Boiling

[Stickyfinger]

I thought Thermostats failed open ?

[lexi]

I think that was usually the case. Modern ones can stick either way in some cases.

[Zelandeth]

They can fail either way - or just jam at an arbitary point.

While it used to be something that you used to hear of now and then they seem to have become increasingly touchy in recent years. One stuck open, one about halfway - which meant the car was fine until you hit the open road when it would suddenly start to get too warm if you tried to go anything over about 50mph.

[Peter.N.]

I thought Thermostats failed open ?

The metal bellows type did but waxstats can fail either way.

Peter

[Old-Guy]

It depends on the design. Over tens of thousands of miles, the wax slowly leaks past the seal until there's not enough left to open the thermostat far enough - that's why the problem rears its ugly head when the cooling system is made to work harder for longer than usual.

As for failing more often, they are probably as (un)reliable as ever. When I was young, a car with 100,000 on the clock was a banger - all that we could afford in the 1970s. Thermostats did used to fail but not that often, the engines often failed first or at the same time! Times have changed, for at least the last 30 years, 25,000 a year for a 'rep-mobile' is not even high mileage.

Between the mid '80s and roughly the new millenium, I used to do 20-30,000 a year. In the beginning I changed my car every 3 years or 100,000, that became 4 years, 125,000, then I became self-employed and it became 5 years, 150,000. The my last bought-new Rover started to die at 170,000 (salt-rotted rad + stuck thermostat + bank holiday Friday evening on the M5 = boiling + two hours on the hard-shoulder waiting for the traffic to clear). During that time service intervals doubled from 6,000 to 12,000 (for the same car!)

[xantia_v6]

I thought Thermostats failed open ?

The metal bellows type did but waxstats can fail either way.

Peter

Every thermostat I have seen has a plastic link holding the valve to the capsule, if the temperature gets over about 130 C the plastic link melts and the valve springs open.

[Stickyfinger]

I thought the wax closed the stat against the spring so that if the piston failed the spring opened it fully ?, like Air brakes, apply air to open the brakes, remove the air pressure they close...a safety feature ?

[xantia_v6]

WAx expands whn hot, so the capsule will always be pushing it open against a spring, but as I said, they always (in my experience) have a safety mechanism that forces them open if a critical temperature is exceeded.

I had one car where the cooling fan thermostat failed, and the temperature got into the red zone on the guage before I noticed (but with no loss of coolant). Subsequently the engine was much slower to warm up, and on inspection the plastic link had deformed so that the main spring was no longer holding the thermostat shut when cold, but had not let-go completely.

[Peter.N.]

The copper bellows type go back to the '50/60s they always failed open because they literally fell apart but I have had one or two relatively modern thermostats fail closed and boil the water although I must admit most have failed open.

Peter

[Bandit12]

I would look at the thermostat too mate. about 5 years ago Mine failed on my previous car (a mondeo) causing exactly the same symptoms and issues on motorway.

Got relayed home as was in Lincolnshire at the time, thermostat had stuck closed, no idea why but only cost me £7 for a new replacement from local motor factors and 15 minutes with a 10mm spanner to replace.

Good luck with it and hope its a simple fix