No Thermostats =?

As I mentioned earlier, upon removing the thermos. cover on the passenger side it appears the PO took the thermostats out (both, unless the driver’s side is still present) a long time ago, based on how rusted up two of the cover bolts were (which, btw, I have still not been able to extract ever since the heads twisted off! :rage: ). I’m wondering if the PO did this in the belief it would make the engine run “cooler”. Indeed, the engine temp, per the gauge, never seems to get up to the operating range. :confused:

Interesting development is that I was mentioning the above facts to a good retired friend of mine in FL, a former J-Ler who has owned many classic cars in his life including two 5.3 XJS convertibles. This is the same bud that lamented to me that 5.3s have “cooling issues” and that, as to his own cats, he had to have the auxiliary fan permanently “jumped” so that it ran whenever the car was running, in order to avoid overheating on hot days in the FL panhandle w. the a.c. running. :angry: He is convinced this is a factory design defect, and, me being ever so the tactful friend, did not argue with him about that point, esp. since he has sworn off the model.

Anyway, I was telling him the above and he told me that, surprisingly, he had been told by a tech at a local classic car repair shop (supposedly nationally-known) that NOT having thermostats in an XJS can actually cause the engine to run even hotter (i.e. overheat). :open_mouth: I have never heard this before, and just can’t wrap my head around how that could be so. ? :confused: Thoughts on this claim, Palmdude (and everyone else)?

My understanding is that the water can flow through the radiator such that the air flow does not remove sufficient heat from the coolant before it is back into the engine.

Having the aux fan come on when the A/C compressor is running is a standard feature on my 1992. So either his set up was broken originally, or it was a later addition by Jaguar that his cars didn’t have.

As it opens the main radiator top hose path, the second valve at the foot of each tstat simultaneously closes off the bypass. The wrong type or dimension tstat (or leaving them out altogether) means the bypass is never blocked off and some of the coolant never goes through the radiator and the car can run hotter.

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Search the archives for the V12 coolant flow diagram …I’m sure many have posted it over the years. It’ll all make more sense then.

On a V12 it isn’t just a matter of the thermostats being opened or closed. It’s also a matter of the thermostats directing where the coolant flows, as Peter mentions.

Cheers
DD

XJ-S cooling diagram

I don’t know how to insert pictures inline apparently.

As mentioned by others this topic has been explained at length in the archives. I have a flow diagram (and explanation) on my website where I offer my 2 cents on the pros and cons of changing from a dual pass to a single pass radiator. If interested it’s at.

http://www.bernardembden.com/xjs/radconvert/index.htm

Check the lower right corner of a post. Just to the left of reply! the “paper clip” symbol, aka attach!!

And on the reply, the upper banc 6 from the gear, the upside down “T”

for upload

go from there.

As like here

Caravatar171_3

You have a 4.0, so only one thermostat.

The car will run inefficiently with no thermostat- taking longer to bring the engine to temperature without it. It’s job is to get the engine hot, fast. It has no ability to cool the car, other than to fully open at its design temperature goal.

The others have chimed in with their other, more valid points.

Bottom line: if the car is overheating, removing the thermostat is not going to prevent that from happening. Once the temp passes a certain temp (188F for example), It’s done everyone it’s designed to do and is fully open, allowing the coolant to flow as freely as it can. The thermostat cannot prevent the car from over heating.

True.

The typical Jag tstat (I6 or V12) is a warm-up accelerator because it modulates the coolant flow between two possible flow paths.

When cool it promotes heating by biasing flow through the bypass route, avoiding the cooling effect of the radiator. Engine gets hot.

When hot, it promotes cooling by closing the bypass and diverting all coolant through the radiator. Engine stops getting hotter.

It cannot bring the temperature down by itself if the cooling system is compromised. Full flow through the radiator is normally effective enough to bring the temp down and cause the thermostat to cycle gradually and keep the temperature in range.

Robin’s concept of water passing through the radiator too quickly to shed heat properly is mistaken. The faster the flow the better and the hotter all over the core the more heat is lost to the air, assuming the cooling system is properly designed and maintained.

With the sole exception to that rule being if the thermostat is stuck closed (i.e. defective) … (right?) :thinking:

But then, using that last point … if there’s no thermostat at all in the system wouldn’t the “flow resistance” be reduced, creating faster coolant flow through the system/radiator = more cooling power (that’s assuming though the “change in direction” were to still take place somehow)? I’m thinking that’s maybe what the PO was thinking when he removed them (or again, they might just have been stuck closed :blush:). :thinking: Would be interesting to see a “flow” chart showing the speed of coolant through a system with thermos fully open vs. no thermos at all. Possibly it only makes a negligible difference.

He actually has both a six and a v12 and his first post in this thread is talking about the v12 car.

Yep, I think I figured out that was what was going on w. VK’s answer … :smile:

This is my next project. I’m fitting four flow sensors to my v12.

kind regards
Marek

I suppose yes, a broken thermostat stuck closed can cause a car to overheat, however if the cooling system was already compromised, then it wouldn’t matter anyways.

My original point being that a (working) thermostat doesn’t cool a car, it helps it get to temperature faster. Once it gets there, it’s job is done and can do nothing more. So a compromised cooling system cannot benefit from a removed thermostat to “maximize flow of coolant”.

Lot of info on the forum last summer, but in a nutshell, for the V12 it’s important that when the thermostats OPEN, that they CLOSE the bypass that flows hot coolant straight back into the engine. Someone did a lot of research last summer on different thermostats, as the thermostat needs to move a certain distance to actually close the bypass. Many failed, this means not all of the coolant is going back into the radiator, so you’re not running at 100% cooling efficiency.

I think only two thermostats reached enough to close the bypass, and one was Gates, which I went out and bought. I didn’t notice any difference in cooling temps, but perhaps it will only make a difference at extremes?

I think that’s the brand I bought (if I can ever get them INSTALLED :roll_eyes:) after I took back the “Fail Safe” model I originally bought. I noticed the F.S. don’t appear to have jiggle pins in them, and I think I read somewhere that it is important with the V-12 engines that thermostats have them. Also, I read a review of such thermos by another major manufacturer of thermos in which they claim that such thermos give a false sense of security b/c they only keep a thermos which is malfunctioning not to close from the open position (i.e. and possibly end up stuck closed) – they do not force a thermos which has failed in the closed position to reopen. :open_mouth: So, in other words, if your thermos is stuck in the closed position (i.e. when first starting the car up) you’re screwed anyway … :grimacing: (maybe they should rename them “False Safe”? :laughing:)

Thing is, if having no thermos in the system would cause it to run hot, why does my temp gauge show not up to running temp. even (assuming nothing is wrong with either the gauge or the sending unit)? :confused:

Reread my post- the point of the thermostat is the opposite. It’s to make it run less cool as quickly as possible. It point is to speed up the time it needs to get to the design temp, which is about 188F.

So without a thermostat, it can’t speed up the time it takes to get to that design temp. On a cool day, it’s possible it never gets there. It might get stuck at 170F or somewhere around that range.

The thermostat is there to restrict the coolant so that it’s able to achieve a balance to maintain an artificially higher temp than it should be capable of getting to on its own.