Gauge works. During cold months temp stays well below "Normal" area. Warmer months, gauge rises to barely inside "Normal" area and stalls and hesitates. From smell, it always seems to burn rich. Replaced: water pump, thermostat, coolant temp sending unit, O2 sensor, MAF valve. You correctly diagnosed fuel pressure unit failure (fuel in vacuum line)a couple of years ago which I replaced with much improvement. Perhaps related electrically (?), seatbelt buzzer operates only occasionally
Low operating temperatures is an indication of an air lock in the cooling system
gauges need the sender unit to be fully immersed in coolant to work but if they are not then the variable resistance material in the sender bulb cannot accurately relate to the coolant temperature
example if you have leak develop in a good system and you do not regularly scan your gauges like a good drive will do. it is possible that you will smell a dying engine yet when yo look at the gauge it will be reading cool to cold because the coolant has dropped to a point where the sender is no longer in contact with the coolant
Testimonial: "OK, but how do I get the sending unit to allow the thermostat to allow the coolant to flow through the system to move the air to the radiator and out? The radiator hoses pass the "squeeze test" and the heater gets slightly warm. The system actually all worked properly for a short time on a very cold day (15). I noticed I had good heat and then the gauge moved above halfway I stopped concerned about overheating. I removed the rad cap and fluid was moving as you would expect. I then checked the gauge. It had not risen. Then it rapidly dropped below the Normal level as it is now. Later, I noticed dirty coolant as if it had flowed through areas it had not been recently"
the temperature sender unit has no connection to the thermostat. It is a unit that changes the resistance to the current flow through the temp gauge in relation to the temperature of the coolant,thus showing a range of currents from cold to hot on the gauge
The thermostat prevents the water circulating through the radiator until the proper operating temperature of the engine is reached ( normally around 85 degrees c
aT this point it will start to open and be in various open positions depending on the temperature it detects in the coolant. So on really cold mornings when the temp reaches 85 degrees it will start to open and pass water and will continue to open until the water that has passed through the radiator ( now very cold) again reaches the unit at which time it will start to close and slow down the rate of flow through the radiator thus maintaining operating temp of the engine. Yes it you take the radiator cap of you will see rate circulating which indicates that the thermostat is open and the water pump is doing its job but as air is always at the highest point it does not mean that there is not an air lock in the system where the water flows under the air but the air is still in the high parts so the air has to be bled out
that air could be in the section around the sender unit and will be enough to stop it working. loosen high hoses off until coolant flows out , remove the sender unit until water flows out . now if there is no air left the next check is the sender unit which in itself may be fault or have a loose connection a test of the gauge is to remove the wire to the sender unit and place in on the block/head and the gauge should immediately read full hot. If it doesn't ( with the ignition on) then there is a wire or gauge problem. If it does then replace the sender unit as the resistance is faulty
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