[eeedelli]

Thanks folks. There are some interesting ideas coming up here.

Just for info, I think the engine temp itself is fine as the thernostat opens, leading to the top hose heating up after a few mins. The problem seems to be the flow rate through the heater. At setting 0 or 1 on the fan, it will provide moderately warm air but increasing the fan speed above that cools the heater core so much that the temperature drops right down. That suggests to me that there's little more than a trickle through the heater core. (It's tricky to get it to run completely cold in summer, too.)

My suspicion is that a previous owner might have replaced the valve and got the bowden wire(s) wrongly positioned, though I'm at a bit of a loss to see how. My cousin also runs a 1989 740 and his is like an oven when on full, so it's something about this particular car.

Regarding the void space. I thought the rubber drain flap for that was only on one side. (Can't remember which side it's on.) Are there actually two and I've missed one? I did a check a while ago that it was clear but I'll have to check it again, once the weather settles down after this storm passing over for the next couple of days.

I've had the driver's side under-dash plastic trays out for some time but can't see any evidence of dribbles down the bulkhead.