This sounds like what my car is doing, except that I get heat after about 5 minutes on cold days. It only seems to heat up on slow climbs at low RPM, like the one to my house on a mountain. It is worse with the AC on.

A few years ago I replaced the radiator and it is better than it was before.

My prime suspect at this point is the fan clutch which I will refill soon. I have stock electric fans that work great.

I just replaced my thermostat because it was leaking! I found a tiny chunk of plastic in the old one, but I don't think it was interfering with its operation.

What is a MONO valve and where is it?

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1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside