Thermostat - How important? - deltaseven
I have replaced the thermostat in my 1989 Rover Metro (1989) several times over the last year, as they keep jamming closed, causing the engine to overheat.

I have flushed the cooling system and replaced all the coolant with new, thinking that a previous owner could have used Radweld or similar to cure a leak.

What side effects am I likely to have if I just remove the thermostat, and run the car permanently without it. I guess the car will just run cold for longer, and never get up to it's 'optimum efficiency temperature', therefore, being slightly less efficient. Am I right, or horribly wrong?

Thermostat - How important? - teabelly
Not only will it run cold for longer you will also find when stationery the car will start to over heat more. The thermostat controls the pressure in the cooling system to some degree and it seems to be more efficient at cooling when the pressure is increased. I didn't have a thermostat in my Vitesse for a while and it tooks ages to warm up, would heat up really quickly while stationery (and over heated several times) and would also cool rapidly when running at over 50 or so. I put a thermostat back in and the overheating problem went away and the car was generally a lot happier. You will also find that petrol consumption is increased because it will be running richer for longer to compensate for being colder. If you have a manual choke you may be able to limit this effect.

If they keep jamming closed then see if you can buy a different make as the one you have sounds to be of dubious quality. If they are brand new thermostats each time they should have some sort of warranty with them. Did you fill up with a pre-mixed anti-freeze solution or mix your own?
Thermostat - How important? - deltaseven
Thanks Teabelly

I hadn't realised that this could CAUSE further overheating.
The Thermostats I have been buing have been from the big Auto accessory chain, where all the boy-racers go (myself included - even in a Metro).
Anitfreeze mix was the same-brand, and I added water too it as per the ratio recommended.
Thermostat - How important? - borasport20

The thermostats you took out, were they stuck open or shut ?
Did you test them (Put thermostat in pan of water and bring to the boil - you should be able to see it open)


I have to grow old - but I don't have to grow up
Thermostat - How important? - lauriew
The thermostats on modern engines are designed to "Fail-safe", ie in the open position.
To have 3 or 4 fail in the closed position is, to say the least, unusual.
If you run without, the engine will take a long time to warm up and will never reach the optimum operating temperature.
This will result in excessive fuel consumption, poor performance and eventually cylinder bore wear.The engine oil will become rapidly contaminated with moisture/condensation and you find "scum" inside the cambox/rockercover.
I would suggest that the problem could be due to some peculiarity with the coolant mixture or incorrect fitting (such as using the wrong thermostat housing gasket preventing the thermostat opening properly).
Thermostat - How important? - Dizzy {P}

Are you certain that modern thermostats fail 'safe'? I asked this question in this forum before but didn't get an answer. I was very familiar with thermostats about ten years ago and they definitely failed 'closed' in those days. These were as fitted to heavy-duty diesel engines but I feel sure were the same as fitted to motor car engines.

Regarding leaving out the thermostat, this can cause a lot of problems. Many (most/all?) modern thermostats control not only the main coolant passage but also the radiator by-pass passage. When one is open the other is closed. That is, when the thermostat opens the main passage, it blanks off the by-pass so that the coolant is forced to flow through the radiator. Without a thermostat in place, not only will the coolant take a long time to warm up but the temperature will then possibly continue to rise to the point of overheating because some of the coolant will be by-passing the radiator.
Thermostat - How important? - Keith S
Buy a main dealer unit and try that perhaps?
Thermostat - How important? - lauriew
During my younger days, I worked in the Engine Research dept of a well known international company. This involved the dynamometer testing of prototype automotive, industrial and marine engines. Most of these engines ran 24/7 at full throttle.
The temperatures (both oil cooler and water), was controlled using standard wax-type thermostats.These were very reliable.As I recall there were only two failures, and these "failed safe". According to the thermostat maker, this was a design feature. The test-beds were set up in conjuction with the engine maker regarding operating temperatures etc.
Thermostat - How important? - Andrew-T
T-b - I'm not sure how a thermostat can do anything to limit pressure. The cooling system is a circuit, so all it can do is prevent circulation, it can't put the lid on anything (though the pump will try to help).

