Winter overcooling

Since a thorough refresh of my V12’s cooling system, the engine does not reach proper operating temperature in the wintertime, ever.

I’m considering blocking the rad with some cardboard. Anybody else have a similar problem?

I’m as confident as I can be that my thermostats are correct and operating properly. I went through plenty of drama to get it that way.

What temperature thermostats do you have installed?

With a properly operating system blocking the rad should not be required, either the stats are the incorrect temp or not closing properly. How long does the engine take to reach normal operating temp? With a properly operating system the time difference between summer and winter times should not vary too much.

I have had a known good proper temperature thermostat (tested by me prior to installation on our kitchen stove with a digital thermometer) fail stuck partially open sometime later (tested by me on the same kitchen stove with the same digital thermometer). So I know that it can happen.


Sorry but the drama is not over Jay, nobody else to blame but the thermostats…
What temp rating are they?

The V12 doesn’t like it much below 90°C, hurts the economy as well.

And as a side note, did you measure that they have enough travel to completely close the bypass?

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@JerryPeck What temperature are the thermostats? Umm, they’re about the same temperature as the coolant.

Kidding aside, I don’t know. I remember reading in the forums here that there are two options, and I chose the hotter one because the consensus seemed to favour those.

@Robin_O_Connor Since thoroughly refreshing the cooling system, it NEVER reaches normal operating temperature, winter or summer. The needle only ever rises to 1/4 on the coolant temp gauge. It takes about the same amount of time regardless of the outside temp.

@Paul_M_Novak Noted. Thank you.

@Aristides Yes, I tested and measured the thermostats before installing them, including measuring how far the bypass blocker thing extends. I tried MANY. These are the only ones that met the requirements I read about here.

Logically, I can only come to the same conclusion: it can only be the thermostats. But I am at a loss … I have done everything I know how to do to make sure I have the right thermostats, yet still…

Are you only using your oem coolant temp gauge to determine operating temperature? How do you know that is accurate? If you have the pre facelift barrel gauge, they can easily be off due to corrosion on instrument cluster.

I have temp gun to help me determine temperature. I also installed aftermarket coolant temp gauges as i did not trust the N gauge.

Also double check your oem temp sensor has good connection. It can get corroded too.


That is a very, very good suggestion. I will determine the coolant’s ACTUAL temperature with an IR thermometer later today. Thank you.

I do suspect that the gauge on the dashboard is accurate, because the heated air coming out of the footwell vents isn’t quite as scorching as I would expect from that big V12. I will see.

Don’t judge coolant temperature by heater warmth, the heaters on these cars are sub par for sure. If you want an amazing heater, buy a Volvo. The Swedes know hot.

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Many moon ago, twenty years ago I reckon, some of us V12 owners did a comparison of barrel gauge readings versus IR readings taken at the thermo housings. There was quite a difference.

From memory the bottom, middle, and top of the “N” on my gauge corresponded to something like 180, 190, and 200ºF on the hand held IR. Others reported as much as 10º difference from my readings. Some hotter, some cooler.

I’ve owned 5 Jags. The only two with marginal heaters were V12s. Both had clogged heater cores (which I’ve never had on any other car) and both have had cooling system air pockets at some time or another. I have no proof or even good explanation but I’ve pretty well convinced myself that’s there’s “something” about V12 cooling systems and heater core problems.


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