Engine running too cool

[TRier]

I’ll check the rad cap pressure for you and come back to you, I don’t recall off the top of my head. As a matter of interest I took the bypass type thermostat out over the weekend and tested it, is was starting to open in the 60+C range, too low obviously. I put a restriction in the bypass loop and an ordinary 82C thermostat in and the car is running approx 10C hotter on a motorway cruise in cool conditions and approx 15C hotter in tickover after say 5 or 10 mins with the stat fully open in other words. The car is running so much better, I’ll see what it’s like on a very hot day but there is plenty of cooling capacity so I think it will be the same.

I have found that if I fill the radiator to the top then on the first drive or so when I park the car it will “cook off” the excess  and release it through the radiator cap, that makes sense as the engine temperature does increase when you park it after a running period and the water circulation stops, I could possibly address that by wiring the electric fan such that it stays on even with the ignition off or by fitting an expansion tank. The expansion tank is probably the right way to go and I’ll do that at some point, for now as per your friends experience, it cooks off the excess once and then the coolant level is correct for the set up.

 

Niall

[RobH]

1 hour ago, TRier said:

have found that if I fill the radiator to the top then on the first drive or so when I park the car it will “cook off” the excess

Niall - the TR3 radiator is not supposed to be filled right to the top and there is no benefit from doing that.  It needs an air space above the water to allow for expansion.  You fill it so there is maybe 1cm of water visible in the base of the filler neck. 

[Z320]

4 hours ago, TRier said:

I have found that if I fill the radiator to the top then on the first drive or so when I park the car it will “cook off” the excess  and release it through the radiator cap, that makes sense as the engine temperature does increase when you park it after a running period and the water circulation stops, I could possibly address that by wiring the electric fan such that it stays on even with the ignition off or by fitting an expansion tank. The expansion tank is probably the right way to go and I’ll do that at some point, for now as per your friends experience, it cooks off the excess once and then the coolant level is correct for the set up.

 

Niall

Hi Niall,

I have no problem with that, this works well also without overflow bottle, but check the pressure of your rad cap.

What this?

Sadly the bellows thermostats are gas- und liquid filled, the liquid becomes to be expanding gas, this opens the thermostat.

But gas can be compressed (wax of a wax thermostst not) and so any system pressure above 4 PSI will close the bellows thermostat more than the heat opens it.

If you use a 7 or 13 PSI cap this is the reason for your raising temperature.

Ciao, Marco

Edited May 4 by Z320

[TRier]

On 5/4/2026 at 11:37 AM, Z320 said:

Hi Niall,

I have no problem with that, this works well also without overflow bottle, but check the pressure of your rad cap.

What this?

Sadly the bellows thermostats are gas- und liquid filled, the liquid becomes to be expanding gas, this opens the thermostat.

But gas can be compressed (wax of a wax thermostst not) and so any system pressure above 4 PSI will close the bellows thermostat more than the heat opens it.

If you use a 7 or 13 PSI cap this is the reason for your raising temperature.

Ciao, Marco

It didnt happen on the bellows type stat, because I assume the temperature was lower, too low. 

 

Niall

[TRier]

On 5/4/2026 at 9:52 AM, RobH said:

Niall - the TR3 radiator is not supposed to be filled right to the top and there is no benefit from doing that.  It needs an air space above the water to allow for expansion.  You fill it so there is maybe 1cm of water visible in the base of the filler neck. 

Yes thats correct and generally I do but sometimes in poor light I can't see the damn thing properly and it can result in an over fill which I dont worry too much about as it just cooks it off on stopping.🙂 Be better if it cooked it off into an expansion tank where it was then available if required but thats for another day. 🙂Thanks for the heads up.