Time to change your thermostat?

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Old-Guy
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Time to change your thermostat?

Post by Old-Guy »

In recent years, I've had two thermostat 'failures' on older cars.

Over time (perhaps more a matter of the number of close-open-close cycles that the thermostat goes through), the wax inside the thermostat, the melting of which causes the thermostat to open, very slowly leaks out so that progressively it opens less and less, eventually to the point where one day the engine overheats.

If you're lucky, like me, it will happen during a hard motorway run, the temperature rising towards the red will be noticed - on a Xantia you can't miss the big red STOP light - and lifting off the loud pedal and pussy-footing along will allow the engine to cool. I suspect that stopping is the worst thing to do as switching off the engine means no pump to force coolant through the partly open thermostat and the residual heat in the over-hot engine will cause it to boil and the head gasket to fail - either then or not long after.

If it gets to the stage where the engine overheats in traffic, I reckon you can expect a well-cooked engine!

Thermostats need changing every 6 years/100,000 miles as a matter of preventative maintenance - a sight cheaper and usually easier than a cam belt change. But failure could well be just as expensive.
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Post by CitroJim »

Good advice that :D

Also, there is a couple of little traps for the unwary.. Some Xantias use a 92 degree 'stat, amongst them Activas. Everyone will try to see you an 88 degree 'stat as a fit-all replacement. Do check and if your engine specifies a 92 one, make sure you fit the same.

Also, other puryeyors don't know the difference between an ordinary 'stat and a bypass 'stat (the one with the appendage on it) If you use an ordinary 'stat where you should use a bypass type, the bypass channel does not get closed off when the engine reaches temperature and this can lead to serious overheating as not all of the coolant flows through the radioator; a large proportion continues to flow around the bypass circuit.
Old Guy wrote:usually easier than a cam belt change.
Unless it's a 2.1TD :evil: On these the 'stat is buried in the water distribution block low down at the back of the block, effectively behind the waterpump. Fun with a capital F to swap one :lol:
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Post by jgra1 »

hmm,

when i did my rebuild in August, I had two thermostats and the second one appeared to open ok in hot water... but since the rebuild the car has been running a fair bit hotter in all conditions....

Will change it for a new one, but interesting, I didn't realise they could get worse over time due to wax leakage, so thanks :twisted:

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Post by davek-uk »

For the sake of a few pounds, I always change the thermostate whenever the cooling system is emptied (coolant change or timing belt).

When my car warms up, there is a distinct build up of temperature (above normal running) and when the stat opens a quick drop. I always look for that to see if it's working properly.

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Post by addo »

An interesting and timely thread; made me wonder if heat stresses had hastened failure of my heater hose fittings. Bought a new Valeo thermostat (non bypass) and seal ring today, $44 (£19.46) - with summer coming down here, it can't hurt.

I'll compare the old and new in hot water, with the infra-red thermometer.

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Post by pete_wood_uk »

Interesting. For what it's worth, both the TD ones that I've replaced in the last few years (and a CX one for that matter) failed the other way - they failed to close properly when cold, so the damn engine takes forever to warm up. Wonder why my experience is different to yours? I don't remember using much of Cambridgeshire's (notoriously hard) tap-water in my coolant... :)

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Post by jgra1 »

I think mine may be failing to close or open properly :twisted:


Will get a new 88C one and fit over weekend.
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Post by f00lzz »

pete_wood_uk wrote:Interesting. For what it's worth, both the TD ones that I've replaced in the last few years (and a CX one for that matter) failed the other way - they failed to close properly when cold, so the damn engine takes forever to warm up. Wonder why my experience is different to yours? I don't remember using much of Cambridgeshire's (notoriously hard) tap-water in my coolant... :)

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I believe that waxstats are 'designed' so that if they do fail it is in the open position.. How this is achieved I don't know and judging from the above posts the design intention is somewhat flawed!!
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Post by addo »

I compared two thermostats to suit my 2 litre 16V. Measurements were taken with a thermocouple probe and multimeter, occasionally checked with an infra-red pyrometer. The two seemed to concur.

That which came out, is a "Calorstat" stamped 83°. It commences opening at 82°C and is fully open by 96°C. Closing starts at 88°C and is complete by 74°C. Construction appears to be a soft stainless alloy.

Replacement I received over the counter is a Valeo P/N 820471. It commences opening at 92°C and is fully open at 98°C. Closing begins at 94°C and is all over by 84°C. Metal is comparably thinner.

Aside from the opening temp differences, what can be noticed is the big variation in how long it takes to go fully open. I suppose the hotter one needs to open faster, while the cooler unit must have more flow control. It was noted that leakage had been occurring past the rubber seal on the old unit.

If there are any ramifications from running the hotter 'stat in an Aussie summer - I'll post up!

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Failed closed

Post by Old-Guy »

Both the wax thermostats that I've changed in recent years (Montego D in 2004/5 and Xantia TD May 2006) failed almost closed. In both cases, testing old and new together in an old saucepan (time for a teas break) both started opening around the same temperature, but the old ones didn't open more than a crack - hence the overheating.

Looking closely at the failed 'stats, there was evidence of wax around the seal where the operating rod goes into the capsule. It seems logical that a lack of wax = lack of movement.

£20 or so for a new 'stat every few years is a awful lot cheaper than a cooked engine.

BTW, the water here in the Cotswold is as hard as rock so I always use (soft) rain water from the garden water butt for making up coolant.
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Post by myglaren »

I usually boil the water to drive out oxygen and chlorine and solidify any calcium salts.

Can't remember when I last changed a thermostat though, probably on one of the dire Hillman Imps that I used to have and hate.
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Post by CitroJim »

I once had a source of demin water and before it came to an end, I filled a huge carboy with the stuff. I stil have about 10 gallons left.

As a cautionary tale of hard water, my old 205GTi (still miss that car :( ) was run on tap water for a long time and this comprehensively killed the original engine, partly through corrosion but mainly through a freeze and a cracked block :( There were corrosion pits in the head that wrote it off.

The repercussions of using pure tap water (we here in North Bucks have tungsten hard water) continued long after I replaced the engine and wrecked TWO heater matrixes in short order due to small particles of rock from the expansion tank running around the cooling system and acting as fine abrasive. This abrasive action wore holes in the thin aluminium tubes of the matrix on the end "U" bends. Looking back, I should have replaced the expansion tank.

It was a good job that 205 matrixes were a doddle to change.

For when the demin stocks run out, I have a big water butt that catches water from the garage roof but it will need filtering before use, again to remove any grains of abrasive material leeched from the concrete roof tiles.
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