However I agree that if pressure were increased, that would prevent the coolant boiling and thus improve heat transfer.
Thermostat - How important? - Flat in Fifth
I'm probably wrong here but I seem to recall that the thermostat in the Metro causes a certain degree of back pressure. Without this the flow of coolant to one cylinder is insufficient. I could be wrong, been known before, but this is one of those odd bits somehow stuck away in the old grey matter.

also unless I have misuderstood how a wax thermostat works, how can it always fail in one position ie open/closed. Surely (Shirley) it fails in whatever position it happens to be in? or?
Thermostat - How important? - Dizzy {P}

The thermostat has a spring holding the valve closed at low temperatures and a capsule containing wax which expands under heat to controllably overcome the force of the spring and open the valve.

The wax capsules are very reliable but when they do fail, usually by leakage of the wax, the spring simply holds the valve closed.
Thermostats - All really failed? - M.M

Important question...

How do you know all these thermostats have failed closed? Have you tested them all in a pan of water and proved they don't open towards/at boiling point?

Thermostats - All really failed? - Cliff Pope
In my years of experience of thermostats in a number of different cars, I have come to realise that there is a lot more to them than simply on/of at the correct temperature. Other contributors have pointed out some of these factors, including the paradoxical one that omitting the thermostat can actually increase overheating.
I have also noticed that the internal designs vary, so that it can be important to get the right one, not just one that fits and has the correct temperature stamped on it.
For example, the degree of constriction varies, so even when fully open as intended, some models apply more back pressure or may be designed for a higher output waterpump.
I know from experience that removing the thermostat is not a cure for failing to rectify some other cause of overheating.
Thermostats - All really failed? - lauriew
It was the earlier "Bellows" type thermostats that was prone to failure in the closed position. These are rarely used today as an equivalent wax type(more accurate and cheaper) will be available.
Regarding the original posting: I would suggest that the radiator pressure cap be changed after ensuring that you have the correct one fitted in the first place. Most of these caps are interchangeable and look the same, but, they will operate at different pressures and it is quite easy to use the wrong one,(particularly if the previous owner has fit it and you swap "like for like").
Remember that the cooling system has been designed to function correctly only if the engine manufacturers intended specs. are maintained for the geographical area where the vehicle is supposed to operate.
Thermostats - All really failed? - Shigg

You could be right about it helping to cool certain cylinders. Indeed it's a good idea with high power A series engines to 'sleeve' the stat housing to 'control' coolant flow. I would suggest looking toward fan side if the cooling system, is it a electric fan your model?

Thermostats - All really failed? - Dizzy {P}
It was the earlier "Bellows" type thermostats that was prone to failure in the closed position. These are rarely used today as an equivalent wax type(more accurate and cheaper) will be available.


Laurie, I'm sorry but I disagree about the failure mode of bellows-type thermostats. I was responsible for re-specifying thermostats during the changeover from 'bellows' to 'waxstat' in the diesel engine manufacturing firm where I worked until a couple of years ago and I will pass on some of what I learned ...

The brass bellows in the thermostat of that type was closed when cold and thus held the valve in the closed position. The bellows expanded under heat and this was helped by a volatile liquid with which it was partially filled. As the bellows expanded, it lifted the valve to the 'open' position. Eventually the bellows would leak/fatigue and fail to return to the closed position, thus it almost always failed OPEN.

The bellows type are NEVER used today, I think they went out in the early to mid 1970s. The main problem with them was the change in their opening rate under different system pressures, and their relatively short life. The replacement wax types overcame those two fundamental problems but they almost always fail 'closed'.

To build on Middleman's point about bleed-holes and jiggle-pins: Thermostats often have a small air-bleed hole which is usually, but not always, fitted with a jiggle-pin. The jiggle-pin is a loose fit and it jiggles (lovely word!) to allow air escape as the system is filled. When the engine is running the jiggle-pin seals off the bleed-hole under the force of the coolant and therefore prevents coolant bypassing the closed thermostat valve.

Onto bypass systems: When the thermostat is closed, the coolant still needs to go around the engine to keep the valves and other parts cool. The normal way of achieving this is to have a thermostat bypass passage which can be blocked by a second valve on the thermostat. This second valve is usually another disc-shaped plate on the bottom of the thermostat but some older designs had a sleeve that rode up and down around the outside of the thermostat and could be prone to sticking. When the main valve is closed to prevent coolant going through the radiator, the second valve opens the way to the bypass passage, and vice versa.

