I have developed a strange problem;
once o month heater starts blowing cold air and engine runs crap when it is cold. Takes ages to worm up.
I do check the water level in the radiator and it is fine.
Then I disconnect the heater hose and run water put everything together car runs great(for a month) and same again.
Any ideas my guess is there is a blockage in the heater core.

Thermostat stuck open? very easy to replace. Try reverse flushing radiator

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Lillee - 1969 Morris Mini K

Are you using inhibiter?

Try flushing the system with a comercially available radiator / cooling system flusher like Wynns (available from Repco, Supercrap etc). Make sure that you have the ON Hot and take the car for a good drive about 30 to minutes with the flushing chemical in.

Drain the system and fill with water, drive again for long enough to get it hot + 5 mins, drain and then fill with a good quality inhibiter.

Also, what condition is your water pump in?

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Sounds like your car is a girl if it's a monthly thing.
It's normal.

Thanks for the ideas guys. Thermostat is new but I will change it anyway just to be sure. Water pump condition, not sure. I have new one in the box I might as well change that too.

That's what I was going to say.
Your little girl is growing up time for a car bra

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Respect mine and I'll respect yours.

Well checking the thermostat is as easy as popping it in a pot of water in on the kitchen stove and slowly heating it up and watching what happens. Use some sort of thermometer to check if it opens at the right temp.

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68 Morris Cooper S Mk1 (*ex 78 1275 LS 4th last built, 70 Morris 1500 OHC & 70 MiniMatic)

i NEVER put a thermostat into any car without first cooking it up on the stovetop.

i had a radiator and cooling system re-do at a radiator shop many moons ago on a pulsar that i'd cooked the head on, they had 3 thermostats in a row, new off the shelf that didn't work... so i learnt from them to test before they go in the car.

i use a coffee thermometer (less than $10 at cooking shops) only goes to about 70C but as i run cold thermostats it works fine as an indication that the stat is opening at about the right temp.

MiniDna, i'd be checking your heater core and radiator core too, could be disintegrating and clogging up...