Pipes, I assume you have the correct thermostat fitted (it can be checked on stove). I cracked a head (in the pre-combustion chambers) on my MQ Nissan once when I hit a hail storm north of Adelaide & the thermostat was jammed open.
If you cover half of your radiator with a sheet of news paper, does the temperature gauge move, car gets warmer?
Macca's advice is tops.

Everything has been checked. I just might blank off half the radiator again, and see how it performs.
Many thanks.
Pipes.

I *very* strongly advise you to use another method of measuring the engine temperature before you start covering the radiator otherwise you may destroy a perfectly good engine to prove the temperature measurement system is faulty.

Older engine temperature measurement was performed by a temperature sensitive resistor causing the current through a meter to vary. The meter/TSR were fed from a crude voltage stabiliser; any one of those or their connectors could be at fault and they were never accurate to begin with.

Beg or borrow a temperature meter which uses a thermocouple (many multimeters have this feature nowadays) and, with a cold engine, remove the radiator cap and place the thermocouple in the coolant. Bring the engine up to temperature (it doesn't sound as though it will reach boiling point) and note the reading from the thermocouple. Also note the temperature gauge in the instrument cluster.

Remove the thermocouple from the radiator and loosen one of the thermostat housing bolts enough to trap the thermocouple under it - retighten. Bring the engine up to working temperature again and note the reading from the thermocouple.

If there is, say, a 15C difference between the two readings you can now establish, with acceptable accuracy, the true temperature of the coolant with an external measurement. Replace the radiator cap and see if that makes a difference.

Check in the workshop manual what the working coolant temperature should be (I'll guess around 95C) and now you can block the radiator off with some confidence you know the true temperature of the coolant.

Keep very much in mind that the relationship between the amount of radiator area blocked and coolant temperature is probably not linear. So a 30% block may be fine on a 25C day on the flat but on a 35C day and a couple of long steep hills it may be too much.

Mazda don't design engines to run cold so if all the bits (radiator, thermostat etc) are the correct parts then I'd be looking to find the real cause not trying to put a Band Aid on it.

PS. Whilst you have the meter/thermocouple put the thermostat and thermocouple in a saucepan of water on the gas stove and note at what temperature the thermostat opens.

-- Edited by Mike Harding on Tuesday 18th of April 2017 04:25:31 PM

Hi pipes

Some good advice given already, but you say it has all been checked. So think outside the box for a bit !!

All engines diesel and petrol have the cooling capacity to stay too cold if the thermostat system does not work as it should. They should stay hot driving in the snow and ice and work OK. Trucks may need some manual radiator covering in extreme cold conditions but we do not have them here in Australia. So the only conclusion to say is your thermostat and cooling system IS NOT WORKING PROPERLY.

Some engines do have a slightly more complicated coolant flow than others. Some thermostats are dual action and open up on one side and close the other side to prevent flow. Go to a Mazda agent and make sure you have the exact one for that engine, not just the temperature setting but the physical design too. Fitting a simple one instead of a dual action one will cause problems. The one shown in the link above is a dual action unit.

It is also possible that the internal heater hoses and connections may be incorrectly fitted so the water is always flowing and cooling it when it should not. The bus had several heater units fitted in the back originally and these may be still connected somewhere underneath or the hoses are incorrectly directed. The internal heaters are connected to the thermostat bypass circuit so will cause overcooling if incorrectly piped. These old buses and trucks, Mazda and (Ford brand too) were good workers and usually no problems. 

Jaahn

     

-- Edited by Jaahn on Wednesday 19th of April 2017 07:58:59 AM

The mechanic's tool he used was a remote infra-red temperature sensor. It would show the temperature of the thermostat housing. The fact that your temperature gauge says 60 Deg C (however inaccurate it is) and that you cannot get any heat inside the cabin tells me that the thermostat is stuck open.

I cannot see any reliable way of diagnosing a thermostat stuck open other than to remove and inspect it. Then stick it in a saucepan of water and boil it up. You should see it closed when it comes out, and then when it gets hot (typically around 78 Deg C), it should start to open . Typically at around 90 Deg C it should be fully open (the valve disc should have moved 8 to 10 mm open). Then, when it cools it should close again. If it doesn't do that, it is stuffed and needs replacing. Since you are getting no heat inside the heater (other than when you partially blank off the radiator), I would say that the thermostat is stuck open and needs replacing.