Finally to testing: There should be a temperature stamped on the thermostat, perhaps 88C or 92C, and this is the Start-to-Open temperature. The thermostat can be tested by immersing it in water that is gradually being heated, preferably holding it away from contact with the saucepan or whatever other container is being used. It should start to open at the temperature stamped on it, or certainly within a very few degrees either way. It should then continue towards the fully open position as the water is heated towards boiling point.

The end.
Thermostats - All really failed? - lauriew
I was going to abandon this thread but others kept it going, so here is my two pennies worth.
A long time ago I was involved with a dissertation on engine cooling for a degree.
Here is a synopsis of the findings.
1) Out of 10 different antifreezes tested, all except one increased the boiling point of the coolant.The actual increase is governed by the type or mixture strength.The odd one reduced the boiling point. This one was also inflammable.
2)By introducing a thermostat, whether working or not or open or not, improves the efficiency of the water pump as it acts as a restrictor in the flow, therefore increasing operating pressure.
3)If a hole is drilled into the bellows so that the contents escape,(the actual contents depends upon the maker), to simulate a fault: the stat will open at a higher temp than previous.The temperature can be as much as 20deg higher.
The stat then remains open, with the bellows "strained" to take on a permanent set caused by the excessive high temp.
The driver of the test fleet vehicle was not aware of any defect, except commented on the heater going hot then cold.
When removed the stat was stuck open.
QUESTION: Did the stat fail open or closed?
Thermostat - How important? - Flat in Fifth
Thanks Dizzy, most of mine that have failed have been stuck partly open, but I follow your explanation.

Thermostat - How important? - John S

I believe you're right! I too seem to recall from many years ago that if an A series engine was run without a thermostat (as some racers were) it was suggested that a blank with a hole about 2cm diameter should be substituted for the thrmostat. Something about a completely open pathway here adversely affecting the coolant flow through the block, which agrees with your thoughts.

That said, I find the fact that several theermostats have failed very odd, and I wouldn't recommend a normal road car be run without one.


John S
Thermostat - How important? - deltaseven
Thanks for all your advice folks! I will visit another parts outlet this weekend and try another brand of thermostat, and see how I go!

Incedentally, my 'diagnosis' of jammed thermostat is as follows:
Temperature guage climbs to red fairly rapidly, engine block very hot, thermostat housing very hot, pipe from thermostat housing to radiator still cool, or mildly warm. I did not test any of the thermostats outside of the engine, but replacing them has always rectified the problem (for a while anyway!).

Thermostat - How important? - wemyss
Delta, One new thermostat failing would be bad luck
Two would be cause for further investigation.
Three failures.. astronomical odds.
As advised earlier fish them out of the dustbin and test them in a saucepan on the cooker.
Almost certainly they will all be OK.
Thermostat - How important? - M.M

Like others I think it is highly likely that these 'stats will turn out to be OK if/when tested.

I bet the system is airlocked under the thermostat and then the thermostat can't open because it needs to be immersed in coolant.

The problem is resolved for a short while each time you fit a new stat because you lose the air...but it steadily builds up again.

Possibilities are minor head gasket failure allowing combustion gasses into system or a hose/joint leak that allows air to be drawn back into the system as it cools...had the latter and it can be a devil to track down with no visible leak. It could even be a buildup of air left in the system when you fill it that is then collecting under the thermostat after a little useage.

A pressure test of the system should prove the fault.

Thermostat - How important? - M.M
Furthermore have your 'stats got jiggle pins? Did the oroginal one you took out have one?

A certain tractor I sometimes repair had all sorts of problems due to this air buildup under the 'stat due to being fitted with a cheap 'stat without a jiggle pin.

Thermostat - How important? - wemyss
MM. In the sixties with the Minor and Mini they used to specify the orientation of the jiggle pin. I think it used to say it should be on the engine block side for some reason.
Thermostat - How important? - Keith S
Genuine Frod stats have a little ball bearing that must do the same job as a jiggle pin. The cheap aftermarket ones from motor parts stores do not.

