Heater not working in '87 Olds 98

The fan works fine, but the air coming out is lukewarm at best. Here's the details.

- I have the digital push-button HVAC
- I replaced the thermostat. No help. The hoses feel appropriately warm at operating temperature.
- I checked the heater core for blockage- water ran through it fine so I think it's okay.
- I disconnected the air control door behind the glovebox and tried it manually in all positions. No help. I did notice that the motor that runs this door only moves when changing the temp from 15ºC to 20ºC and after that it moves no more. But it seems to be fully open at 20º so I don't think that's the problem.

Is there a coolant flow regulator somewhere that maybe isn't doing its job? I couldn't find one on the engine side of the firewall, and I'm not anxious to start tearing the dash apart on a wild goose chase.

This is a beater car, so I'm not opposed to jury-rigging something into whatever position I need to make it work.

I had the same problem on my '86 Olds.

Go to Wal-Mart buy a bottle of "CLR", it is a calcuim, lime, and rust remover.

Disconect the heater core from under the hood, blow the coolant out and replace it with CLR and let set for about 30 minutes, then flush with water.
Repeat until use the entire bottle of CLR. Then reconnect heater core.

If radiator stop leak has been used on vehicle it tends to plug-up the heater core. Hope this soves your problem.

Thanks for the reply. I'm leery about pouring CLR into my heater core, so I bought the rad flush adapters and backflushed the heater core. The water ran though with no problems and I left the tap on for about 5 minutes. No difference to the heat output.

alot of things in my 98 dash a re vacuum controlled, my heat went out because my car used to sit and mice got in there and chewed the vacuum lines, i had to tear everything apart and patch them, then a few weeks later my heater wasnt really blowing heat like it used to, i replaced the core and it was more hot than it ever was.

I finally decided to tear the ducting apart under the dash to get at the heater core. It was hot on the top half, but the bottom half was cool as a cucumber. So for those of you who said it was my heater core, I think you were right.

I'm going to try some rad flush to see if that improves it. It looks like a b*tch to get at the core hoses on firewall so I don't want to try replacing it unless I really have to.