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....

Good idea to start this thread TC. Here's a stat stripped down for anyone who hasn't seen one to ponder on. This one was 11 years old, had done 300k miles and was still operating well, opening at 88C.

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.

Yes it's a pretty standard description with one or two over-generalisations I think. For example, Wiki says that wax stats have a bleed through to allow hot coolant to soak around the capsule and operate the valve. It seems that the diesel stat is 'water tight' when closed (ref. Mike_Noc's tests). In many cases, a ball valve/giggle (jiggle) pin is used simply for air bleeding. I suspect that even a sealed stat will open at the correct temperature since the inlet hose always has a large bore where there will be sufficient turbulence from thermal gradients/vibration/pump pulsation to ensure the hot coolant does the job at the right time. That's why the in-line mod works OK. (Sorry, shouldn't have mentioned that).

TC

(see image above)

What does the second valve seat on? Is it the chamber within the pump inlet? I'm trying to do some photo manipulation from your previous pictures and the Xpart images of the stat housing and water pump. Putting them together may help orientate our minds onto what goes on inside.

TC

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

Right, now I see where you are coming from with the suggestion that it is two valves in one. I agree it does appear to be two valves on the one shaft. One a watertight seal (unusual) and one a fairly leaky seal, more likely intended to just restrict flow rather than provide a seal.

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

How much force is required to compress the small spring say half way? Is this likely to be similar to the pressure drop across the water pump? The spring looks relatively hefty, but I've got little feel for the pumping pressure. I've not seen the spring in a PRT for example, but I do know the 1.8 PRT starts opening when the engine's only at 2,000rpm. I guess the pump will be running at 2-3 times that rate looking at the drive diameters. Running the engine up to 3 -3.5K rpm or even 6K rpm seems untennable for a water pump. (???)

TC