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Hey guys, there's an ongoing discussion in the E9 board about adapting an aftermarket heater control valve to the E9's since otherwise the hot coolant continually circulates thus combating the cooling power of the AC.

Everyone feels a bypass valve is needed in order to not block that flow of coolant. I feel there might be sufficient flow for all that coolant to just go down to the block --same flow, different path.

Does the heater control valve on the E12 bypass the coolant flow or simply blocks it?

E12s simply block it, as do E23s, E24s and E28s. E32s and E34s plumbed it differently and completely blocked off flow from the back of the head. Blocking it off will not cause problems.

In other words, the fact that I can't turn of the heating totally is normal?

Falkenberg wrote:In other words, the fact that I can't turn of the heating totally is normal?

No, it should block water flow completely.

hm. I last month I spent 100euros on a new valve, and I still can't turn the heating off completely. Sometimes almost, other times it's almost hot in the car. At least it is not leaking anymore.

Texas dealers in the 70's took to putting a simple open/close valve ahead of the windshield in the heater line. Customers who'd spent a lot on these new German machines were all over their dealers on how hot the cars were with the AC wide open. The dealers invited company reps from the factory to Texas and gave 'em a ride in the cars during hot weather, windows up, etc. to get their point across. So, the customer would bring the car in before summer, the shop would shut off the line, then in the fall, they'd open it back up. That's the story, anyway. Seems like a glaring oversight, not being able to turn off the hot coolant.

Falkenberg wrote:hm. I last month I spent 100euros on a new valve, and I still can't turn the heating off completely. Sometimes almost, other times it's almost hot in the car. At least it is not leaking anymore.

I've heard there is a new design for replacements which I haven't seen or used, that could be the problem. Also, are the heater hoses hooked up correctly? The input from the back of the head should be on the bottom at the firewall. Even with the OE type you can bypass some hot water if they're hooked up wrong, and the redesigned replacements may be even worse. Check installation first.

FWIW, Bavs (E3s) and probably E9s didn't have a heater valve, they just directed air differently. And Bavs had terrible A/C, but not because of the heater valve, it was just an inadequate design.

Mike W. wrote: Also, are the heater hoses hooked up correctly? The input from the back of the head should be on the bottom at the firewall.

Thought about that yesterday before reading your response, and it could explain the issue. I've opened the old valve, it is basically a disc that has holes of different sizes. By rotating the disc you control the flow of coolant by positioning certain holes over a hole (is flange the correct word?). The disc is pressed on the flange by a weak spring. So if one would reverse the flow of coolant, maybe the pressure difference of the coolant could overcome the force of the spring, and move the disc away from the flange, which would cause coolant to flow through the heater.

I indeed have the hose from the back of the head going to the upper pipe of the heater. That's how I got the car, and I think I've seen other cars having it like this as well (I've wondered before if it should be like this). I'll swap the 2 hoses and see what happens.

My 'new' valve had 1995 stamped on it

I think you have a fighting chance then. On my first E12, I hooked it up wrong and after I installed a heavy duty water pump I started getting a buzzing sound over about 65MPH. It was the disc in the heater valve fluttering away. I think as long as your new one hasn't been installed since '95 with 180F coolant running thru it baking the O ring you will be ok.

So yes, after switching the hoses I can now turn off the heating. Such luxury!