Leaking heater core?

[irext]

My AU series one 6cyl has started regularly fogging up the windscreen when the demister is on the first two fan speeds with or without heat turned up. I think it's worse with heat wound up but not conclusive. If the fan speed is set to three or four it defogs o/k but the noise drives me nuts. Running the aircon fixes it also. No coolant loss is evident from the radiator reserve tank so if it is a leak it must be a pinhole. It never used to do this so something has changed but before I pull the dash and everything else out to get to the heater core I thought I would see if anyone else has had a similar experience. The car has done 240000 very reliable kms so far but I guess it's at an age where these things can start to fail. I am somewhat reluctant to simply send it somewhere to get fixed and have someone possibly do a rush job of refitting the dash and create all sorts of other problems so I'll almost certainly do it myself. (Ive been burnt before).

[FGII-XR6]

if you've been in one of the areas hit by bad storms / flooding you may have water in your ducting and if some of the sealing foam is wet it may take a while to dry

[blacklabel]

I know my AU wouldn't demist my windscreen with any less than 3 or 4 on the speed dial and I just kept doing this for months wondering why it wouldn't defog/demist on just low,or 1 on the dial,as it really should.

Well you know what fixed it?
Simply changing the Fresh/Recirculated air selector button on your dash!
I had always had it set on "recirculate"- to stop fumes and smells coming into the cabin when driving in traffic.
As soon as I pushed the "Fresh" button (instead of "Recirculate"),and set my temp control dial to 2 red notches past blue, my windscreen demisted instantly.
I think, that particularly with winter driving,your carpets and mats etc...tend to stay damp and the "recirculate" button simply pushes all that humid,damp air around your cabin and up against your windscreen,making it hard to demist.
As soon as you hit "Fresh" it does the trick.
Even the Ford handbook agrees!

Might not solve your problem,but then it just might.

[Au2falcons]

I don't think you have a heater core leak, otherwise you would smell the coolant in the cabin and get a leak in passanger side footwell.

If you can't defog with the first two fan speeds then check when the air flow is set to screen that air isn't flowing out of the feet/face vents. If air is leaking to other vents then that would explain why you have to turn the fan speed up high. That would point to a fault with the air flow control (vacuum operated).

Take blacklabel's advice and make sure it's set to use fresh air as much as possible, the A/C well help defog quickly. If find if I use the A/C for too long that you tend to get an annoying fogging of the outside of the windscreen on a wet/moist day.

[irext]

Tried fresh and recirculate without much difference. Interestingly when the fan speed is set to 3 or 4 it's forced to fresh anyway. I did check that air isn't being directed elsewhere when demist is on and all seems well although not a huge amount of airflow hits the srcreen. I'm not sure what it was like before. I'll get a pressure test done on the cooling system to see if a leak exists. (please don't be the heater core!!!!!)

[bodes-sh]

i guess the other thing to check first would be if you have a dash mat, that the mat is aligned properly and not blocking or partially blocking the demister holes