It may not be the heat. Maybe the AC is staying on and what you are getting is heated air that has been cooled again. Maybe the baffles are not operating properly so that the heated air is directed into the cab.
I have a 1989 Mercedes 190 that the heater only blows warm air when hot is
> selected, the drivers side is the worst, almost stone cold.
>
> I have so far.. Installed a new thermostat and water pump ( the water flow
> is poor ), checked the heater valve on the firewall and its opening, checked
> water flow through the heater core, flow tested the radiator, checked for
> blocked hoses, checked the flaps that control the heat settings, bleed the
> system free of air pockets. Also replaced a faulty temp sensor on the engine
> ( controls the radiator fan)
>
> The last owner has been contacted, and he told me he just put up with a cold
> heater :-)
>
> I have run out of ideas, the water flow seems to me, to be the problem. The
> pump and thermostat are both genuine Mercedes parts.
>
> Regards, Paul
>
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