Mike,

Sorry I didn't get back to you before now. I connected the pressure gauge to
the coolant level sensing plug which inserts into the coolant expansion tank.
Details in the archives. Added a swage standoff to the plug, filled with
radiator seal epoxy. Added a needle valve between the line to the gauge and
the coolant tank, and also plumbed in a valve and nipple between the gauge
valve and the coolant tank so I could independently evacuate the system or
pressurize it. Bought 1/8 inch OD nylon pressure tubing at FLAPS to run to
spare tire area. Spliced the line from the gauge to this line and cable tied
both to the radiator hose.

The details of the gauge I used and other possible mounting points are all in
the archives.

It is rather easy to see air in the coolant tank because:

1) The system pressure doesn't rise sharply as the engine heats up in the
morning.

2) Minimum cold overnight pressure doesn't go below about negative 0.2 to 0.4
psi.

3) System pressure decrease upon hard acceleration is about 5 to 7 psi
instead of 1 to 3 psi. (note: the engine compartment cooling system (I-4
valid comment only) are very soft and expand and contract with both positive
and negative pressure. A fully evacuated system has harder sidewall
containment (less air). When the water pump is accelerated, a pressure
differential across the inlet to outlet of the pump can develop depending on
the rigidity and the flow rate. The higher the pressure differential, the
lower the volume of water that the pump will move at any given impeller
velocity. In other words, overcoming a pressure differential takes work away
from that which can be exerted to push the water through.) This is the
phenomena that leads to reduced system cooling capacity when air is present.
This is further exaggerated when air is mixed with the coolant, leading to
bubble formation (significantly reduced heat capacity) and hot spot
cavitation (rapid onset of local boiling in the head for example). The
heterogeneous phase equilibria (air and coolant mixing) are strongly
mitigated by surfactants such as Redline's Water Wetter.

4) For major amounts of air in the tank (air gap of 1.5 inches or so - water
level electrodes just covered) the maximum pressure in the system will only
get up to 5 to 8 psi, instead of the cap limit of about 12.5 psi. Sure, sure
the cap says 14 psi but they lie!

Finally, I would suggest that the coolant pressure gauge (and an exhaust
temperature gauge) quickly show a fingerprint of characteristics that you
will recognize as nominal performance. You will learn to recognize signs of
failure or system degradation quickly.

I'll post this response to the list in case others may be interested.

Good luck,

Frank Grunthaner