As the geniume ones are only about £9 it is hardly worth buying the cheap rubbish.
Thermostat - How important? - Dizzy {P}

Interesting news about genuine Frod thermostats with ball bearings! :-)

In my rather long missive posted at 21.10, I described the function of the jiggle-pin. What I didn't mention was that, in jiggling, this little item keeps the bleed-hole clean from rust and crud. If the cooling system is absolutely clean and filled with pure water I can see no problem with the ball-bearing idea but I wonder how well it can handle limescale, etc., in an old system on tap water? I am always uneasy about Ford's cost-cutting ideas, having read the book AND seen the film many times over the years!
Thermostat - How important? - Flat in Fifth
>>I am always uneasy about
Ford's cost-cutting ideas, having read the book AND seen the film
many times over the years!

Dizzy that film wouldn't be Die Hard Value Engineering and its follow up Lethal Value Engineering Weapon by any chance.

You know the ones where Bruce Willis and Mel Gibson disguised as Value Engineers go into factories and totally screw manufacturers and the engineering industry in general.

not all that seriously (it is Friday)
Thermostat - How important? - Dizzy {P}
Alvin, also some engines have horizontal thermostats and these need the bleed-hole/jiggle-pin to be at the top, for obvious reasons.

Fif, Haynes manuals talk of wax pellet thermostats failing 'open' but I have never come across this myself. You've now made me wonder if 'backstreet' makes can sometimes jam partly open due to poor design and/or manufacture. I feel sure this wouldn't happen with good quality stuff like Wahler.

Thermostat - How important? - Shigg
Sorry but I can't let this debate end without mentioning the bypass hose on old minis. Did moggys have them as well?

Thermostat - How important? - Dizzy {P}
Yes, Steve, I'm sure all the old 'A' Series had them. I remember buying a hose with convolutions in it so you could longitudinally compress it for fitting without lifting the cylinder head!
Thermostat - How important? - Shigg
Dizzy, I seem to remember back in the early eighties all car shops had loads of these things in stock, fastened blister style onto a large piece of card. Thank god for the A+!

Thermostat - How important? - wemyss
Dizzy, I had a Series11 Morris Minor 1955 803cc and of course like every one else had a Mini.
The Mini had the short solid rubber pipe which by cutting a small amount off could just be fitted with patience, bruised fingers and a bladed tool to slide under it.
However after doing this I don't think anyone would do it again and went for the convoluted pipe you describe.
This job was one of the much discussed quirks of the Mini but I don't think it applied to the Minor or it would have stuck in my mind. However you're usually right so I will have a look in the official Morris Minor works manual next time I'm up in the roof space.
I also recall making a tool in those days to assist in replacing a small plastic seat for the Moulton rubber cone in the rear suspension. It had to be compressed up to get it out. Do you remember that one?.
Jiggle pins - How important? - M.M
If you have a stat without a jiggle pin and know/think it should have one simply drill a small hole in the stat flange halfway between the edge and middle. Into this fit a small split pin so it's loose and can....err...jiggle.

If the stat sits other than dead level locate the jiggle pin to the highest part of the housing.

As a matter of interest I've just changed the stat on a Mercedes this week and that had a ball type jiggler.

Thermostat - How important? - John S

Dizzy is right. All the OHV Minors with the A series engine feature the bypass hose. When I built the engine on mine, I couldn't get the straight hose, but only the convoluted type. I wasn't pleased, especially when I found afterwards that I could have used a section of heater hose, which has the right internal diameter.


John S
Thermostat - How important? - simonsmith_523
A bit confused here? The coolant system is pressurised. The thermostat only opens the radiator circuit. No thermostat will make the engine overheat more???? Not really, it will confuse the engine sensors going to the ECU , as the engine will not reach operating temperature, the ECU will still think the engine is warm up mode.
Thermostat - How important? - Cyd
Sure you haven't been putting them in upside down? The wax capsule needs to be pointing down to be in the hot water flow.

Sure there isn't a blockage in the rad or pipes which is stopping the water flow? Try reverse flushing the rad with a garden hose.

