I have a 1988 BMW 735i that is having heater problems. I checked the thermostat already, and I had been leaking coolant, but when I replaced the fuses for the heater system even though they were good, the car got a little warmer although not nearly as warm as last year. I’m wondering if this could be anything but the heater core, or if there are any suggestions as to what to do to fix this issue.
Find the heater hoses. With the heater at the max temp and the blower at max the input hose going in should be the same temp as the output hose.
You say you “checked” the thermostat. How exactly did you check it? If it is running a little cool, then that could be the problem. Considering the cost of thermostats, I can’t see checking them, I would just replace one if I suspected a problem. They do get tired in time and replacing is not difficult on most cars.
In addition to the heater core there is a blend door the controls the air flow and therefore the heat. It may be partly closed or something (like a mouse nest or insulation) may be blocking air flow.
I’m guessing you had some coolant loss when you checked the thermostat. It’s possible you have an airlock in the heater matrix, these can be a bit of a nuisance to clear. In effect the hot air is being heated by hot air in the matrix rather than water, doesn’t do the job very well.
Remove the coolant filler cap - rad cap is better if you have one, check if your engine has a bleed valve. Set the heater to hot, start the engine and leave to idle.
Top up coolant and check for bubbles, this could take 15 - 20 minutes, you can increase engine revs slightly to help clear the airlock.
If this doesn’t improve heating dramatically, you may have a jammed vent as mentioned by a previous poster.