Sounds like you have a bad thermostat. It's probably come apart and not allowing the engine to warm up much. Try that first it's easy, cheap & quick. Keep the cap on the expansion tank and you won't loose much antifreeze.

I had a similiar problem because the heater core was partially obstructed. Feel the two hoses that go into the firewall. If one is significantly cooler, the heat isn't circulating through the core well. I put two hose clamp pliers on the hoses, removed them and back flushed the core with a hose. Make sure it is back flushed by attaching the hose to the colder hose.

Man, did the heat improve!
--
Paul NW Indiana '89 740 Turbo 120,000

I had the same problem with my 89 740 N/A. The water pump was on it's way out and leaking through the front seal. It had lost enough coolant (about 2L) so that it would only get pumped into the heater core when the pump was spinning madly.

Have you checked your coolant level?

Best Regards,

Mike H.

89 740 B230F M47

I think the 700 series have a check valve similar to the ones on the 240's. It should be on top of the intake manifold and have a vacuum line connected to it. Maybe this has gone bad.