Here'* how I understand it. Coolant enters the LIM from the cylinder heads on the lower sides, and flows to the ends of the LIM.
At the coolant elbow, coolant exits to feed the heater core through passages in the tensioner assembly.
At the throttle body end, coolant flows out the firewall side LIM hole behind the stovepipe bore up into the plastic passage of the UIM and into the throttle body. From the throttle body, coolant flows back down through the radiator side hole in front of the stovepipe bore into the thermostat chamber.
Under the throttle body feed port in the LIM, coolant flows around the outside of the LIM through the little casting into the thermostat chamber about 1.5" below the LIM deck. The thermostat chamber is isolated from the rest of the LIM except for these two passages. Coolant exits the throttle body end of the LIM through the thermostat when it opens.
It is obvious that the highest points in the cooling system most likely to trap air, are the critical coolant passages around the hot stovepipe that carry coolant to and from the throttle body. When air is trapped in the coolant passages around the hot stovepipe, temperature of the plastic can rise much higher than coolant temperatures and contribute to rapid deterioration of the UIM. This is why I recommend that L36 owners drill an air-bleed hole in the thermostat flange and take extra care to fill and bleed the engine according to the procedure described in Techinfo: http://www.bonnevilleclub.com/forum/...=article&k=100
When the LIM gasket fails, coolant short circuits from the pressurized coolant side to the vacuum of one or more of the intake ports; or, enters the crankcase; or, weeps to the outside; or wicks up the LIM bolts, or some combination of those. Coolant is available at all four square passages where the LIM mates to the cylinder heads.
When the UIM fails, coolant under pressure enters the intake around the stovepipe or the throttle body gasket and gets sucked into the combustion chambers. In the image below, the smaller round holes mate with coolant passages on the deck of the LIM to carry coolant to and from the throttle body and around the stovepipe. You can see the perforations from the crumbling stovepipe bore into the coolant passage and throttle body gasket in the failed UIM below. When coolant enters the UIM at the stovepipe, cylinders #3 on the front bank and #6 on the rear bank, are the closest for catastrophic leaks and hydrostatic lock.