Thread: Thermostat 300TDI

less than opt temp doesn't do the fuel consumption any good.

Originally Posted by pjlhat

Is it practical to run a 300tdi in a 130 defender during summer without a thermostate

If you are trying to mask another problem then its probably not a good idea
Sort the cooling system and a 300 tdi will not overheat

As above, sort the system out.
But I have in the past managed to turn the thermostat spring in such a way that it held the thermostat wide open whilst trouble shooting.
It is my belief that the thermostat is required to give some restriction to the coolant flow, to allow a controlled flow through the radiator giving time for heat exchange.

Does that make sense?.

Also the thermostat has the plunger at the other end that blocks the 3/4 bypass hose from the water pump.

My problem was a faulty vdo electric gauge. It would climb to 95 ok, but then would keep rising.

Originally Posted by jboot51

It is my belief that the thermostat is required to give some restriction to the coolant flow, to allow a controlled flow through the radiator giving time for heat exchange.

Does that make sense?

That, sadly, is what I call an "Old Mechanics Tale". Water that speeds through a radiator also speeds through an engine, it all works out for more cooling.

Originally Posted by jboot51

Also the thermostat has the plunger at the other end that blocks the 3/4 bypass hose from the water pump.

But that however is the big issue. Without the plunger you waste the flow rate through the bypass, hampering cooling. A brass plug jammed in there would slow the bypass flow enough to get the radiator flow up to useful levels.

A fast flow is no good if the heat can't be removed effectively.

Originally Posted by loanrangie

A fast flow is no good if the heat can't be removed effectively.

If coolant flows slowly through your engine then the coolant will be considerably hotter at the top of the engine than at the bottom. Large (and fluctuating) temperature differences can lead to cylinder head gasket failure and head cracking. The stock 300TDi thermostat allows fast flow through the bypass and adding small amounts of cold coolant as needed to keep the temperature relatively even. When the thermostat is fully open and blocking the bypass the restriction of the tubes and radiator hoses is more of a deal than the maximum size of the thermostat opening. At no time does the coolant flow rate in a normal engine reach speeds where cooling is inhibited by a larger thermostat opening.

Another way of looking at it: If 1 litre per second of coolant goes in the top of the radiator at 100C and leaves the bottom at 60C, the average temperature of the radiator core is 80C. If 2 litres per second go in at 100C and leaves at 80C, the average temp of the radiator core is 90C. That means that there is a higher gradient of temperature between the incoming airflow (say 40C) and the radiator core, leading to higher heat removal.

Heat transfer is a function of both temperature drop and volume flow.

Thanks for all the input, to really get a handle on this i need to be sure just what the temperature is! what do people advise for a good gauge and is capillary better than electric and is the original measuring point the best place to take the reading from ? Peter

I run a vdo capillary at the original location.