The Function and Malfunction of the Diesel Thermostat

[blacknwhite]

I'm sorry T-Cut, I just think everyone it looking for a complicated answer.

On the bedding/seating point (should the shank bend or buckle), once the stat has been removed, you'd never know. You'd only be looking at half the problem which wouldn't be at all obvious.

[blacknwhite]

T-cut I simply don't know. I'm only throwing up ideas.

[HarryM1BYT]

Originally Posted by T-Cut

The pump is located as shown in the Rimmer drawing.



The port I mentioned is clearly visible at upper left of the mounting face.

Looking at the way the pump is orientated, suggests to me that the secondary/inner valve stops flow coming into the stat housing from the port. Does this port connect internally with the head? I assume the output from the pump is from the upper right of the casting, especially from the shape of the gasket, but I can't figure out where it goes into the engine.

TC

Looking at the above diagram - the second 'valve' doesn't seem to restrict or close anything, the disk simply moves into an open pocket in the pump.

[Mike Noc]

Its a bit strange that we are always hearing of failed diesel thermostats, but so far no-one who has a failed stat has joined in.

There may be one or more reasons for them to fail, so we need to look at failed examples.

Do the CDT stats fail any more often than other makes, or is it just that we hear about them because they take so long to change?

As I've mentioned before the last stat I changed was on an Astra, and was just as difficult, as the crank pulley and cam gear wheels, engine mount, cambelt and cam back cover all had to come off to get to it. Nice one Vauxhall!

Mike

[HarryM1BYT]

Originally Posted by Mike Noc

Its a bit strange that we are always hearing of failed diesel thermostats, but so far no-one who has a failed stat has joined in.

There may be one or more reasons for them to fail, so we need to look at failed examples.

Mine had drifted in opening temperature at 96K miles - holding the temperature 8 to 10 degrees low, so I had it swapped out (OEM). I had intended to ask for the old stat back, to investigate myself, but forgot.

So all I can definately say, is that mine 'failed'.

[T-Cut]

Originally Posted by HarryM1BYT

Looking at the above diagram - the second 'valve' doesn't seem to restrict or close anything, the disk simply moves into an open pocket in the pump.

If you study the photos in Post 30, there is a valve seat machined into the inside of the pump case where the stat sits. I think the secondary valve seals the flow when the primary is fully open. That's why it it's floating on the small spring. There are pressure marks on the disc in Mike_Nocs photo.

TC

[T-Cut]

Originally Posted by Mike Noc

Its a bit strange that we are always hearing of failed diesel thermostats, but so far no-one who has a failed stat has joined in.

Yes, where's Jules when you want him. He said he'd replaced hundreds of these and had not found a single one failed 'open'.

There may be one or more reasons for them to fail, so we need to look at failed examples.

Agreed. I suspect it's nothing to do with the wax propulsion system but a simple mechanical change somewhere in the works. You mentioned that the rubber sealing collar around the primary valve was bubbling off I think (?). Maybe it's just something as simple as that. The sealing rubber deteriorates so the valve doesn't seal properly. This will allow cold coolant to bleed into the housing and as we know from that other thread, a very small bleed has a significant effect on running temperature. Maybe the plastic seat for the primary warps over time due to the loadings? Your photos does show uneven water/calcium staining around the valve and seat.
(See Post 30)

TC

[T-Cut]

Here's another suggestion. It seems to me that the flow in and out of the pump with the thermostat 'shut' has to be as proposed below. We know that the flow from the heater is unrestricted at all times and moves as indicated. The pump exit route has yet to be figured out.



As the stat opens, the secondary valve closes the flow from the engine port and at the same time allows flow from the radiator to mix with the flow from the heater. All I can guess from this is that the port is connected internally to the head. When that stops, the engine circulates to the radiator via the exit on the front of the block.

TC

[HarryM1BYT]

Originally Posted by T-Cut

If you study the photos in Post 30, there is a valve seat machined into the inside of the pump case where the stat sits. I think the secondary valve seals the flow when the primary is fully open. That's why it it's floating on the small spring. There are pressure marks on the disc in Mike_Nocs photo.

TC

Mmmm, see what you mean - I certainly agree now, but I'm puzzled...

If it shuts the flow off to the engine, sure that will cause the engine to rapidly over heat?

[Matt1960]

Originally Posted by Mike Noc

Its a bit strange that we are always hearing of failed diesel thermostats, but so far no-one who has a failed stat has joined in.

There may be one or more reasons for them to fail, so we need to look at failed examples.

Do the CDT stats fail any more often than other makes, or is it just that we hear about them because they take so long to change?

As I've mentioned before the last stat I changed was on an Astra, and was just as difficult, as the crank and cam gear wheels, engine mount, cambelt and cam back cover all had to come off to get to it. Nice one Vauxhall!

Mike

I have changed two of these on different cars. One with a mileage of 113,000 and the other 155,000. I doubt if either had been changed before, and both were opening in the low 70s.
On inspection, they both looked like new.

My opinion is that they are no less reliable that many other makes but that the job to replace is so awkward that they get left, and because the fbh ensures that the temp gauge gets to halfway when its cold.
I guess its those people with faulty or no fbhs that notice the faulty thermostat first.

Matt