[CptMike]

Hi cg084,

I drive a 2.8L 30v... I'm not familiar with the 4.2L. But I think you might just be right about air in the system.

Temp sensors seem to almost always be mounted at the highest point in the coolant flow, excluding the upper hose and the upper part of the radiator. When a thermostat opens, coolant is pumped through the system until it reaches a designated temperature, and then thermostat closes again. Any air still being held in the cooling jackets and passages within the engine block with rise to the top, probably about where your cooling sensor is located. That would account for the low gauge readings - there is no coolant actually touching the sensor, only warm air. When the temperature in the block rises enough to transmit heat to the thermostat, the coolant starts flowing again, air bubbles and all, and the cycle will repeat. The gauge will go up for a while... The heat being on or off just varies the level of coolant that is actually flowing through the system. Air in the system will cause the problems you have described.

I ran into this same problem on an '85 Mercedes 300D Turbo Diesel. I replaced all the hoses once - there were 23 of them of various sizes. What a pain! When all was done I bled off the air through the bleed valve as per the shop manual. But my temp gauge and heat were very erratic. I felt like there was air in the system that could not escape and leave the actual coolant path when the thermostat was closed, even though it was a pressurized system. With engine off and forgoing the bleed valve, I disconnected the top radiator hose at the top of the radiator and, holding the open end as high as was possible, poured it full of coolant mixture. Bubbles did rise from the block. I then held my hand over the end of the hose and positioned it right in front of the radiator hose neck at the top of the radiator. Quickly removing my hand and shoving the hose on the neck, I tried to keep coolant loss at a minimum. Tightened everything back up and found that I had evidently eliminated the air pocket in the system. No problems after that.

With everything else that you have already done, I would give the "fill it up from the radiator hose" method a try. It would cost nothing but a little time an a small amount of coolant spilled when placing the hose back on the neck.

Hope you get it solved...