If by "does little" you mean moves 7500 cubic feet of air across the radiator reducing the coolant temp at the water pump by a measurable amount.

You imply that it is completely local and the heat transfer between the coolant itself is negligible, yet we are concrete proof that it does because the cooling happening at the radiator affects coolant traveling all the way up and back to the switch on the water pump. If it is that obvious that several degrees centigrade of heat transfer occurs from the fan cooling the lower portion of the radiator all the way up the coolant hose and in the water pump itself, then it should be very clearly that heat transfer is happening at a significant enough rate for coolant in the rest of the system to be affected.

 

You're right, I never implied anything about a lower temp switch.
I stated quite clearly that running the fan for a longer period would accomplish little and exceeds what the engineers who designed the vehicle determined to be appropriate.

There's my point. I'm really not clear on why people are on a quest to reduce post shut-down heat soak. It's essentially benign, other than possibly reducing the life of plastic components. The engine doesn't get hotter after shutdown, just the coolant as reflected by the gauge reading.

Relatively few cars of the era had electric fans- most were still mechanical- and fewer still had an automatic feature to run them after engine shutdown. They all survived just fine.

If there's another factor I'm missing, please clue me in.

 

Well, not missing exactly; but I do think you are slightly under-playing the damage that high under bonnet temps do to the loom and other components liable to heat damaging effects after a hot stop. The engine bay air temps do get substantially higher after shutdown, as a number of posters have mentioned. And there are absolutely loads of posts that attest to the troubles caused by cooked injector looms, hoses, cooked amp looms, and cooked dizzy low tension wires, to name but a few items.
Whether dropped valve seats are caused by it maybe moot, but loads of posters consider they are.
Lastly, I do not agree that the XJS, with its notoriously hot and crowded engine bay, its marginal airflow through the rad stack unless in tip top condition and its 5.3 litre V12, is honestly comparable with other cars of he era, even big engined ones. My original model Range Rover had so much space round the V8 you could have shoved another on in there!
Funnily enough, the Jensen Interceptor, even though it has a much less crowded engine bay, also benefited from a louvred bonnet, which was offered on certain versions OEM, and according to my mate who has one, the louvres make a huge difference to under bonnet air temps and component longevity.
Greg

 

Then ya got me.

I moved near ALL the electrickery items subjest to heat sensitivity OUT of the engine bay.

That was in 1998 when I began tracing a wiring issue, as we V12ers all do when we are bored.

Rewired the whole under bonnet section.

Basically unplugged the loom at the bulkhead connectors, hung it from the shed rafters, and started again, Grants way.

The wiring in there is now wrapped in "Header wrap" and is stil soft and pliable now as when I finished the job.

The hoses, well, they had to stay, too many coolant leaks otherwise, but I change my hoses every 5 years, and I mean fuel as well, so that little slackness never gets me caught out. The old are NOT hard, ever, but where and when I drive down here, a breakdown will cost ya your life in summer, and I am far to precious to cut that short by slack maintaning of a Jag V12.

Under bonnet temps are basically what they are, and some small things, like "exit air" improvements whilst moving, will assist, but when the beast is shutdown "hot air rises", and that beautiful bonnet stops its escape.

I looked at louvres, and all myriad of things. Summer temps of 47c mostly has one looking at strange things in the midday heat, but never went for them. They look sweet on the E Type.

One note that may be worth thinking about. Your choice, When I rebuild this thing, I took it for a blast with the bonnet OFF, yeh I know, but then again, I do what I do. The running temps were duly noted, new engine had me glued to that crazy gauge, and it sat ON the N. I put the bonnet back on, cops were NOT impressed with a no bonnet car, went for the same blast, that crazy gauge sat BELOW the N, almost 2 needle widths, so there is more to this airflow than my brain can handle, and I have moved on. The beast does what it does very well, and has never let me down. It is driven HARD mostly, why else is there e Redline on the tacho.

 

Fair enough. My experience is with 60s and 70s Corvettes and to a lesser degree, Sunbeam Tigers. These are similar to the XJS with having a lot of engine stuffed into a tiny space but dissimilar by not having a fan that operated after engine shutdown. Aside from some anecdotal info about fuel percolation on the carbureted versions or the occasional weak starter solenoid, there's no known issues with soakback heat.

I've read the theory of valve seats dropping after shutdown and still have serious doubts about this being the result of soakback. The heads and seats start to cool the moment the ignition is cut, not get hotter as per theory. In any case, running an electric fan without the coolant circulating is not an efficient method to cool the heads if the first few minutes are critical, again as per theory.

I doubt that the Jag engineers 30 odd years ago were concerned about long term component life, and also question how much damage is done to them by normal running temps and years of age vs after shutdown soakback.

This leads me to wonder why they incorporated the fan feature at all. Could it be to mitigate coolant boil-overs due to overpressure?

 

As to the oft-cooked wiring.....

I've always felt this was caused primarily by Jaguar's crazy idea of actually clamping the looms to the engine itself, at the bottom the vee.

I don't think any amount of after-cooling can offset that.

On my current V12 project is easy to determine which wires were in the vee and which weren't. The ones that lived in the vee were crispy and the ones that didn't were still pliable.

When I've made up new V12 engine wiring I've always used 'high temp' wire and some sort of extra heat sheathing....but I'm pretty well convinced that just re-routing the wires out of the vee is what makes 90% of the difference.

Cheers
DD

 

Just as a little bit of real life data that can be seen as analogous to the cars, I am working in my picobrewery today and noted something of relevance.

I have a vessel containing about 10 gallons of water being held at 200*F. Let's think of that as being similar to an engine containing coolant. Attached to the vessel are a series of pipes, some being similar in size to the ID of a rad hose. Downstream of the vessel, at approximately the same distance as the length of a car's rad hose, located in one of pipes is a thermocouple.

Despite the liquid in the vessel being at 200* and the pipes being made of very themally conductive copper, the temp at the thermocouple never exceeded 75*, basically room temp.

The effects of convection or conduction are minimal.