Ford Ranger / B-Series :: 1997 - Engine Will Not Reach Proper Operating Temperature

I have a 97 Ranger 2.3 standard shift, No A/C, a pretty simple Ranger with 158,000 miles. About a month ago while do some checking around under the hood I noticed my coolant overflow tank was empty, so I filled it up to the proper level with some anti-freeze mixture. Thinking if the system was low on anti-freeze it would suck what it needed from the recovery tank, wrong! About two weeks ago I noticed that my temperature gauge was NOT registering in its normal position and was considerably lower towards the Cold mark. Heat coming out of the heater was normal.

I opened the radiator cap and everything looked okay. After a few days this would not go away and replaced the thermostat thinking the thermostat was stuck in the open position, wrong again! I burped the system by letting the engine run with the radiator cap off and filled the radiator with anti-freeze mixture every time some of the air was expelled. I did this for a few days and noticed that the engine would not reach normal operating temperature at idle speed! I had to put something on the gas pedal to increase idle speed so the engine would get hot, this is not normal. Finally I got to a point where the radiator would not take anymore anti-freeze mixture, but the problem still exists.

Two things I noticed that will get the engine temperature up to normal, engine speed (RPM's) and not having the heater on. Let me explain, at normal idle the engine will not reach its proper temperature, but when driving down the highway the temperature goes up to normal (about halfway on the scale) no matter if the heat is on or not. When stopped at a light or just idling I can watch the gauge slowly go down towards the cold mark, if I turn off the heater it takes longer to go down to cold. I do know the heater fan is cooling the coolant to a degree (no pun intended), but it shouldn't cool it so much as to make the temperature drop so dramatically. Okay, heater off, riding down the highway, everything normal, come to a stop and idle temperature starts falling slowly, but when I turn on the heater the temperature starts falling faster.

My diagnosis: I have no leaks of anti-freeze, checked and re-checked. I have no symptoms of a blown head gasket, no milky white appearance on dip stick indicating anti-freeze mixing with engine oil, no clouds of white smoke coming out of exhaust, checked tail pipe for anti-freeze, none found, no loss of power and no loss of gas mileage. I changed the thermostat, like I said. Do not think it could be the radiator pressure cap, normally if the cap goes bad the system will have no pressure and the engine will overheat, the opposite of the problem. Water pump, normally if the water pumps goes the coolant will not circulate through engine and will over heat, again, not the problem I am having. Could it be:

1. A long shot, I bought a defective thermostat and it is still staying open.
2. The temperature sending unit has gone bad, very improbable since it does register the correct temperature when truck is moving down the highway.
3. The coolant is spent, again unlikely, it has been in the engine only for about 3 years and still looks clean and green.
4. Have to have a cylinder pressure check to determine actually if the head gasket is really blown or not, which I doubt.

My conclusion is that I still have a huge air lock somewhere in the system and the engine is still not completely filled with coolant and when the rpm's drop not enough hot coolant is reaching the temperature gauge sending unit, but if that much coolant is not in the system, why am I still getting lots of heat blowing from the heater? The vehicles cooling system is not that complicated and there are few parts involved in its proper operation, I think I have covered them all, but somewhere I am missing something to pin point my problem.