Yup, you need a heater control valve.

I could say, they all do that, but I have proof otherwise. The 86 I just sold actually still worked. The 82 245T I used to own, that one regulated fine.

Curiously both cars were from Texas, where I suspect the heat was used infrequently.

In any case, none of the cars I've driven daily have had heat that regulated, and I've never desired to replace a heater control valve. I spend enough time under my dashboards as it is.
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Rob Bareiss, New London CT ::: 92 244-M47-208K ::: Bilsteins, Turbo swaybars, Virgos, Lowered, Group A wing, Tach, 6-Disc, Satellite Radio, Keyless Entry ::: Coming Soon: B-Cam!