Morris Minor Owners Club

Water levels are fine when the engine is cold, and even warmish (60-70 deg). However, as soon as it get's hot sitting in traffic or heavy load on the motorway it looses water fast.

I thought it might be boiling away, as the only evidence I can see of water getting out is through the overflow. It's looks like it's being thrown out quite violently (enough to push off the pipe) so I figured it was boiling.

However, last trip I noticed it happening the temperature gauge never went over 90 degrees... So I don't see how it could be boiling. The cap should should also mean it doesn't boil until 106 or something too right?

I've taken a look and revved it and can't spot a leak anywhere, and there doesn't seem to be much evidence of a leak... but I lost 2 litres today on a short-ish trip around town.

The engine is a 1275, recently installed (though I did have the same problem once, on a very hot day during a 3 hour motorway trip with the old 1098) with all new hoses. The only thing the same with the cooling system is the radiator.

Any ideas? I thought the radiators on morris mnors were really oversized, the temperature doesn't seem to be getting high enough to boil it away though so... leak?

First check - is the rad cap correct type? There are long and short caps - and obviously a short cap in a long neck rad won't seal! Try a catch bottle on the end of the overflow pipe - and don't overfill the rad - leave a good inch or so clear at the top - when cold. It is of course entirely possible that the rad is blocked internally - a good flushing both directions with a garden hose won't do any harm. And - are the fins ok and not blocked? If none of this does any good -you may need to lift the head to inspect the gasket.

likely to be a head gasket leak,which is presurising the system and pushing water out,if you take it to an mot station or any garage that has the emission testing equipment ,they can do a sniff test at the rad filler neck with the engine running to detect exhaust gasses.This will eliminate or prove a head gasket problem

If you run the engine up tp temperature - with the rad cap off - then if the gasket is leaking into the water passages you will see a steady stream of gas bubbles coming up through the water........ DO NOT run it up to temperature and then remove the cap!

No bubbles at temperature and compression test came back 180-190 across all cylinders. Still worth taking the head off?

Could it be the thermostat stuck open?

It can't be the stat stuck OPEN (maybe stuck closed - but unlikely) - and if no bubbles - then NO! Good compressions there! You sure it's not just the wrong rad cap?? Give the rad a good flushing - or try another.

I always make sure to bleed the cooling system by removing the heater return hose and running engine at fast tickover.

I use a length of clear plastic tubing pushed into the 'open' hose and resting into the top of the rad - no mess that way and you can see when it runs clear of air bubbles.

I don't think this is an obvious cause of your problem but you never know.

As a wild punt in the dark (and from having a friend who had something very similar happening to him) what is the state of the inside of the radiator?

He tried pretty much everything, but swapping his old radiator with a newer one from my car cured it. His radiator had become clogged up with hard mineral deposits and, as a result, had diminished the cooling efficiency.

Might be worth considering ...

He's already been advised to flush out the rad ...........

If you have decided that its the head off route,take it to an mot station as previously posted,my local garage says they NEVER take a head off,where overheating is an issue BEFORE they do the sniff test.Will confirm gasses in the system if the gasket is leaking ,even in very small concentrations.

You don't need a sniff test - the bubbles will be obvious if gas in entering the water jacket......

not always obvious,get the test done,its the Modern way roy. a couple of years ago,a guy brought a Rover into my brothers garage,with similar issues,he had replaced the rad, ,there was slight bubbling,not much , the owner wanted him to do a head gasket(Very common fault on the small Rovers) He did the sniff test,no sign of hyrocarbons from the filler. It turned out to be a hairline crack in a metal water connector,obviously sucking air into the system,with the cap on and the system pressurised it then had a slight leak but not easily spotted by the owner. but obvious from under the ramp,Cap off, to do the visual check,there were the bubbles.So Bill for new pipe and labour considerable less then taking the head off.
Taught me a lesson,if its the garage you always use would think they would do it free,literally takes seconds.
OP is considering taking the head off,but says there are no bubbles,the test would set his mind at rest.

I take your point - but some of us NEVER use a garage ......so finding one with 'sniffer' gear willing to do this for free may not be easy... No bubbles = no need to lift the head...

No bubbles every time I've had the cap off. Since it lost all water on the motorway I've only had it loose water once. About two litres this time when the traffic to work was bad. Temperature gauge never went above 90...

To check for bubbles - take the rad cap off and fill the rad right to the top while the engine is warming up to operating temperature. There may be a few bubbles initially as air locks escape, don't worry about them.. - what you don't want to see is a steady stream of bubbles coming up - and getting worse if you lift the revs.....

If you don't top it up, does it continue to lose water? As long as the water level is above the core it will be OK.
As Roy said in his first post "and don't overfill the rad - leave a good inch or so clear at the top - when cold"
I have a suspicion that you're overfilling it, you say it never overheats.

I go back to an early post - is it the correct rad cap??

4 pound esm cap, identical to the one from my other car. Swapped them just to make sure it's not a faulty one.

Rregarding overfilling: definatley not the case. I've overfilled before and I know the level it drops down to. It did overheat on the motorway and afterwards took more than 3 or 4 litres to fill. The other time in traffic was just shy of 2 litres. Both cases it was lower then the core!

The caps may be identical - but are the rads identical? When you fit the cap - do you need to press it down tightly before twist locking it? Have you tried the collection bottle on the overflow pipe? And if you brim it - ARE THERE ANY bubbles coming up when you leave it running? If you have another car - swap the rads over - does it still overheat??