Thermostat ?

[gefary]

My 04 cdti 36000 miles is due it's annual service.
The temperature is about 80C so I may change the thermostat at the service.

Questions. 1 Is the original thermostat just bad design ?
2 Is it good design but made to open at the wrong temperature ?
3 Do thermostats deteriorate for any particular reason ? 4 What temperature opening did the manufacturer recommend ?

[James.uk]

For a thermo to fail at such a low mileage on what should be a luxury vehicle is rather a poor reflection on the quality of our cars. Unfortunately it is also a horrible, and rather expensive job replacing it..

Even my cheapo 1994 ZX has managed 100k with it's original thermo still working well...
...

[FrenchMike]

Yes,i am curious too knowing what happend when one sink a bad thermostat in boiling water ?
Maybe some dirt avoiding it to close entirely?

Basically it must open at 88 degres, the melting wax temperature.


Mike

[T-Cut]

I believe the lower running temperature of the diesel engine is related to it's considerable mass. The petrol engines use a thermostat with the exact same operating temperature 88C nominal, but typically run up to 20C hotter. This isn't due to the diesel's 'high thermal efficiency' factor that many people see as the explanation. The diesel engine will overheat if there's insufficient airflow to cool it. With a duff fan and stationary traffic, the diesel will go into the red. So, there's certainly enough heat being generated to take the temperature way up. The question is, what's happening to all that heat when the car's being driven? Logically, the thermostat acts as a thermal valve for coolant flow, so you'd expect the coolant to stabilise at the same running value across all engine types. So, there's some factor about diesels which makes them run cooler, too cool in most cases. One possibility is that the radiator is simply too efficient/large for this engine.

Anyone else have thoughts on this?

TC

[James.uk]

Hi T-cut. My lil ZX runs much hotter than my 75, in fact you could probably get blisters if you held you hand over a heater vent for long with everything turned up to max heat. And the ZX has TWO huge cooling fans up front!!

The engine in a ZX is the Peugeot 1.9D and it is used in a LOT of vehicles..
...

[marley]

My thermostat went at about 90 odd thousand. Got a new one from Rimmers , for about 35 quid if I remember, local mechanic charged just 20 quid too! If I remember correctly!

[Jakg]

Originally Posted by marley

My thermostat went at about 90 odd thousand. Got a new one from Rimmers , for about 35 quid if I remember, local mechanic charged just 20 quid too! If I remember correctly!

If your mechanic can change the thermostat for less than £100 all-in, i'll book it in tomorrow, I don't care if it's in Wales!

[rrobson]

Originally Posted by T-Cut

I believe the lower running temperature of the diesel engine is related to it's considerable mass. The petrol engines use a thermostat with the exact same operating temperature 88C nominal, but typically run up to 20C hotter. This isn't due to the diesel's 'high thermal efficiency' factor that many people see as the explanation. The diesel engine will overheat if there's insufficient airflow to cool it. With a duff fan and stationary traffic, the diesel will go into the red. So, there's certainly enough heat being generated to take the temperature way up. The question is, what's happening to all that heat when the car's being driven? Logically, the thermostat acts as a thermal valve for coolant flow, so you'd expect the coolant to stabilise at the same running value across all engine types. So, there's some factor about diesels which makes them run cooler, too cool in most cases. One possibility is that the radiator is simply too efficient/large for this engine.

Anyone else have thoughts on this?

TC

sounds like a very plausable theory. it would be worth finding out the size and volume of similar sized engines' radiators. also, there are lower running speeds and the combustion process is different. diesels are generally used to cover more miles, and a lot more air is passing over the engine at 70 then at 30 or 40.

can a thermostat of a higher opening temperature be installed, or could the radiator be partially covered to restrict cold air passing over it?

[kaiser]

Originally Posted by T-Cut

I believe the lower running temperature of the diesel engine is related to it's considerable mass. The petrol engines use a thermostat with the exact same operating temperature 88C nominal, but typically run up to 20C hotter. This isn't due to the diesel's 'high thermal efficiency' factor that many people see as the explanation. The diesel engine will overheat if there's insufficient airflow to cool it. With a duff fan and stationary traffic, the diesel will go into the red. So, there's certainly enough heat being generated to take the temperature way up. The question is, what's happening to all that heat when the car's being driven? Logically, the thermostat acts as a thermal valve for coolant flow, so you'd expect the coolant to stabilise at the same running value across all engine types. So, there's some factor about diesels which makes them run cooler, too cool in most cases. One possibility is that the radiator is simply too efficient/large for this engine.

Anyone else have thoughts on this?

TC

Diesel engines are more efficient than petrol engines. That is a fact. This means that they put more of the energy in the fuel on to the tarmac, to put it simple, which means that less go to the exhaust, and, tah tah, less to the cooling water. That is as simple as it is. They thus tend to run cooler everything else equal. In a good petrol engine you get roughly 1/3 each to usable power, exhaust and water. In a good Diesel unit you can get up to 40 percent as mechanical energy.
The mass of an engine has got nothing to do with the temperature after you have completed the initial warm up. Having a lump of iron or one of aluminium is largely of no consequence, except for the spread of the heat in the block. Once you have heated the lump, you can leave it out of the question.
The thermostat in an engine is there to keep it warm, in essence, because if you had no thermostat, the equilibrium temperature would vary and be determined by mass flow and temperature through the radiator and load on the engine.
Having a thermostat in the picture allows you to create a buffer of cool water to be released as and when the thermostat senses higher temperatures. You can thus, approximately, create an environment of constant temperature around the engine, largely unaffected by load and outside temperature, which makes it easier to design optimal structures and processes.
The lower temperatures of a Diesel engine has everything to do with its higher thermal efficiency, and therefore lower heat losses.

[T-Cut]

Originally Posted by kaiser

The lower temperatures of a Diesel engine has everything to do with its higher thermal efficiency and therefore lower heat losses.

But they can still overheat if the fan fails. They clearly generate enough heat to do so.

TC