toybul,

I took my heater core out of the '82 because I was replacing the evaporator. The new heater core does make a big difference, at least in my case. The fins were not blocked. The evaporator had enough dirt to plant a garden.

Things to check -
- unplug monovalve electrical connection - bad monovalve = lukewarm heat
- check aux pump and make sure its doing its thing

To be fair, at the same time I replaced my heater core I replaced my water pump and radiator, so my experience is not completely scientific.

BUT, it too the car heat from lukewarm to burn your face off...passengers beg for mercy.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car