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" Q: My 1999 Pontiac Sunfire coupe is doing something something I can&amp;#039;t easily explain. The car has 240,000 km on it and it has the stock engine, although I think I bought it with a K filter charger. While driving the temperature gauge will plummet from about 177 degrees to -7, and the engine is trying to keep up with temperature drop, because I&amp;#039;m getting worse fuel economy. I think it&amp;#039;s a mass airflow or fuel pressure problem. I don&amp;#039;t think it&amp;#039;s the thermostat, but could it be the temperature sending unit? I drive about 1000 km a week.  A: Well, the water temperature obviously isn&amp;#039;t going from a normal 177 deg to well below zero while the engine is running. I&amp;#039;d start by checking the temperature sender&amp;#039;s connector and wiring, and then use a voltmeter to see what the voltage at the sender is before and after the temperature swing to confirm a bad sender. Or you could just throw caution to the winds and screw in a new $30 sender, because that&amp;#039;s probably the problem. Shop for the part at the auto parts store before you go pay top dollar at the dealership.  Follow up Q: Thanks Mike! I installed the new sender and started the car up. The temperature in the car yesterday on the way to and from work was -40 both ways. So after installing it and waiting for the computer to cycle the engine temp was still showing -40. Will it take a while for the new sender to work or is it a bad sender? Follow up A: Either your new sender is bad (not likely) or the wiring or connector to it is open. There&amp;#039;s no waiting time\u2014the temperature should read correctly immediately. From the Service Manual: &amp;quot;The Engine Coolant Temperature sensor (3) is a thermistor, a resistor which changes value based on temperature, mounted in the engine coolant stream. Low coolant temperature produces a high resistance (100,000 ohms at -40\u00B0C (-40\u00B0F) , while high temperature causes low resistance (70 ohms at 130\u00B0C/266\u00B0F). The PCM supplies a 5 volt signal to the ECT sensor through a resistor in the PCM and monitors the terminal voltage. Since this forms a series circuit to ground through the ECT sensor, high sensor resistance (low temperature) will result in high PCM terminal voltage. When the resistance of the ECT sensor is low (high temperature), the terminal voltage will be drawn lower. This terminal voltage indicates engine coolant temperature to the PCM. A hard fault in the ECT sensor circuit should set either a DTC P0117 or P0118. Remember, these DTCs indicate a malfunction in the engine coolant temperature circuit, so proper use of the DTC table may lead to either repairing a wiring problem or replacing the sensor, to properly repair a problem.&amp;quot; Your \u201340 deg reading says to me the circuit is open, so look for a connector with a pushed-back pin, a broken wire or corrosion that keeps the connection from being made. "^^<http://www.w3.org/2001/XMLSchema#string>
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