Someone has either reversed the wires on the temperature gauge or damaged the wires from the engine to the gauge and spliced them back backwards. It is a simple reverse of polarity.
First, check your thermostat, your car may actually be overheating slightly due to the thermostat sticking. If it sticks then no anti-freeze flows through the heater core(providing the cab with heat), which will cause overheating. If it eventually opens, allowing flow, and cooling off the fluid/engine (gauge drops) then fluid flows again giving you anti-freeze through the heater core and heat pours out. Check it by removing it and putting it in a pan of water and boiling the water with a thermometer in the pan (try to hold them both off the pan surface with tongs, it may screw up your reading), watch close for the temp at which it opens, should be 180F, but check for your car.
Second step would be to check your water pump, low flow from a water pump could also cause the same problems.
After those two, "you're on your own kid"
If you are absolutely sure you do not have air trapped in your cooling system then you may have a defective gauge if you are getting heat through your heater. Usually I would say you have a coolant flow problem with either a stuck thermostat or a bad water pump but if you have heat from your heater, then you must have circulating coolant in your system. I would check the gauge or the sensor.
Check the coolant level, be sure it is to the full mark on the reservoir. Check the temperature gauge, when warmed up it should read aprox 200 degrees F, if not the thermostat may be stuck open, replace it. If the reservoir is full, with the engine running and the temp gauge reads aprox 200 degrees, feel the heater hoses with your hands, they should both be hot. If one is hot and the other is warm or cold, the heater is plugged and needs to be flushed. If both heater hoses are hot I would suspect the temperature blend door is at fault.
1600 watts is unusually low for electric baseboard heaters! These usually are rated at about 6000 watts.Typical electric baseboard heaters operate on 220 volts thus you heater would pull about 7.3 amperes and 16 gauge wire can easily handle this.A standard 6000 watt baseboard heater pulls about 27 amperes and needs 10 gauge wire.
Simple check is to ground the fuel sending wire at the fuel tank.If the gauge reads FULL then it is a bad fuel sending unit and you need to drop the fuel tank to replace it.If the gauge reads empty when you ground the wiring then it means you have a bad fuel gauge.I would lean strongly towards the sending unit be bad my 87 the gauge started reading about 1/4 tank less before it failed.
This is a quote from a popular auto repair book:"It is rare for the temperature gauge to fail,but if it is erratic in operation check the lead from the (temp. sensor) switch for broken insulation and earthing.If the gauge reads maximum,suspect the sender switch(temp. sensor)." The switch can only be tested with an ohm meter or by substitution of a new unit.I just replaced my tem. switch and it didn't fix my gauge!So...
Many things can cause this problem. The Jetta needs to be seen by a licensed mechanic to determine the exact cause.Assuming you are not getting much heat from the heater, the heater core may be plugged or the temp blend door mechanism is faulty. With the engine warmed up and running, open the hood, locate both heater hoses, feel each heater hose with your hand, if both hoses are hot you probably have a temp blend door problem. If one hose is hot and the other is only warm you probably have a plugged heater core.**seems to have proper heat from heater even when the temp gauge is low. The hoses I checked and both were warm. when I let it idle it gets up to temp but as soon as I start moving the temp jumps way down. Any more ideas?? ***Borrow an infared temp gun and measure the temp at the thermostat housing. See if it matches what the vehicles temp gauge says. I'm thinking maybe the temp sending unit is faulty. If the vehicle's temp gauge reads low and the heater is putting out good heat my guess is the gauge is off. Hope this helps.
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