Ooh, the plot thickens. I've never really had to mess with my S-Type's thermostat. I had to dig out my manuals this morning, faced with the horrifying prospect that I was less correct than usual. (The W word is never to be used...) Here's what I learned. For anybody interested, get some coffee and I'll bore you to tears.

For background, the S-Type thermostat is definitely an oddball, compared to the standard 'on/off' device seen in most cars, older ones at least. On your father's Chevrolet or Vauxhall, hot coolant left the engine and was sent to the radiator inlet for cooling. That style of thermostat would close when cold and open when hot, or modulate somewhere in between. But basically it was an on/off valve.

The S-Type thermostat is sort of an on/off valve, but I think a better term would be to call it an either/or valve, or maybe a shuttle valve, because of how it fits in the thermostat housing. The housing is a giant tee fitting, with two inputs. The base of the tee is the common return to the water pump.

Here's a picture lifted from the '02 service manual. I'm using the V6, as the diagram was less cluttered, but the V8 is basically the same.

1) The red arrow shows hot coolant leaving the engine block.
2) The green arrow shows coolant returning to the engine (via the water pump #1).
3) The red circle shows the thermostat housing #10. Note the two inputs and the common return (green arrow) to the engine.





Here's where it gets interesting, at least to my geek sense. In the image above, note how the guts of the thermostat look like a spool. It's like two discs on opposite ends of a common shaft. Fitting inside the thermostat housing, this spool can be shifted to block one input and open the other. If sitting in the middle, both inputs are partially open, but there's no way to block off both inputs at the same time.

When cold, the spool is shifted down and the radiator return hose #9 is blocked. The thermostat's other inlet port (the radiator bypass passage, right above the thermostat) is open. Coolant leaving the engine (red arrow) is routed directly back to the water pump, bypassing the radiator.

When hot, the spool is shifted up and radiator return hose #9 is now open to the water pump. The radiator bypass tube is now closed and coolant flows through the radiator.

In practice, the thermostat probably modulates somewhere in the middle once at operating temperature, connecting both inputs to the common return.

Please note when I mention shifting the spool up or down, I'm talking about relative to the above diagram from the service manual. I'm not sure of the thermostat's orientation on the vehicle.

So let's shift gears and look at the actual thermostat, removed from the housing. Remember, the thermostat is two discs fixed on opposite ends of a common shaft, able to block off one input passage at a time, but not both.

Here's a purloined image of a thermostat, flipped to match the orientation of the diagram:





Kinda cool, huh? That's your father's basic thermostat upside down at the bottom. That's the portion that opens and closes to control the return from the radiator. That's the bottom of the spool mentioned above. The picture is a cold thermostat obviously, so this passage is closed.

The extra disc at the top, not found on your father's thermostat? That's the top of the spool mentioned above. When shifted up, it's the portion that meets the thermostat housing and closes the radiator bypass passage.

So Mikey, yep, you're totally right and I was in left field again. If looking at a thermostat in your hand, it's going to be closed when cold, just like you said. I got confused because once installed, it changes from a simple on/off valve to more of an either/or valve.

Have I lost everybody yet? Hope this helps somebody. I'll go refill my coffee now and share with anybody else who finds this stuff interesting. Let's see, it's a 12 ounce coffee cup, divided by the number of people interested, leaving me 12 ounces...

 

Harrumph!!! Rammer rammmer rammmer....Karl...
All that meticulous research and you didn't post the temps which trigger high and low fan speeds according to coolant temp!

 

In mid-summer 2016, I drove my '01 Jag S-Type 3.0L from New Jersey to Florida…covering 1200 miles in 2 days during some brutally hot and humid weather.

Ambient temps ranged from 92°-100°F (33-38°C), steadily increasing the further south I drove; A/C on "Auto" the entire trip.

With the O/D at 150,000 miles, my pre-trip prep included a new thermostat & housing, hoses & coolant, plus and a ton of other goodies installed to ensure the trip would be as trouble-free as possible. It was.

Throughout the entire trip I monitored my OBDII tool's sensor readings on my iPad screen as neurotically as a surgical team monitors a patient's vital signs. Engine temp concerned me most, given the car's age, mileage, length of my trip, the 70mph interstate speeds, and the heat & humidity outside.

So I can tell you that, in my case, at steady speeds of 65-70mph, A/C always on, and typical ambient temps as above, my engine temp consistently hovered around 204°F (96°C).

In heavier, slower traffic engine temp was usually at about 210°F ( 99°C); in purely stop 'n go traffic my engine temp never rose above 212°F (100°C).

Never during this trip, even after arriving in the sweltering heat and heavy traffic of Fort Lauderdale, Florida, did the engine temp ever rise above 212°F.

I also noted that the cooling fan worked properly while the car was running and that it shut off as soon as I shut the car off.

In fact I've never heard the fan on any of my 3.0L S-Types ('01, '03, '05) run after shutting the car off. I don't know if this is normal behavior for all 3.0L S-Type fan systems or not, but it is for all of mine.

One off-topic note - During the 1200 mile NJ to FL trip I averaged roughly 30 mpg @ 65-70mph, using cruise control. There is one stretch of road in the Carolinas where you can literally gas up, drive hundreds of miles non-stop at 70mph, then fill up again. In this stretch of road I averaged 32mpg, an all-time high for this car. I calculated the MPG by hand, not using the onboard trip mpg numbers.

 

How's your hearing?

On my '02 V6, I've noticed something about the fan running after shutdown. On a warm day, if I've been in stop and go traffic (low ram airflow), the fan might run for a few minutes after shutdown. If I'd been moving at a good clip before turning onto my street, the fan won't run.

This always seemed normal to me. With the scenario of low ram airflow through the radiator, the coolant temp must have climbed just a smidge above the trigger point for running after shutdown.

 

Huh? What?

 

I said, "HOW'S YOUR HEARING?"

 

From my experience when the fan runs after shutdown (for any period of time) this is the first sign of an impending cooling system issue on the horizon.

 

I had this issue for several years, changed thermostat but it did not fix it. Finally I cleaned the radiator with power washer and all is well ever since.