Front bulkhead design

[XenonJohn]

A long term annoying problem:

I have an Elite with a Rover V8 conversion. I have done most jobs on the car over the years but there is one annoying one left.

An (old stye) Mini heater system has been fitted behind centre console, presumably as it is compact though a bit feeble. Heater matrix was new about 5 years ago. Car always garaged.

The problem is that whenever car is driven, summer or winter, air from demister vents badly steams up the windscreen. This happens whether water from engine is flowing through the matrix or not. Happens when the demister fan is off or full on, warm day or cold day.

Matrix might be leaking, however, no water loss from cooling circuit, always used correct anti-corrosion in cooling circuit, car does not steam up on hot day with windows closed (sign of leaky matrix and damp carpets), no dampness in car carpets etc. Plus it is a relatively new matrix.

If matrix not leaking then humid air is getting into heater unit and coming out to hit cooler glass - but where from? Air inlet to heater is via a wide duct from a round hole approx 10cm diameter in front bulkhead just to left of steering column under dash made up of lots of smaller perforated holes. This air inlet looks like it was present in the car at manufacture, not a later mod.

I thought maybe the double bulkhead under the middle 2/3 of screen had a blocked drain hole and was holding water. I took the black mesh cover off and no water present. I noted just one single moulded louvre each side in this bulkhead under the screen. What are these for? They don't seem to go anywhere linked to the heating/ventilation system at all.

I expected to find the disc of perforated holes that heater air inlet is connected to, emerging into this double bulkhead under the screen, where it would pick up clean air (which would then be damp if drain hole was blocked) However it does not, so that theory is out of the window too.

Q: Does this air inlet go straight through bulkhead into lower engine compartment, under master cylinders somewhere? or is there another separate double bulkhead there? It must go through at a point to the right and maybe lower than the double bulkhead that is under centre 2/3 of the screen. I cannot see it emerging into engine compartment as too much engine in the way.

This is a silly problem compared to all the others I have had to fix but it is really annoying to have the screen mist up as soon as you start driving. If you keep side windows open halfway, it will very slowly get better as you drive - only very slowly though, even on a hot day.

All the normal causes seem unlikely.

I plan to pull off the air inlet pipe so it is just cabin air entering heater/demister unit to see if that improves things. It's a strange problem though I have never had on any other car, and I've had a lot of old cars.

The only other factor that might be relevant is that until recently one of the rocker cover engine breathers just had a small K and N style air filter fixed to top of it. When cold or at tickover fumes would come out and find their way into the car - perhaps through the 2 louvres in the double bulkhead under the screen. I just wonder if these have somehow coated inside of window with something oily but invisible that makes it steam up really easily.

I now have a proper catch tank and plumbing to air filter.

Any bright ideas would be much appreciated in the meantime though while I do these experiments.

Thanks

John