Try flushing again using a strong flush compound - Holts do one which comes in packet form with three packets of different chemicals to be used in order.
Thermostat - How important? - Aretas
Deltaseven, are you really sure the thermostat is at fault. If the radiator cap is faulty it will allow the coolant to boil at a lower temperature than the engine is designed for. i.e. it will boil at 100C rather than around 115C.

Mike Holland
Thermostat - How important? - simonsmith_523
isn't that what antifreeze/coolant is for? to lower the boiling point of the coolant mixture. The pressure cap is designed to release pressure when the coolant gets very very hot.
Thermostat - How important? - Dizzy {P}

The purpose of antifreeze/coolant is to lower the freezing point, not the boiling point, as well as inhibit corrosion within the engine.

The pressure cap is there to *retain* pressure, not release it (except under very extreme circumstances). It is a scientific fact that the higher the pressure, the higher the boiling point, and the pressure cap allows the engine to run at over 100C which is needed for efficient engine operation.
Thermostat - How important? - Cliff Pope
This has been a fascinating thread. It has inspired me to go out to the garage and retrieve my box of 'useful' old thermostats and do some tests.
1)All the dud ones (wax type) have stuck in the closed position.
2)The opening temperatures vary considerably from those stamped on the thermostat
3) The extent to which they stay open after the temperature has fallen varies a lot. Some shut the instant the temperature drops below the stated figure, others linger open about 10 degrees.
4) The degree of throat or constriction varies too
5) The adjuster screw that some versions have does not alter the opening temperature, only the extent of the gap once it has opened.

This will all be basic stuff to anyone doing a degree in thermostatology.
Thermostat - How important? - Andrew-T
If you double the pressure over boiling water, the boiling temp. rises from 100 to about 120°C. The pressure-cap does just that. An engine normally runs at 80-90°C, so the pressure-cap extends the working range while providing a safety-valve when things get really hot, otherwise a hose would do that job!

If 25% antifreeze is being used, the boiling point may rise by about 2° because dissolved substances raise the b.p. unless they are themselves volatile. That is why respectable antifreeze contains glycol which boils about 200°C, not isopropanol, which would azeotrope with water and lower the b.p.
Thermostat - How important? - simonsmith_523
Wrong, antifreeze/coolant does raise the boiling point of water as well as stop it from freezing.
Thermostat - How important? - M.M
I'm not really sure who is "wrong" SS.

Dizzy describes the basics and Andrew says correctly that there is a small benefit from quality glycol in raising the boiling point.

I would say at normal system pressures a decent glycol would raise the boiling point by about 4 degC at 33% concentration and 8 degC at 50%.

This is a benefit but I'm not sure how much system designers rely on it. I do understand it may be a help in preventing localised boiling in engine blocks where the design runs close to the limit.

Historically with old engines it was in the handbooks to use plain water in the summer and then re-fill with the glycol mix only for the winter...obviously they didn't need the extra range that glycol gave then.

Thermostat - How important? - Andrew-T
SS - I can't tell who you are saying is wrong. But the effect antifreeze has in raising b.p. is marginal compared to anything else.
Thermostat - How important? - simonsmith_523
Your absolutly right my son. I'm gonna drain my coolant tomorrow, and just refill with water.
Thermostat - How important? - Shigg
What about corrosion? I've always been under the impression that a major reason for antifreeze was to stop this, maybe someone with a knackered Triumph Stag engine would care to comment?

Thermostat - How important? - simonsmith_523
What about corrosion? MiddleMan says its alright
Thermostat - How important? - Shigg
Maybe Iron engines are more tolerant than alloy ones (that's why I mentioned Stags - loads of them had their alloy engines ruined by corrosion), but personally I'd never run without a corrosion inhibiter. Look at it this way, would you run a water filled heating system without an inhibiter? I wouldn't and no heating system engineer would recommend it either, do also bare in mind that heating systems don't run in such adverse conditions as acr engines and are made up of generally corrosion resistant metals - boiler excepted, they're usually cast iron. Pay your money (or not) take your chance, or lose your warrenty!

Thermostat - How important? - Armitage Shanks{P}
Fascinating thread! We have the total UK knowledge on thermostats, in one place, and for free! My small offering is that corrosion is a major problem when you have an electric effect caused by 2 different metals in a liquid. A single metal in a liquid would not corrode much, if at all. But an alloy head on an iron block, even when seperated by a gasket, can cause major problems. Wasn't the Stag V8 a case in point?
Plain water and trolls. - M.M
For the benefit of those who may be misled...

