got the authoritative version so that I can go back to my father-in-law and settle the disagreement. And it seems that one or two people reading this thread might have had a few misconceptions sorted out - providing they believe Mr Lees of course! (Somehow I doubt they all will!!)

[petrolhead611]

apropos of proprietary parts, I bought a new steel A arm and all the bushings and ball joint from a non-franchised parts supplier( "German & Swedish", for the benefit of other UK forum members) and was amazed at how cheap the parts were. NB I re-used the original sway bar mounting metalwork as it was more substantial than the new item, and the new rubber fitted it perfectly.

[flash968]

thanks dave

[Dean924s]

Great info!!!

I should have suggested that you take the T Stat and put it in water on the stove and bring up the temp and watch the valve with a thermometer in the water (the classic test to make sure that they are working. This would provide with out a doubt that the info you obtained. In your case a "show me demonstration" may be the clincher.

[AKCJ]

Dave - you da man!

Thanks for taking the time. I think you (and most of us) already knew that you were right but now you have the proof. I don't think I would have pursued this with my father in law - I don't mind knowing that I'm right and he's wrong without proving it to him.

One more thing. It may be worth noting that with most valves (especially valves built like an automotive thermostat) the change in flow is not linear with the change in valve position. This depends on many factors but pressure drop across the valve is the main one.

Anyway, good job.