check the water pump and the thermostat too.
There was a series of water pumps for some models of BMW with plastic impellers. These plastic impellers got lose on the shaft. So the belt rorated the shaft but the impeller actually just slipped on it and had no real RPM as required.
Remove the water pump and check that.
The thermostat you can check as following. Just put it in a pot of water and heat the water up on your kitchen oven, depending on the thermostat type it should then awitch to open position open between 75-80 degree Celsiuis. That you can easily check.

shogun >>E32 750iL<< 11/88
7 series Tech tips
http://tinyurl.com/3486c

• heater is only blowing hot when engine is at higher rpm's - thankinson 2006-03-10 20:02:42 (545 views)
  • Sounds like it might be the hot water valve/pump (url) - davidc 2006-03-11 04:23:28 (560 views)
    • Re: Sounds like it might be the hot water valve/pump - leeway 2006-03-24 19:47:28 (458 views)
    • if it gets hot in idling - shogun 2006-03-12 22:43:18 (472 views)