Diesel thermostat definative answer

[T-Cut]

The paraffin waxes used in stats is a 'narrow cut' fraction with a very narrow melting range. Different cuts have different ranges. These waxes expand dramatically when they melt, more so than water does for example. So, the solid pellet of wax is trapped behind a sealed piston. This is pushed out against the return spring pressure when the wax melts and the diaphragm valve opens. Nothing can stop that force, it's tremendous. When the wax cools, it contracts and resolidifies below the set point of the stat, so closing the valve again. The return spring ensures a nice even closure.

So, let's consider what might happen if the stat breaks down.

A. Loss of wax past the seals.
If that happened, there would be insufficient volumetric expansion to open the valve fully. Engine would overheat.

B. Spring tension is lost in some way.
The valve will open whatever the spring tension, but there will be less efficient closure. The wax may solidify without the piston fully retracting. The stat fails open. This is not observed according to Jules experiences.

C. Water gets into the wax.
Not really sure how this could happen, but wax would have to be displaced from the capsule to let water in. Water/coolant and wax have no chemical inter-reaction. Water doesn't expand much when heated, so the valve would only open partially when the residual wax melts. Result, overheated engine.

D. Can't think of it.

So, there seem to be no obvious mechanism by which the stat will open fully below the set temperature due to wax/water factor. Other possibilities are that the outer seal tends to fail allowing coolant to bypass the stat. Seems very slim possibility, but a mechanism like that would explain the observations.

TC

[Matt1960]

TC,
Thanks for the explanations.
So, if there were bypass of the outer seal, then when testing a failed stat in hot water, it would open exactly the same as a new one, wouldnt it?
I just wish I had some to test.
Matt

[T-Cut]

Originally Posted by Matt1960

So, if there were bypass of the outer seal, then when testing a failed stat in hot water, it would open exactly the same as a new one, wouldnt it?

I would expect so. Maybe Jules will send you some to test?

TC

[Frank Incensed]

Originally Posted by T-Cut

So, there seem to be no obvious mechanism by which the stat will open fully below the set temperature due to wax/water factor. Other possibilities are that the outer seal tends to fail allowing coolant to bypass the stat. Seems very slim possibility, but a mechanism like that would explain the observations.
TC

Seems like half the problems with Rover engines are the result of the failure of silicone rubber seals.

I've had a response from thermostat manufacturers Behr, today. They seem to be saying that they don't supply the thermostat on its own without the housing and they don't make one with an alternative operating temperature. They've referred me to their UK distributor, Hans Motors in Birmingham. I'll send Hans Motors a copy of my original e-mail as, judging by the reply from Behr, something may have got lost in translation. I used to know someone who translated technical instructions and information from English to German and vice versa. Wish I was still in touch with him.

[T-Cut]

Most Germans speak English a lot better than a many of the English.

TC

[Frank Incensed]

Originally Posted by T-Cut

Most Germans speak English a lot better than a many of the English.

TC

Can't deny that TC. More probably an unwillingness to help than a lack of understanding. I hope it's a different story with Wahler, given their boast that they relish a challenge.

[DerekS]

T-cut.. thats a good statement of cause and effect, and I would agree with you on your explanations ..... BUT my experience is different .

I have had several cars with 'waxstats' (mini/1100/maxi) and found that whenever the temp rose above normal for a little while ( I tow a small caravan), then the thermostat was bejiggered, and had to be replaced (easy jobs on those cars). It ALWAYS stuck open . My experience anyway. Are modern stats made different ?

[T-Cut]

Originally Posted by DerekS

I have had several cars with 'waxstats' (mini/1100/maxi) and found that whenever the temp rose above normal for a little while ( I tow a small caravan), then the thermostat was bejiggered, and had to be replaced (easy jobs on those cars). It ALWAYS stuck open. My experience anyway. Are modern stats made different ?

Logic tells you if the engine doesn't warm up to the stat's setting, then it must be stuck open. That's what I always assumed. However, member Jules' replacement of dozens of diesel thermostats shows otherwise. None have been stuck open.

It seems the true cause of this mystery has still to be found.

TC

[James.uk]

Could it be that the jammed thermo closes again when the system is drained, or when the housings fixings are being removed? i.e. stresses being relieved?
...

[rossocorsa]

Originally Posted by DerekS

T-cut.. thats a good statement of cause and effect, and I would agree with you on your explanations ..... BUT my experience is different .

I have had several cars with 'waxstats' (mini/1100/maxi) and found that whenever the temp rose above normal for a little while ( I tow a small caravan), then the thermostat was bejiggered, and had to be replaced (easy jobs on those cars). It ALWAYS stuck open . My experience anyway. Are modern stats made different ?

possibly a fail safe built in but set a bit too low? wouldn't explain a stat like mine though that works but will not hold a temp above 83C can't decide whether to bother changing it or not but mpg is a bit lame only hit 40mpg on a long run with a back wind!