Do you get hot air from the heater? If so, I'd suspect a problem with the gauge. They are electromechanical gauges with heating wire wrapped around a bimetallic strip. As the wire heats the strip, it bends and pulls on the pointer. There are two adjustments on the back. One sets the pointer's position, and the other one sets the range from high to low by varying the tension on the pointer.
I had a similar problem with the fuel gauge on my '72 Challenger. It appears the heat-conductive insulator material breaks down and reduces the heating effect on the bimetallic strip. I adjusted the pointer numerous times, and each time it would read "full" for about two weeks, then go right back to reading 7/8 tank when it was full.
I would suggest first using a thermometer in the neck of the radiator to determine the coolant temperature. If it is indeed normal, as I suspect, you can try adjusting the gauge. The sensors cause very little trouble, so another possibility would be to modify the circuit by adding a resistor in parallel to the sensor.
If your fuel gauge also reads low, replace the gauge voltage regulator. It's a small three-terminal metal box plugged into the back of the dash board circuit board. It has a bimeallic strip that cycles on and off like a signal flasher. It turns battery voltage on and off so over time, its output averages 5 volts. From experience I can tell you, it is not possible to connect the gauges to a 5 volt power supply so you can make the adjustments with the gauge removed from the dash. The constant 5 volts will heat the strip too much, then it will read too low when put back in the dash. The built in regulator switches the 12 volts on for only about 40 percent of the time, then switches to 0 volts the rest of the time. The heating wire heats up very quickly and cools down slowly. You can see this yourself. When you turn the ignition switch on, the fuel and temperature gauges go up very quickly. When you turn the switch off, it takes quite a while for the gauges to go down.
The temperature gauge is easier to adjust than the fuel gauge if you choose to try. The fuel gauge must read correctly at empty and full. The temperature gauge spends all its time in one position so it only has to read accurately when the engine is warm.
Of course this all assumes the coolant temperature really is around 180 - 190 degrees at the top of the radiator. If it is cooler than that there's only two things I can think of, and both are unlikely. The burnng fuel absolutely will result in heat that must be removed. If ignition timing is severely retarded, the burn process will be completed after the fuel leaves the cylinders. One some engines with thin steel exhaust manifolds, they will actually glow orange from the excessive heat. The engine will not produce its full normal power either.
The other possibility is the heater core is acting as a really efficient radiator. This could explain why blocking the radiator with cardboard doesn't help. Pinch one of the heater hoses, then watch to see if the coolant temperature goes up. If it does, add a restriction to one of the hoses. This would be weird, and I've never heard of this happening although I had the opposite problem with my '88 Grand Caravan in the summer. It ran too hot at anything above 60 degrees and 60 mph due to deteriorated and crumbling cooling fins on the radiator. Running the front and rear heaters on the hottest settings and medium fan speeds kept the engine cool, so that shows how efficient the two heater cores are at removing heat from the coolant.
You can also try pinching the upper radiator hose for a few minutes. If the temperature comes up, the thermostat is not closing.
Tuesday, February 16th, 2010 AT 4:27 AM