"""the lower disc closes off the bypass that allows coolant to continuously
flow throughout the engine.""""
Not on the 1.9L Dennis, i have a 1.9L WP housing in my hand, and i don't see
how this could happen, i agree on the 2.1L. As for the 1.9L coolant
circulation in the water jacket, i agree, but i did some test on a 1.9L
(brother in law) putting an Aircool head temp thermo coupler on each
cylinder... the funny thing is that #2 and #4 where slightly hotter....

Now, what about joining the left head front output to the outgoing (going
front) main line????

Anyway, we will see as i'm putting an extra coolant temps sender on that
"blocked" left front outlet plate (on the head), i will be able to switch
with the original one. I like the simplicity of the 1.9L cooling system, les
 possible leaks...

Cheers
ps.: no news yet

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The engine can run hot without a thermostat. As the thermostat opens to
allow coolant to flow from the radiator, the lower disc closes off the
bypass that allows coolant to continuously flow throughout the engine. So
the flow to the radiator is basically short circuited without the
thermostat. If the even is an automatic, it is even worse as the trany oil
cooler is a parallel loop with the radiator always taking some of the
available flow. Then to make matters worse, the 1.9 cooling system does not
flow coolant through the engine. The in and out are both on the rear. The
water jacket being filled and convection cool the front cylinders. With the
heater valve open, coolant then flow through the right side of the engine.
Not left front outlet is there unless someone added it, (easy to do). So it
is possible to have a good gauge reading and still be overheating the right
side if vapor bound or something.

Dennis