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MG MGA - Thermostat

Can the thermostat stop opening suddenly?
I ran the car about two or three weeks ago, and TBH I did think the thermostat opened slightly later than normal (ie when temp guage was a little higher). In the past, I believe the guage showed the thermostat opening at just below 190F.
But today, after fitting a new heater valve, I started it and let it idle at about 1500 rpm (too high I know) and it just got hotter and hotter. I turned it off when it started approaching 210F.
The thermostat must be at least 20 years (or more) old.
I have ordered a new thermostat anyway.
Graham V

Graham. Yes, thermostats can fail suddenly and they most commonly fail closed. I have, twice, had this happen when driving, a reminder that the occasional glance at the temperature gauge is an excellent idea. The most common immediate action is to pull over to the side of the road, remove the thermostat, refill the radiator, then continue your journey. Remember, when the thermostat is operating correctly, it only controls the minimum operating temperature of the engine, not the maximum temperature.

Les
Les Bengtson

Thanks Les for the informative response
I understand that the thermostat’s main purpose is to help the engine to heat up to normal operating temperature. Is it ok to run it permanently without the thermostat to eliminate the risk of the thermostat failing?

I appreciate in your case, you were referring to getting home in an emergency. But just wonder if the car is mainly for summers (in the uk!), would that avoid the danger of a thermostat failing ?

I understand that that ideally you would also need a blanking sleeve.
Graham V

Hi Graham, I once had a car which had had the thermostat removed. It ran far better when I replaced it.
P Willows

Hello Graham----Thermostats don't play fair, they can and will fail anytime they like and usually not when you want them to-
The new thermostats that we get here (Failsafe) tend to stick slightly open when they fail, causing slower than normal warm ups and lower cooling temps mostly but don't overheat.
I wouldn't run a road car without one, it'll take ages to warm up without one---the temp would be all over the place depending on load and when you get out on the highway the gauge will drop right off-ALSO- the cooling system is designed to have a restriction(thermostat) to help the water pump push the coolant up into the little pockets in the cylinder head, If there's no restriction there can be little, or not so little, hot spots formed which can cause cylinder head failure especially in the area above the exhaust ports.
Put a thermostat in it and don't go crazy with the temp. 76c works well for a good allround temp, it will run about 180 on your gauge--

willy
William Revit

PW - Thanks interesting to hear. As you will see from below, I am beginning to understand that would be the case

Wily - Thanks. I see myself as v lucky that it failed while I was at home checking for leaks after replacing the heater valve. I have put a new thermostat in, 82C and on my short test run could clearly see a lot of action on the temp guage, with it opening and closing quite a lot to maintain the right temperature range. I dont remember seeing that much action before, maybe I just hadnt noticed, or maybe it had already started to fail.
But it was clear that every time the stat opened. the temperature quickly came down, so your advice, as always, is certainly correct.
Graham V

Graham, The gauge should settle down after the car warms up a bit---IF your gauge keeps fluctuating after it's had a bit more of a warm through, you might need to drill a small 1/16"(max.) hole in the flat of the thermostat to allow a small amount of circulation. This acts like a bit of a shockabsorber for temperature flutctuation--- Shouldn't really need it but if the gauge keeps wriggling around after it's all warm then I'd do it.

cheers willy
William Revit

Thanks Wily. It came with a pre-drilled hole with a small “widget” hanging in it. I wondered what that was for. I won’t get a chance to take it for a proper drive for a couple of weeks but will see what happens then.
As far as I can tell, you can’t get the fail safe thermostats here which is a shame
Graham V

Graham
That little donger/widget in the hole is there as an air bleed, it's there to prevent, or help prevent air locks when you refill the coolant---they work well and will correctly block the hole off as soon as the system is full and running.---If you remove the donger then that empty hole remaining would be too large and will/would ruin the operation of the thermostat.
IF in the rare occasion you do have an issue with the gauge fluctuating still when nicely up to temp. then a tiny 1/16" maximum sized hole drilled somewhere in that same area as the donger valve will settle it down-----Mistakingly removing that little valve to do this will leave a hole too large, it only needs a tiny hole.

cheers
William Revit

Thanks Willy, appreciate it
Graham V

Graham. I have, twice, had to drive a vehicle without a thermostat when the installed thermostat failed in the closed position. I prefer not to do that, but each trip (about 100 miles) was made with no noted problems.

