The Function and Malfunction of the Diesel Thermostat

[Jules]

That is a very typical degradation of temp against time.
I've seen 100's of them following this pattern.

And just to remind new members just because the gauge is reading mid scale, may only mean your engine is running in the low 70's !

[NigelOBB]

Originally Posted by Jules

And just to remind new members just because the gauge is reading mid scale, may only mean your engine is running in the low 70's !

I agree Jules at 70c the top of the needle on my temp gauge is just starting to touch the bottom of the half way marker..

[HarryM1BYT]

Originally Posted by Jules

That is a very typical degradation of temp against time.
I've seen 100's of them following this pattern.

And just to remind new members just because the gauge is reading mid scale, may only mean your engine is running in the low 70's !

..and if you want to know the actual running temperature, you have to use the instrument diagnostics which does provide accurate numbers.

[Trevor1975]

Originally Posted by NigelOBB

I agree Jules at 70c the top of the needle on my temp gauge is just starting to touch the bottom of the half way marker..

Mine is reading 76 degrees at 9 o'clock. Need to find another 14 degrees!

Link to diagnostics instructions:


http://forums.mg-rover.org/showthread.php?t=346274

[Jules]

Originally Posted by NigelOBB

I agree Jules at 70c the top of the needle on my temp gauge is just starting to touch the bottom of the half way marker..

Think you mean quarter way marker Nigel

[NigelOBB]

Originally Posted by Jules

Think you mean quarter way marker Nigel

You keep me right Jules 😜

[Union Wheels]

Originally Posted by jim lee

Further to my post above. I have spent an enjoyable wet afternoon in the garage grubbing around with my old thermostat. First I again heated ii in a pan of water and measured its lift. The results are:

Temp Lift

Cold Zero
72Deg C Just begins to open
86Deg C 1.5mm
90Deg C 2mm
94Deg C 3.5mm
96Deg C 6.5mm
98Deg C 8.5mm
99Deg C 9.5mm

Thus all appears well apart from the movement before 86 Deg C. If that could be eliminated, it would appear it would then open at about the right temperature.

Withis in mind, and following my thoughts in the post above, I filed 1.5mm from the end of the central rod. The unit was then reassembled and the heating test repeated. The results are:

Temp Lift

Cold Closed
86Deg C 0.5mm (I could not detect opening before this temp, but clearly it did so)
90DegC 1.5mm
94Deg C 2.5mm
96Deg C 6.5mm
98Deg C 7mm
I could not manage to get 99Deg C this time as the gas cannister on my heater began to run out.

I think these results show that the opening temperature can be adjusted by altering the effecgtive length of the central pushrod. One obvious way this could hppen in service is due to the compression or degradation of the sealing ring on the main valve. As only small dimensional alterations are required to bring about a change of opening temperature, this may also be affectd by plastic creep of the housing, especialy around the valve seating area.

I offer these thoughts and results to any who may be interested.

I have just finished reading this long thread and I am wondering if the hard solids found within the wax by T-CUT are causing the push rod to start from a position further out of the wax capsule.

This would reduce the gap between the push rod and the plastic housing and so cause the thermostat to open at a lower temperature as Jim Lee has shown.

I note the measurement of 28mm of rod coming out of the wax capsule and wondered if this was constant in all cases including new thermostats

Cheers, Pete

[T-Cut]

Originally Posted by Union Wheels

I am wondering if the hard solids found within the wax by T-CUT are causing the push rod to start from a position further out of the wax capsule.
This would reduce the gap between the push rod and the plastic housing and so cause the thermostat to open at a lower temperature as Jim Lee has shown.

The total volume of the wax plus 'solids' doesn't change over time, so the 'gap' you imagine remains constant. It's shape may alter as seen in the photos, but that in itself won't affect the result. Some areas may be 'tighter' while others will get 'slacker'. Net effect nil.

I note the measurement of 28mm of rod coming out of the wax capsule and wondered if this was constant in all cases including new thermostats

I don't see any significance in the 28mm measurement. That was simply the rebound measurement after allowing air behind the piston. It showed that the seal of the envelope is pretty good, but it's not perfect. When the piston is held compressed for a while. the protrusion will be less because the air will have escaped.

While it's not been proven in these experiments. it's well recognised that the influx of even a trace of moisture into the plastic envelope will reduce the opening temperature. There's a reference to a patent on this in the write up. Also the photographic evidence suggests something volatile gets into the wax over time and creates voids between the wax and the envelope. Maybe the wax itself degrades over time to create the solid stuff and a gaseous byproduct. That will reduce the opening point. Similarly, if the volatile material was water, you have the answer to it all.

TC

[Trevor1975]

For info, My car was getting short infrequent journeys by a previous owner. The Thermostat was constantly keeping the engine at 72-74 degrees. now after a few months with some decent runs it has improved to 80-82. this is not to do with the weather etc... it just seems that through use it has recovered?

[T-Cut]

Maybe it never really got warmed up before? I doubt a failing stat will make any sort of recovery, but circumstances maybe suggest it did.

TC