Maybe you should edit your profile so your city and state show up so we'd have an idea of where in the world Flickenger's is ... just a thought .

 

Please help me to figure out where the main problem is.

I have replaced a thermostat (calorstat to wahler) 1 week ago. Before that most of time the temp gauge was pointing at 4 o'clock), but yesterday I've got my car throw the coolant out of the reservoir. The temp gauge pointed "at 3 o'clock". I opened the hood and the coolant reservoir was ready to make a BOOM. By the way, I just realized I haven't hear the main cooling fan working.
Today I tried to heat up my car not moving, with no air through radiator. The temp gauge was "at 4" most of the time after thermostat opened the long turn (hot upped hose). The FAN was silent. I suspected the dead FAN, but after launching air conditioning system I got my FAN turned on and working.

So, the main question is: Could be true that fatigued and not adequate ECT doesn't give a prompt command to turn the FAN ON, and that thing leads to my coolant boiling? I mean, the thermostat goes to it's opening temperature, but ECT things that the engine is still not hot enough. That why I get lower miles per gallon, the cooling fan silent and coolant boiling?

Is that true??? Any ideas, please?

 

The needle may not be 100% right, sometime people take them off when fixing the ODO and don't put them back on right. You can verify the temp with an OBD 2 tool, reading the live data. You can run the AC all the time to keep the fan on. Other than that the fan should come on itself at 216°.

If you coolant reservoir is trying to blow its lid, you may have a bad head gasket. If the car has been overheated just once it could be breached.

 

Sorry, I haven't provide details about car.
I own '94 2.5 liter NA Volvo 850. No OBD2 socket available.
The coolant reservoir doesn't try to blow under normal conditions (regular driving in town). I got it one, yesterday, in traffic gap, but it was OK today during test ride.