Okay, that sounds promising.
While moving cars about last night, i left the Truck idling, 30 minutes later went to jump in and switch off. I noticed the defrost was cold and the temp gauge reading a touch under the red. I cut the engine to hear that ominous sound of boiling coolant and it blowing out the overflow. I fired up the motor and kept the revs up around 2500 and immediately watched the temperature gauge drop to halfway and heat coming from the defrost vents. Leaving it running the coolant level was halfway down from the full mark in the header tank, hoses were nice and pliable to touch, not hot, sides of the radiator too not hot, center of the radiator was stone cold, both Heater pipes cold. The heater was blow cold inside, but applying revs, the heater will blow hot, back at idle it was cold again.
I'm guessing that the heater operation is working and now my thoughts are turning to the water pump is on early failure. not noisy or leaking, just not pumping enough coolant.

 

is how I repaired my blend door. I used a Oscillating multi-tool to make a cleaner cut to enlarge the opening.

 

Many thanks for this link, well done. We are going to try one more option before checking the door flap, as the one I check previously must have been the hot and cold heater control **** which seemed to work fine.
One other thing, If this recirculating door is broken would it still open and close by increasing the engine rpm's?
The heater vents blow hot air, after idle the heat goes and cold air blows out. Take the revs above 4000 and the heater blows hot again.
We are going to look at fitting a new water pump first as I don't believe there is enough flow, and I have noticed a howling noise coming from the front of the motor when cold up to normal temp.

 

My door broke and fell covering the blower motor and was always pulling in outside air.

 

Everything is as it should be, heater controls all opening and closing. Our problem is a blocked heater matrix. The main radiator was 80% blocked, so we took our thoughts to the matrix. Coolant isn't circulating unless hi revs are applied then there is some coolant coming through but not a descent flow.

 

So is it fixed?

 

No, not yet. It's on hold as I have other work that's taking priority.

 

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