I was referring to "all cast" engines from many years ago when saying the handbooks would say to run plain water in the summer. Say diesel tractors from the 1960s.

Never run plain water in modern vehicles.

"simon smith" knows this I guess but observant visitors will have noticed he/she plays the troll game.

Plain water and trolls. - wemyss
MM is cpmpletely correct and I think SS is simply playing games.
In the 60s the handbooks used to tell you that you shouldn't forget to drain off your anti-freeze, flush through and refill with water.
And anti-freeze of that era never used to mention corrosion inhibitors included in it and was simply to prevent water freezing.
With modern cars with disimilar metals and materials it appears to be imperative that we must use them as per the handbook.
Plain water and trolls. - Mark (RLBS)

Be nice. Disagree by all means, but be nice.

The alternative is you having to go through the registration process all over again with a different e-mail account - but then you'd know that.

Plain water and trolls. - simonsmith_523
I wouldn't dare disagree with you middleman
Plain water and trolls. - Dizzy {P}
SS, Does your mummy know that you're not in bed yet?
Plain water and trolls. - simonsmith_523
Don't know mate, haven't lived with mummy for years. What about you grandad?
Plain water and trolls. - Mark (RLBS)
You are the weakest link.

Plain water and trolls. - PhilW
For some strange reason that amused me greatly!! I actually LOLed
So whats the verdict then? - Inadequate
So we haven't really come across an answer and we seem to have gone down the antifreeze raising boiling point route, I was always under the impression that the sealed system raises the boiling point and that the thermostat just acts like a valve allowing the coolant to run through the radiator when its nice and hot. The little jiggle pin is just there to stop air locks. Regards the bypass hoses mentioned earlier, I think all cars have a form of this fitted as this is what is fed off to run the heater matrix.
Plain water and trolls. - HF
I just wonder why people do this, and what's the point?!
Plain water and trolls. - Peter D
I aggree, very entertaining.
However the problem is not rhe stat. The stat does not control the pressure in the system at all as sugessested in the first responses it just controls flow versus temperature. You may have a block radiator. Run the car in the drive and check the rad temp all over the surface, new rad only £25, check that one of the hoses ahs not collapsed ot delaminated inside. If you have or can borrow a digi meter with a thermocouple measure the temp in and out of the radiator with the can going. If they are only 5 or 6 degrees different then the flo is too low either blocked or a collapes/delaminated hose or a corroded impeller in the pump. I have dismantled these to find the impeller has corroded off the end of the shaft. Good Luck
Plain water and trolls. - Chris1DH
Fascinating thread. First visit here, prompted by queries about wax thermostats.
Have some relevant experiences - 'A' series byasses (aargh!) temp sensors around engines, corroded aluminium in Astra, overheating Escort... metallurgy degree.
Now I'm a plumber, sorry Heating Engineer. Some Combi boilers (max 3 bar, so water boils at 156) use wax stats. The one prompting me here operates a diverter valve to direct boiler(radiator) water either to the rads, or to a secondary heat exchanger to heat water for the taps. The valve in question has a shaft with discs and springs along it which open/close waterways when the shaft is moves longitudinally, about the same distance a car stat opens, from what I remember watching car stats in saucepans.
You can (now) only buy the whole valve, retail about £111. Often it's only the wax element which has failed, "cold". I don't suppose for a minute I'll find a Frod bit which fits, but does any one of you who has delved deepest into these things know who makes the waxy bits of thermostats? 100 at £5 each, yes please!

By the way there are those who say that "sealed" heating systems don't need corrosion inhibitors, even though we get copper, steel, aluminium, cast iron & brass all together sometimes. But there is mushrooming activity dealing with boilers and systems sludged-up with corrosion products. "Power flushing" devices can cost around £1000 - glorified pumps in buckets. Mystery surrounds the constituents of corrosion inhibitors and desludging chemicals.
To NOT install and maintain corrosion inhibitors you have to be one of
1)a complete dork
2)a cowboy who doesn't care
3)a cowboy who does care but wants to maximise future work
4)British Gas, plc.


Value my car