Low Heat Output

[b2002mw]

I have a '87 325e and the blower works fine and I tested the heater core for high pressure and high volume, both flow well thru it.
I can't figure out why I get hot air when I'm in high RPM and moderate hot when lower RPM.
Is there a temp control adjustmet or some thing controling water volume before the fire wall.

What controls the temp dial for mixing between hot and cold?

Any help would be great....

Thanks

[M-technik-3]

Sounds like thermostat stuck wide open.