The Function and Malfunction of the Diesel Thermostat
In spite of lengthy discussions and interjections, the true functionality of the diesel thermostat remains to be established. It seems almost certain that it is different from what we know about 'ordinary' wax stats. It has two valves for a start. Well, it has a standard flow sealing valve/disc and a second, co-operating disc, moving on the same axis. The function of this second valve has yet to be identified. Since the stat is placed in the coolant flow to the pump, the usual understanding isn't valid.
The ultimate reason for this thread is to identify why so many diesel stats appear to malfunction. They seem to open early or begin opening at too low a temperature. This causes the already cool-running engine to run too cool, even tepid in many instances. That there is a change in the stat's workings seems to be undoubted, since replacing it with a new one restores the running temperature it originally had. Please note, this thread is not to discuss the in-line modifications. This thread does need your thinking head on. TC |
The "thinking head" of Wikapedia seems worth reading.
Under the heading Radiator engine cooling sub section Waterflow control. |
All I can say is that my 2005 tourer (german spec) has done over 100.000 miles on the same stat. The engine warms up very quickly and I have really good heat into the car also quickly. Have the inline mod ready if needed but the car doest need anything doing to it....
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http://i1108.photobucket.com/albums/...12990Small.jpg You can see that the main valve spring pushes against the bracket when the valve opens, and the smaller spring will allow the smaller valve to seat against the seat as it closes, taking up any overtravel. The 2 valve discs move together until the smaller valve closes completely. I stripped it to heat the spring up and then re-assemble and test to see if an overheated spring, as mentioned in a previous thread, will make any difference to its opening temperature. Not that I think it will mind. :) Will report the results when completed. Mike Edit: As spotted by blacknwhite the bracket is shown here the opposite way round to how it sits in the thermostat. |
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Good question but unfortunately I threw my old water pump out so can only agree with the earlier suggestion that it is part of a pressure relief system.
That would make sense with the difference in diameters of the radiator cooling and heater matrix circuit pipework, but it needs confirming. It also allows the thermostat wax bulb to be in the direct flow of the closed circuit of the heater matrix so there is no need for any holes in the thermostat main valve. Mike |
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I still do not see what flow it might need to restrict though. |
Probably due to the difference in the pipe diameters of the two circuits.
The second valve will bleed off excess pressure when the coolant is only flowing through the smaller heater matrix pipes, and allow full pump flow when coolant is flowing through the larger radiator pipes. Mike |
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