Engine running too cool

[Tim T]

I have a 5 which is running very well but, the engine does not appear to be achieving the correct running temperature, even on a long run in the summer. Could anybody suggest why this may be and what should I be checking further. Top radiator hose gets hot but is still comfortable to hold/touch. Heater works but not as it used to.
Any suggestions gratefully received.
Thanks,
Tim

[Scott M]

I’d guess the thermostat is stuck , but I’m sure someone more knowledgeable will know for definite 

[Peter V W]

Tim,

Do you have the correct temperature sensor?

Should have a black not red top.

Peter

[Tim T]

Thanks both.

Scott, I have tested the thermostat and it was good.
Peter,the sensor has been changed. I will check that it is the black one. I have a vague recollection that it was red as I was told previously that it should be red? But, that will only affect the gauge reading, won’t it? Surely I shouldn’t be able to hold the top hose comfortably?
Many thanks 
Tim
 

Edited March 28 by Tim T

[RobH]

 Do you have an electrical  thermometer to check the temperature of the thermostat housing ?  Either a surface-measuring one (thermocouple or thermistor) or an infrared 'gun' ?   If the temperature really is low then Scott's suggestion is the most likely. 

 

edit- posts crossed and I see you have tested the thermostat. Still worth an independent measurement though - the wrong sensor for the meter won't make any difference to the actual heat. . 

Edited March 28 by RobH

[Tim T]

1 minute ago, RobH said:

 Do you have an electrical  thermometer to check the temperature of the thermostat housing ?  Either a surface-measuring one (thermocouple or thermistor) or an infrared 'gun' ?   If the temperature really is low then Scott's suggestion is the most likely. 

Rob,I have both so will check again. If the thermostat is stuck open, surely it would get to temperature eventually?
Thanks,
Tim

[TR NIALL]

33 minutes ago, Tim T said:

Rob,I have both so will check again. If the thermostat is stuck open, surely it would get to temperature eventually?
Thanks,
Tim

Don’t think so as the Water will be circulating all the time and cooling.

Maybe block off the front of the Rad and leave it Ticking Over see will Temp rise….it should.

[RobH]

1 hour ago, Tim T said:

surely it would get to temperature eventually?

No it won't.  Not unless the engine heat output is greater than the radiator cooling capacity, and the system is designed with enough cooling in hand to prevent that under all normal circumstances. Most of the time the thermostat should be regulating the temperature by varying the coolant flow.  Without it the system will reach some lower equilibrium depending on how hard the engine is working and how much airflow there is at the time. 

  Niall's suggestion would prove the point but a measurement is maybe a bit more scientific .        

[Ralph Whitaker]

Thermostats are available in a range of operating temperatures, maybe you have one of the lower temp items and need to change it for one that opens at a higher temperature. Other than that other factors can affect the temp an engine runs at such as timing and mixture.

Ralph

[Steve-B]

A classic sign of stuck thermostat is when hoses are unequally hot or cold when the engine is up to temp. I discovered this in my TR6 when the water pump was recently replaced that my thermostat was 75% stuck closed. Replaced it when new water pump installed and my engine hoses all same team, and engine definitely running cooler on the dashboard gauge 

Edited March 29 by Steve-B

[stuart]

22 hours ago, Tim T said:

Thanks both.

Scott, I have tested the thermostat and it was good.
Peter,the sensor has been changed. I will check that it is the black one. I have a vague recollection that it was red as I was told previously that it should be red? But, that will only affect the gauge reading, won’t it? Surely I shouldn’t be able to hold the top hose comfortably?
Many thanks 
Tim
 

FWIW The red top one is for TR7 although a lot of the usual suspects sell it as being for 4/5/6 The black one is what you want though this isn’t your problem in this case

Stuart

Edited March 29 by stuart

[Tim T]

Many thanks for all responses.
Temperature sender is definitely the correct black one.
I haven't checked the thermostat yet, waiting for a replacement and gaskets to arrive.
I have checked some temperatures with the engine at its usual running temperature after 15 minutes or so ticking over.

Thermostat housing    77 deg C
Centre of top hose      55 deg C
Top of bottom hose    43 deg C
Top heater hose          62 deg C
Bottom heater hose    42 deg C
Top of Radiator           65 deg C     

Do these readings suggest an stuck open thermostat?

Cheers,
Tim

 

Edited April 2 by Tim T

[RobH]

1 hour ago, Tim T said:

Do these readings suggest an stuck open thermostat?

