Engine Heating Problems

Hi, I had a thermostat removed from my 90 2.5 TD this summer, as the temp gauge was going right into the black zone virtually in no time immediately after starting it from cold. At the same time, I fixed (seized up) the fan, visco never engaged even when the gauge was almost in red. Summers here can be very hot! This solved the problem, the needle never again came even to the half.
However, with the cold days now, I put the stat back, and problems with the temp started again. To me this logically means, though I'm ignorant, that the thermostat is faulty, even though I checked it by putting it in a pot with hot water, when it opens, and again closes when out of water.
The fan has remained fixed. Just put new antifreeze in the rad, it took about 8.5 litres, shouldn't it be around 11 litres?
I know there are similar threads here, but please help what I should do.

What temperature did it open at though?

It will always take less to fill the system than it says because the bottom of the block will still have water in it unless you take out the block drain plug.

I would replace the stat, in the mean time as an experiment, take the little jiggle pin out of the stat and drill the hole out slightly larger, perhaps another one at the opposite side.

See if that makes a bit of a difference, but get another one ordered up ASAP.

Thanks, bought a new stat, just need to find some time to install it. Btw, I got the one that opens at 82 dgs, perhaps it was better to take the other one which opens earlier?