After observing my vain efforts to solve my overheating problem T_Max finally figured it out!
It appears that in SOME cars the thermostat is not effectively sealing the rear bypass flow from the engine to the water pump when the thermostat is open. That explains why SOME cars appear to run at normal operating temperature while other, similar cars, run hot.
As the top picture shows, he used a heat gun to open his thermostat all the way and wrapped a piece of copper tubing around the stem so that it would STAY open all the time. This had the effect of permanently sealing the engine bypass channel from the water pump.
The middle two pictures diagram the function of the thermostat and show the parts and water channels.
The bottom picture shows an ambient temperature of 102F and the gauge reading in the normal range.

What this DOES NOT EXPLAIN is why the thermostat fails to seal the rear channel in SOME cars when the seal is new, the normal function of the thermostat has been confirmed, and the installation and assembly are done properly.

 

Yay!!!!! Congratulations. Well, even if it is anticlimactic. Sometimes you get them first try, and sometimes after a bunch of work. Good for you, for not quitting. That's an interesting picture. I didn't know the newer cars show the actual temperature. Porsche sure knows how to do it right. I love a success story.

 

Gregg,
The temperature reading on the dash is the outside temperature. The temperature gauge on the dash is showing about 187F

 

ErnestSW - how did you fix your car? The same way as pictured?

 

I'll let you know tomorrow.

 

I don't think I would have used the term "Figured out", yet! But thanks for the credit that you certainly deserve equal amounts of as co-researcher and the driving force behind this project.

Although I think we are on the right track, there are still a few variable to be eliminated in order to arrive at a conclusion (IMHO)!

 

Ernest,

Well done you are 'on the track' to the final solution, for of course to block the thermostat fully open is only a temporary solution

To make your drawing complete, you need an outlet from the cross pipe to the top of the radiator - This passage is always open (Later edition, this outlet is now added)

When the thermostat opens it close the short way passage from the cross pipe return to the pump and then, the hot cooling water has to take the longer way via the radiator and return to the pump. This means, the thermostat seal has to overcome some pressure equal to the pressure used to send the cooling water via the radiator.

That means, we are playing with two possible reason for your problem:
1. The radiator has (to) high resistance
2. The thermostat 'engine' is weak so it cannot overcome the pressure and therefore not close the hole 100%

I take as granted that the rubber seal behind the thermostat is OK (These phenomena is mush worse on pre 1984 models because, they do not have the rubber seal, but only a aluminium seat who had tendency to corrode)

 

Erik,
Done.
I don't THINK the problem is one of water pressure vs the force of the thermostat "engine". I THINK the problem is a combination of factors.
1. The manual specifies a movement (throw) of "at least 8mm" for the thermostat beginning at 83C and ending at 98C.
2. At what temperature does the thermostat seal the rear passage? If it's at the end of its throw at the full 8mm, the thermostat would have to see a constant temperature of 98C or 208F (11C or 21F above normal operating temperature) BEFORE it seals.
3. Where does the thermostat "read"? The coolant coming from the radiator is cooler than the coolant on the engine side. If the radiator coolant is less than 98C does it partially close the thermostat allowing recirculation from the engine?
4. Why do SOME cars run cool and some run hot? Are there subtle differences in thermostat housing measurements (I doubt it) or do some thermostats work as Porsche intended while some do not?
5. How does one identify the "correct" thermostat?
I'd love to be able to chart the "actuation curve" of movement vs temperature on the thermostat from a "cool" running car against that of a "hot" running car.

 

Ernest,
I buy your statement with the following comments
1. OK
2. I don't know at what temperature, but you can measure the house and the thermostat with different temperature, then you get the answer
3. The thermostat do not read it fells and is certainly on a constant move forwards and backward for controlling the working temperature for the engine (When the thermostat is totally open the cooling water comes from the radiator and it will start closing until it gets hotter water direct from the engine and so on)
4. Because not two engines are 100% equal and specially not all the accessories as thermostat, radiator, temperature indicator ect.ect
5. I think the best and most safe is to buy an original Porsche thermostat, they are all tested to fulfil the Porsche specification - All others can be sold as OEM

 

Erik,
I hope you are correct.
Ernie

 

I would only use an OME (Porsche only) thermostat, O ring and rear seal. This is so you are insured that the manufacturing process is within the engineering specs sent by Porsche. We know that the t stat begins to open the return passage from the radiator at 83 degrees Celsius. Once the thermostat starts to open, the disk starts to close the bypass line (rear rubber seal). Once the coolant has reached a temperature of 98 degrees Celsius, the thermostat has reached its opening limit. The opening travel distance from cold (83 C) to hot (98 C) of the thermostat disk is 8 millimeters. The bypass line (where the rubber seals sit in the thermostat housing) should, in theory, be closed, thus allowing the flow of the cool coolant from the radiators return line (top hose) into the engine completing the hot cooling cycle.

If the thermostat disk travel is not at least 8 millimeters, we have potentially two problems.

1. The return colder coolant is restricted from entering the engine
2. The rear bypass line is not fully closed, allowing some of the hot coolant to circulate in the block and not reach the radiator.

Fine tune your thermostat.

Remove the thermostat housing from the engine. Leave everything on it (sensors, thermostat, and o ring). With a cold thermostat, if you pour water into the return housing (upper radiator hose). No water should flow out of any of the holes on the housing. Submerge the housing in hot water, until it reaches a temp of 98 degrees. The thermostat should be fully open by now, blocking the bypass line. Once again pour some warm water into the upper radiator hose housing. No water should come out of the two square ports on the bottom of the housing that bolts up to the intake manifold. If water leaks out, it means the disk is not sealing against the rear seal. Fine tune the thermostat by installing a thinner o ring in front of it, allowing it to seat further into the rear seal.

Once you have established that the thermostat is working as designed and you car sending unit still reads hot, I would install a stand alone water temperature gauge. Install the new sending unit where the original one goes (on the top of the housing). You can then determine if the cars temperature gauge is accurate or not.

 

Steve,
Good explanation and suggestions - Only I have comments to the following:
---------
Quote
If water leaks out, it means the disk is not sealing against the rear seal. Fine tune the thermostat by installing a thicker o ring in front of it, allowing it to seat further into the rear seal.
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The O-ring is placed in front of the thermostat and not behind!

 

Steve,
The O ring serves only ONE purpose and that is to prevent the thermostat housing from leaking. The thermostst sits in a seat in the housing and will not move foreward or back if the O ring effectively seals the housing from leaking.
The question you are asking is whether the thermostat is sealing the rear bypass. It's not.

 

Erik,

Yes the o ring is placed on the front of the T SAT, if the o ring is thin it would allow the T SAT to float, move in the housing not allowing the disc to seat properly against the rear seat. The T sat has to sit in a specific located with in the housing with no movement that is one of the jobs of the O ring.

 

Steve, I never thought about the O-Ring being on the thin side of it's specification (even as new). This is definitely an idea to be looked into further.