 

Maybe.   You'll find out for sure when you fit the replacement.

Something is obviously getting through since the rad is warm.  

Edited April 2 by RobH

[Z320]

Interesting issue against all saying:

no cooling problems on this TR6 (also not on others).

Are cooling problems tale spinning?

[BlueTR3A-5EKT]

17 minutes ago, Z320 said:

Interesting issue against all saying:

no cooling problems on this TR6 (also not on others).

Are cooling problems tale spinning?

Well spotted Marco.

Something to debate over a pint in the pub,  

We have done it for years and you have just spotted our specialist subject.

[Tim T]

Parts books say that the standard thermostat is 82 degrees C, should it in fact be 88 degrees C?
Thanks,
Tim

Edited April 6 by Tim T

[john.r.davies]

Could be the thetmostat, but modern replacenent sensors are unreliable.

Test the sensor by suspending it in a pan of cold water on a camp stove.  Connect it to the gauge and earth by extension wires.   Heat the water, measuring the temp wirh a cook's thermometer or an infrared 'gun', note the reading at each stage.

Now you know what the guage reading really means!

John

Edited April 6 by john.r.davies

[Lebro]

I have used an 88° one summer & winter for a good many years now. It puts the temp gauge nicely in the middle, & has caused me no issues.

Bob

[Tim T]

26 minutes ago, Lebro said:

I have used an 88° one summer & winter for a good many years now. It puts the temp gauge nicely in the middle, & has caused me no issues.

Bob

I like the sound of that Bob!
Tim

[Andy Moltu]

It always struck me as slightly odd to have different stat temps for summer and winter. One will result in a higher reading on the gauge. Who knows which one Triumph calibrated them for!
 

The optimal operating temp for the engine should be much the same whatever the ambient temp. A higher thermostat temp should mean the water entering the heater would be hotter which is beneficial in the winter.

Either the engine is over cooling (stat opening too far) or the gauge is under reading. (Gauge, sender or voltage regulator can be responsible) Change the stat and if the readings at the gauge are the same the issue is the reading being incorrect. 

[trevor]

After reading some replies and the mention of temperature senders I thought I would check mine and its Red, though a well known "Great British" parts supplier lists it as the correct one and the Black for TR5/6 Spitfire, Vitesse, so I'm a little confused.

[TRier]

I have a similar issue on my TR3, its annoying because I went to the trouble of buying one of those ridiculously expensive bypass type stats and I don't think it has worked properly since the day I got it. The car has been fitted with an uprated water pump and rad so it has a lot of cooling capacity and as part of that series of upgrades I replaced the bog standard type of stat for the bypass type, its always over cooled since. I'm very sure it is the stat on the basis that no matter how large the cooling capacity the stat should regulate the coolant flow to achieve its design temperature. For interest I'm going to stick the bypass stat in a container and heat it up and see what happens but I think no matter what I see there I'm going to re fit the standard type stat, timely post this one as I'm planning on doing that this weekend, my missus tootsies are getting cold!

I'll take a few photos and show my findings in case its any use to anyone else although I think this subject has been well covered. 

 

Niall

[Lebro]

I have the correct sleeved bellows type stat in my '3 (88°) Have had no such issues.

Bob

[iain]

19 hours ago, Lebro said:

I have the correct sleeved bellows type stat in my '3 (88°) Have had no such issues.

Bob

Same.

[Z320]

On 5/1/2026 at 11:24 AM, TRier said:

I have a similar issue on my TR3, its annoying because I went to the trouble of buying one of those ridiculously expensive bypass type stats and I don't think it has worked properly since the day I got it. The car has been fitted with an uprated water pump and rad so it has a lot of cooling capacity and as part of that series of upgrades I replaced the bog standard type of stat for the bypass type, its always over cooled since. I'm very sure it is the stat on the basis that no matter how large the cooling capacity the stat should regulate the coolant flow to achieve its design temperature. For interest I'm going to stick the bypass stat in a container and heat it up and see what happens but I think no matter what I see there I'm going to re fit the standard type stat, timely post this one as I'm planning on doing that this weekend, my missus tootsies are getting cold!

I'll take a few photos and show my findings in case its any use to anyone else although I think this subject has been well covered. 

 

Niall

What pressure is your rad cap, please?

And how much coolant do you fill in?

A friend of mine tops the rad up occasionlly on his TR3,

with the result of an puddle under the car (like yesterday 😀).

This way he knows he has enough coolant in there 😂

Ciao, Marco