In my 1989 240 DL the heater only puts out luke warm air. Lots of air flow. AC doesn't work so it isn't a factor. Changed thermostat didn't help. Hose coming down to heater valve is hot. Hose that runs under the center console is hot also, but doesn't seem as hot. Recently the firewall to engine block hose gave out and I replaced it. Had to replace the gasket at the water pump for the coolant bypass tube also. Radiator was replaced last summer.
I am thinking that I hook the hoses together bypassing the valve to determine if it is the valve or the core? Doesn't look like it will be a lot of fun though.
I have seen on this site how to replace the valve with a "new style round one" from FCP Groton.
Any suggestions, helpful hints, better ideas, Volvo elves to do the work???

1. Should I try backflushing the valve and core from the firewall?
2. Anything else to check before I start taking things apart?
3. Does the heater core have a history of trouble?