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conventional thermostat failure modes

conventional thermostat failure modes

conventional thermostat failure modes

(OP)
I've had (wax pellet?) thermostats stick partially open, and one whose metal frame broke, leaving it open.

Is there a "stuck closed" failure mode to worry about, too?

Dan T

RE: conventional thermostat failure modes

Tmoose,

The stuck closed failure for the wax pellet actuator would be due to diaphragm rupture and loss of the wax fill.  Not a common failure mode, but can happen, most especially if the diaphragm gets overheated (but it would take temperatures over 250 to 300 F for that to happen), or lots of cycles at middling-high temperatures.

RE: conventional thermostat failure modes

Interesting.  I've had two failures in something over a half century of driving.  Oddly (perhaps?), both were "stuck closed"!

Rod

RE: conventional thermostat failure modes

I've also had two stuck closed.  

I've discovered a few stuck open or broken open, mostly during teardowns for other reasons.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: conventional thermostat failure modes

Not seeing much "modern" about that...looks just like all the others I've seen over the years.

Regarding stuck closed failure mode, and diaphragm ruptures...it's a tricky problem.  The best seal for hot water is EPDM.  But EPDM is attacked by paraffins (e.g. hot molten wax).  So the diaphragm is usually a nitrile rubber, sometimes a Viton or similar.  It's basically then a race as to whether the diaphragm rots away due to hydrolysis attack from hot water, or gets dissolved/softened by hot molten wax.

There are some more "modern" designs with a viton diaphragm, and a seperate rod seal or wiper of EPDM.  

Oh, another failure mode is a stuck-open one, and happens when the piston is allowed to stroke a little too far, and the diaphragm inverts.  Reverting the diaphragm is sometimes possible, but takes a lot more force than the return spring usually has in it.  This mode is usually an infant mortality failure (one cycle), unless something else breaks or deforms first.

RE: conventional thermostat failure modes

The only time this ever happened to me, the thermostat stuck closed. A coolant hose blew off on the motorway. The AA guy removed the thermostat, refilled the coolant and off I went minus the thermostat and completed my journey.

Was an 80's car.

Regards, Ian

RE: conventional thermostat failure modes

I have seen both, but fail open or early open/not fully close are by far the most common.

As Pat remarked, fail closed does tend to get immediate attention!

Regards

Nick

RE: conventional thermostat failure modes

"stuck closed is much more likely to get the operators attention rather quickly,"

Ya got that right!

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