The T’stat, a relatively new item for Ic engines, is as stated above. designed to warm the engine faster, to a point. early on, not for the engine’s benefit, but for the occupants. Heat in cold climes…
But, definitely a warm engine benefits. Not to mention the emmision benefits… .
Decades ago, when our F150 came to us, it ran very much on the cold side of the guage. But, one day, it got hot, real gurgling hot! I had procrastinated on a t’stat swap. Nope, now is the time, A bit fiddly, but got it done.
Fixed. Warmed up nicely,. no over heat, just right.
Somewhere I read that Ford’s 5.0 not the best design in that facet. T’s stat stick could be on the closed jsut as much as open. The former surely not good. .Teh ltter only a bit better…
We know the engine generates the heat and that the radiator extracts the heat. We also know that it must be a design feature that the radiator has a greater capacity to remove heat than the engine has to generate it. If that were not so, then the car would most certainly overheat all of the time.
When there is plenty of airflow and water flow, the radiator’s overcooling always wins this battle and the car runs cooler, perhaps even too cold, despite the fact that some already hot water simply goes straight back round to the engine again through the bypass.
The problem comes when there is little water flow and no airflow. The little water which goes via the radiator to get cold isn’t enough to outweigh the volume of hot water which takes the easy short route via the bypass and so the temperature creeps upwards.
The thermostats role in this is to bias the water flow towards the radiator more as the temperature gets higher. It has a spring loaded foot, so if hot enough, it should make that split 100% to the radiator route and 0% to the bypass.
Not running a temperature controlled water splitter means you have no control over temperature. It’ll simply move about all over the scale until it the rate of heat generation equals the rate of heat dissiption.
Paul, in case Greg doesn’t have that info, here is the complete list-
Gates 33188
Gates 33188s (Superstat)
Stant 13398
Stant 45398 (Superstat)
Dayco DT18A
Quinton Hazel QTH 104
Unipart GTS101
Any of these will close the bypass port when fully open.
Thanks very much, Dave. I’ll compare with the ones I bought to replace the “False Safe” pair, if they’re not on there, it’s back to Autozone yet again. Undoubtedly, they will think, “that crazy Jaguar owner” …
O.K., wondering now who put together that list. First of all, the only ones carried by Autozone, O’Reilly or for sale on Ebay are the Stant duo. What’s odd is that O’Reilly’s price for them is only $5 and $9 apiece, which is lower than the other brands and seems mighty cheap for what I would think is a top-quality thermostat. Also, neither version has jiggle pins, which seems to be an important part of thermostats and something mentioned in the XJS factory workshop materials. Further, there is a 1/8" height difference between the flanges of the two models. Are ya’ll sure these Stants are O.K. to use?
Paul, the thermostat numbers were compiled by Mike Frank. The original thread started as “Motorad High-Flow thermostats”. dated 8/15/2019. Mike did a lot of research on cooling systems and thermostats.
You will find the thread interesting, should you decide to read the whole thing.
As the 'stats on my own car have some years of use on them, I have ordered a pair of the Stant45398’s.
I can follow up if you wish.
Funny, in that the ones I bought (latest) from AZ are stamped with Motorad’s name on them. So I take it they aren’t “acceptable”, since their name is not on the list? btw, can you please provide a link to the original thread? Thanks.
Noted your comment above a few weeks ago, and thought - that’s odd, thought my wife’s '92 did not do that. So, I went out to make sure I had remembered correctly. Sure enough, the Aux electric fan does not come on until needed (same as her previous '89). Did you do the mod on your '92 or did it come that way? We are not having any XJS cooling problems here in Miami with record temps and high humidity - which kind of surprised me (180F thermostats). Anyway, as long as it cools OK that way, I prefer to leave it that way since I do not think running the Aux fan constantly seems prudent, especially when cruising down the pike with adequate air flow through the radiator. Nonetheless - whatever works!
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
37
Yep. IMHO, that was a stupid decision on someone’s part. Even if the system is working with the fan not running, it would work more efficiently if it were running. So I – and others, I believe – have recommended that that circuit be modified so the fan does come on whenever the compressor is running, just as it did on earlier models.
Now I’m confused … I thought the aux fan is supposed to come on whenever the a.c. is on, at least on Superblack and Superblue. In fact, it is the fact that whenever I turn on the a.c. the aux fan fuse blows (or whenever I short the fan switch relay connections with a paperclip) that has been driving me nuts ever since I got Superblue. Her fan appeared to be frozen (from disuse?) when I bought her, but quickly freed up when I gave the blades a yank by hand. I can’t use the a.c . w/o the fan running, or the engine quickly overheats. So, yes, I continue to suffer in 104+ Texas temps this week w/o a.c. in her still … I think Gregmatic recommended earlier to try ramping the factory fuse amperage up by 5 amps, so I guess I’ll try that next …
Yes, my 88 turns aux fan on with AC, but its not when compressor runs, it’s when you turn on the air system to cool. I removed my compressor, and aux fan will still come on if i lower temp control.
I actually found wire on relay to pull so that won’t happen now.
If , at some time, the car begins to have an overheating problem with the A/C on in local traffic, then I may do that. In the meantime, I see no point in having the Aux fan running full time while taking an 8 hour trip down the road at 70 mph and unnecessarily burdening an already marginal OEM alternator.
Thanks for your response – Take care and stay safe!
First of all, the only ones carried by Autozone, O’Reilly or for sale on Ebay are the Stant duo. What’s odd is that O’Reilly’s price for them is only $5 and $9 apiece, which is lower than the other brands and seems mighty cheap for what I would think is a top-quality thermostat. 
Also, neither version has jiggle pins, which seems to be an important part of thermostats and something mentioned in the XJS factory workshop materials. 
Further, there is a 1/8" height difference between the flanges of the two models. Are ya’ll sure these Stants are O.K. to use? 
Must have been a slow week (month?) around that time last year on the XJS forum … 
Her fan appeared to be frozen (from disuse?) when I bought her, but quickly freed up when I gave the blades a yank by hand. I can’t use the a.c . w/o the fan running, or the engine quickly overheats. So, yes, I continue to suffer in 104+ Texas temps this week w/o a.c. in her still … 
I think Gregmatic recommended earlier to try ramping the factory fuse amperage up by 5 amps, so I guess I’ll try that next …