In the days of my youth, several engines I owned had what was called a "bypass hose". This was a hose between the block (just above the water pump) and either the intake manifold (Ford 289) or the cylinder head (BMC A series 948cc). I would note that both of these hosed connected to stubs that had an internal diameter of about .375" or 3/8". Some what larger in cross section than the 1/16" hole drilled into a thermostat. As a result, I have to question how much the size of the hole really matters until I see a series of experiments covering the subject.

Remember, the thermostat only governs (generally) the minimum operating temperature of the water, not the maximum operating temperature which is set by several factors including the coolant mixture, load on the engine, fuel mixture, and efficiency of the installed water pump. In cold weather, even a thermostat of higher rating may not keep an engine operating at a sufficiently high temperture of optimal performance. Many times, when living in Germany, I saw people driving Mercedes sedans with the majority of the radiator blanked off in winter. Often, only a small, about 4"x4" hole allowed air into the engine compartment. Something worth considering.

Les
Les Bengtson

Les
The hole in the thermostat and a bypass are completely different animals.
The bypass, which is built into the casting of the cylinder head on a 'B' series engine, circulates coolant from behind the 'closed' thermostat, through the casting orifice in the cylinder head, through a hole in the engine block ,back into the water pump, creating a circulation in the engine only --with no circulation from the radiator at all--When the thermostat opens at various amounts then hot coolant is allowed to circulate/escape to the radiator for cooling down and fresh cooled coolant from the radiator enters the water pump to reduce the temp in the engine back to the thermostat controlled temperature. Until the coolant in the whole system has warmed a bit this opening and closing of the thermostat is where Graham is seeing his gauge fluctuate--Normally when the system is at working temp the fluctuation is barely noticeable on the gauge. The bypass is also open on these engines at all times which helps even out temp changes when the thermostat operates by mixing some of the heated coolant with the incoming cooler coolant from the radiator.

Having a small hole in the thermostat works differently to this. It allows a small amount of cooled coolant into the engine at all times and also a small amount of coolant to pass through the thermostat back into the radiator and acts as a bit of a shock absorber on the thermostat preventing sudden temp changes--The hole has to be very small, otherwise if it's too large it will act like the thermostat is partially open and cause slow warmups and can cause the engine temp to drop off out on the highway.

NORMALLY- If the engine's bypass system is working properly , when the car gets up to temp there isn't an issue with the gauge flicking about and it won't need a hole drilling---
but I've seen homemade water pump gaskets with no bypass hole and even cheapy pumps with casting flashing blocking the bypass hole and the gauge goes crazy on them
William Revit

When I swapped the 3-bearing MGB engine in my MGA for a 5-bearing 1950 engine a few years ago, I decided to replace the 82 degree thermostat, even though it seemed to still be working perfectly.

When I removed the older thermostat I decided to test it with the new one and I immersed both of them in boiling water.

Although they both opened up at the same temperature, I was quite surprised to see that the new one opened up much further than the old, it probably had twice the gap that the old one did.

So, it seems to be sensible to replace the thermostat every now and then.

Cheers
Colyn
Colyn Firth

Thanks for all these interesting posts and discussion. I dont ever recall seeing any "up and down" fluctuations on the gauge in the past, other than once when the thermostat first opens, I am guessing about 5 minutes into the drive.
Maybe I just didnt watch it as carefully, or maybe it was because the thermostat was already starting to fail.
I havent taken the car for a longer drive yet, and I am unlikely to be able to do so for another week or so, but it sounds logical that it should settle down as the water reserve warms up. I will confirm in due course
Graham V

So just resuscitating this thread as I hate it when people dont report back. So I have at long last had a chance of taking it for drive and as Willy predicted, it settled down after about 15 or 20 minutes.
Funny I have never noticed anything other than the initial fluctuation before but maybe I didnt look closely enough!
Graham V

This thread was discussed between 03/04/2025 and 24/04/2025

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