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The stock valve has a spring loaded door, which allows a small amount of pressure to flow in the reverse direction. The result is a certain amount of backflow...which ensures the heater core is always warm, so there will be instant heat, if needed. (German cars, colder climate.) There's a door inside the HVAC that completely isolates the A/C evaporator and the heater core. Most all of these doors have long lost their foam and leak...allowing heat in. (The door can be adjusted at the main HVAC "computer", BTW.Sometimes this helps. Very common for us to add a second heater control valve to shut off the "back flow", so the heater doesn't get any hot water, when running.
Of course, if one of the interior vacuum pods is leaking (super common), this must be repaired first , so that the heater control valve/valves can get enough vacuum to close. Always start with testing the vacuum source and check valve. Move inside to testing the vacuum pods, heater control vakve, and vacuum switches. Then make sure the door between the heater and the A/C closes mechanically. If you still get heat, the foam on the door isn't sealing and you can then add a second heater control valve.
Greg, thanks for the good insight on the foam on the doors/flaps inside the HVAC airbox. Is that what I am seeing here in this diagram detailed in red? I've already been through the checks folks have suggested (Vacuum leaks, Solenoid testing, flap position testing/adjusting) and heater valve function. But I am still getting warm air. Does anyone have a real picture of of the this part?
Here is another view for a photo I found from a google search but I can't see the flaps. I'd appreciate seeing a detailed view of the foam.
Last edited by Michael Benno; 06-26-2018 at 04:58 PM.
Greg, thanks for the good insight on the foam on the doors/flaps inside the HVAC airbox. Is that what I am seeing here in this diagram detailed in red? I've already been through the checks folks have suggested (Vacuum leaks, Solenoid testing, flap position testing/adjusting) and heater valve function. But I am still getting warm air. Does anyone have a real picture of of the this part?
Here is another view for a photo I found from a google search but I can't see the flaps. I'd appreciate seeing a detailed view of the foam.
Actuation rod #48 controls the flap that separates the heater from the A/C and generally can be adjusted some to shut off the heat, even with the foam missing. Be careful not to over adjust this rod, as the lever it attaches to can (and does) break. That lever is part of the HVAC box....not a separate part. (Someones needs to "print" some sturdy replacements.....for the 1-2 we find broken every year.....another in a long line of "extremely low volume, never make back the time it takes to make them" 928 parts.)
Thanks, Rob, for the foam pictures.
BTW....The "clue" for deteriorating foam is turning your HVAC on high....those "chunks" that come flying out are the old foam.
Actuation rod #48 controls the flap that separates the heater from the A/C and generally can be adjusted some to shut off the heat, even with the foam missing. Be careful not to over adjust this rod, as the lever it attaches to can (and does) break. That lever is part of the HVAC box....not a separate part. (Someones needs to "print" some sturdy replacements.....for the 1-2 we find broken every year.....another in a long line of "extremely low volume, never make back the time it takes to make them" 928 parts.)
Thanks, Rob, for the foam pictures.
BTW....The "clue" for deteriorating foam is turning your HVAC on high....those "chunks" that come flying out are the old foam.
If someone used a high res laser 3D scanner it could be prototyped pretty quickly with thicker, stronger material.
Nest time I'm at Mark's I'll see whether the lever that the mixing motor rod attaches to is conserved across years. I have a spare '89 box that should have that lever on it.
I had a bunch of these problems on my 90 GT when in Florida and driving it in the wicked heat. The research that I did led me to conclude that the Germans design philosophy was "default to heat" if there were any faults in the system. This led me to replace the valve, re-foam the air doors and install a pull cable to positively shut the airflow over the heater core when needed. Accessing the vacuum solenoids takes some work and plenty of time and patience!
Not sure how Carl's product works, but the stock valve has a spring loaded door, which allows a small amount of pressure to flow in the reverse direction.
The result is a certain amount of backflow...which ensures the heater core is always warm, so there will be instant heat, if needed. (German cars, colder climate.)
There's a door inside the HVAC that completely isolates the A/C evaporator and the heater core. Most all of these doors have long lost their foam and leak...allowing heat in. (The door can be adjusted at the main HVAC "computer", BTW.Sometimes this helps.
Very common for us to add a second heater control valve to shut off the "back flow", so the heater doesn't get any hot water, when running.
Of course, if one of the interior vacuum pods is leaking (super common), this must be repaired first , so that the heater control valve/valves can get enough vacuum to close.
Always start with testing the vacuum source and check valve. Move inside to testing the vacuum pods, heater control vakve, and vacuum switches. Then make sure the door between the heater and the A/C closes mechanically. If you still get heat, the foam on the door isn't sealing and you can then add a second heater control valve.
Beware: There are many different heater control valves, out there. I had a car come in, last week, that had a valve which closed with no vacuum and opened with vacuum....completely opposite of what is needed.
Greg, when adding the second valve do you just tee off the existing valve? Use another stock valve and just orient the other way. I know it sounds simple but I want to be sure.
That was my thinking to T off of the existing valve. I'm also curious where the second valve is installed. Do you cut the return ling above the transmission or do it more towards the right side manifold? How the hell do you get your hands in there? Tips and pics appreciated.
There's only two hoses going to the heater. One hose already has a valve. Simply put the valve into the second hose. You want to shut off the backflow in that hose, so install the valve with the arrow towards the heater core.
I install the valve directly behind and below the fuel cooler....lots of room right there....and the hose you need to put it in is already right there (should be there).
Tee off of the vacuum supply to the stock heater control valve....you obviously want the same signal.....i.e. vacuum to close both valves. (Note that the prerequisite is a functional interior vacuum supply.....virtually none of these cars have 5 existing diaphragms that are intact, unless they have already been replaced.)
Note that these cars shut off the vacuum to these valves when the ignition is turned off. This will open the valves and allow heat soaking of the heater core, which will allow heat to escape past the bad foam on the door, on restart. This "restart" heat will have to be disapated out if the heater core. (You will still get heat after a hot engine restart.)
Note that you will be using my idea and instructions. At the very least. It would be polite to order the second heater contol valve kit (yes, we do this often enough to have a kit) from me. My prices compete with every other supplier...and you don't have to make sure they understand what you need.
06-26-2018, 04:27 PM
