Morris Minor Owners Club

Since I replaced the head gasket of my 1275 Midget engine I have enjoyed about 150 trouble free hot weather miles.

When I refilled with anti freeze mixture I topped up the header tank with some plain water. I checked the level today to find that the header tank still contained clean plain water and the level was unchanged. When I pulled off the bottom hose anti freeze mixture came out.

Why hasn't the coolant mixed and why hasn't the engine displayed any signs of overheating (as far as I can tell)? I would be grateful to anyone who can come up with a solution.

If you squeeze the bottom hose with rad cap removed do you see the level rise and fall?

Why hasn't the coolant mixed and why hasn't the engine displayed any signs of overheating (as far as I can tell)? I would be grateful to anyone who can come up with a solution.

Maybe you have not read any signs? Possibly you 150 miles has been of short trips where the engine has not achieved a high temperature?

Unlikely, but maybe the car heater has actually been in use and dissipated sufficient heat to cool the engine sufficiently on these 150 miles?

Does the thermostat have a blocked “bleed hole” or is not operating at all? Short trips may result in higher engine temperatures which are not actually sufficiently excessive with regard to overheating.

Ymmv, but you need to check things out before it does become an issue.

Is this the standard Morris Minor radiator or a radiator with a separate plastic expansion tank connected via a small bore hose often used by BL in the 70’s.

Regards John

Well it's possible, but really not probable, that the Radiator is blocked.
What's certainly probable and most likely is that the Thermostat hasn't opened to allow the water past.
If you haven't got a Temp Gauge, you'd be unlikely to notice until something happened.
You have to remember that with the Thermostat stuck, or not open due to low temp, the water pump is always pushing against the 'stat'.
So if it never opens, the expansion of the water won't affect the pump operation, it will simply expand back towards the Radiator Cap and once over 4psi come out of the overflow...

Any sign of water escaping from the overflow pipe?
When did you last/ever have the thermostat out for a look?
You could try a short drive of perhaps 10 miles and see how hot the heater air is...Hotter than expected?

I have a temp gauge and my car carries number plate size POLICE signs front and rear (see my profile avatar).
The front one covers about 1/2 of the grill, but with a working thermostat the cooling system copes without any issues.

Thanks for the responses.

The longest trip was a reasonably vigorous 24 miles (by Google maps) in hot weather and several of 17 miles, all with the heater off. It has a standard radiator and no expansion vessel. I usually open the bonnet when I stop and have found nothing untoward (nothing felt particularly hot and the header was at a reasonable temperature). I haven't yet the checked the thermostat, but I've just bought a new one in case. When I squeeze the bottom hose there is water movement.

I've warmed up the engine with the radiator cap off and the heat has spread normally, if a little slowly, with the water in the header becoming hot and the bottom hose warming up nicely. However, there was no visible sign of water flow and only a slight sign of movement when I revved the engine. So, a classic sign of a faulty thermostat. But no overheating!

Are the impellors on the water pump ok?
I have known cases where the steel impellor has rusted away which has resulted is no water circulation.
The minor is so overcooled that even with no water pump the engine has a hard job to overheat.

Thank you.

I shall remove the thermostat, reassemble without it and check for water flow, if that seems sensible.

I’m sure you will be putting the stat back but if decide to leave it out, there used to be a special tube affair that was recommended to be fitted in place of it, I don’t know if they are still available though. If I recall correctly, it ensured water reached the rear cylinder that could suffer if run without the stat.

Still available:-
https://www.minispares.com/product/Clas ... o%20search

Thanks for that Phil, mine is years old.

OK. I intend to replace the thermostat as soon as I've discovered whether the water pump is working. However all your suggestions have raised two more questions in my mind.

These are:
Is the blanking sleeve for competition use only, given, as Philthehill says, the radiator's ability to overcool the engine?
And:
What thermostat is best for a 1275 Midget engine?

The sleeve can be used and is suitable for use on non competition engines but fitting the thermostat is best.
A 82 degree thermostat is most suitable for a 1275cc Midget engine.

What thermostat is best for a 1275 Midget engine?

88 Celsius. I’ve never used anything else in any of my vehicles over more than five decades - unless one was fitted by someone else.

I personally cannot see any advantage of running an engine cooler in the summer than winter.

82 degrees centigrade is the standard thermostat for the MG Midget.
The MG Midget has a coolant capacity of 6 pints and whilst the Minor may have a larger coolant capacity (8.75 pints/1098cc) the thermostat should still keep the coolant in the engine block at 82 degrees centigrade.

Mr. V wrote: ↑Tue Aug 27, 2024 6:16 pm Thank you.

I shall remove the thermostat, reassemble without it and check for water flow, if that seems sensible.

Well when you've removed the thermostat, just place it in some near boiling water and you can watch it open....

Or better still - place the thermostat in some cold water, heat the water and see what temp the thermostat opens. Use a jam thermometer (or similar) to check the opening temp.
The thermostat opening temp is stamped on the thermostat.

Right. The thermostat checks out. In fact, it seems to open further than the new one I bought. However, the old one doesn't have the little valve in the disc, if that's relevant.

Without the thermostat, and on a fast idle, there is circulation in the radiator, but I suppose a faulty pump would be likely to show something, given the absence of a restrictive 'stat in the system.

So my guess is a faulty pump. Any more suggestions? I'm utterly flummoxed. It's the lack of water loss and no signs of both mixing in the header and overheating that's contributing to the flummoxment.

As a side note, how does the restrictor sleeve improve the flow to 3 & 4?

PS. I continue to be extremely grateful for all your contributions. Thank you all.

I take it that the fan belt tension is correct so that the pump pulley is turning properly? Removal of the pump will show if the impellors have rusted away, or something else might become obvious.

If coolant is circulating, I would not suspect the pump. Particularly if the heater is working OK.

Still seems likely it has never got hot enough to need cooling.

The hole in the thermostat (possibly with a jiggle pin) is there to bleed the radiator when filling - and as it would always allow some coolant through, help the stat to reach the engine temperature.