QuestionI'm sorry. I should have worded that differently. The temperature will go back to normal when the heat is turned off.
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The text above is a follow-up to ...
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My wife's 99 Elantra's check engine light came on about 2 weeks ago. This past week, we had to start running heater because of weather. After car warms up and is running with heat on, it will turn cold and temperature gauge will start going up. After heat is turned off, temperature gauge will go back on. Checked fan, it constantly runs when the car is running.
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I think I follow you up until the point where you say the "temperatuer gauge will go back on."
You should check the diagnostic trouble codes to see why the check engine lamp came on. While the code typically won't tell you the specific thing to do to repair the problem, it should give insight into what things to check.
Since you're having issues with the heat and engine temperature, you should check your coolant level and the thermostat. Low coolant level will cause erratic engine and heater temperatures. If your thermostat is stuck open, it'll cause the heater and engine temperatures to remain or become cold when the vehicle is in motion at any significant speed. These issues can also cause the check engine lamp to come on.
To enable me to have a better understanding, I'd like to know what you mean by the temperature gauge going back "on." I'm presuming that the gauge "going up" means that it's registering a higher temperature. I'm also presuming that you mean that the heat turns cold while the coolant temperature gauge indicates a warmer temperature.
AnswerNo problem.
Rising engine temps with heat on and lowering engine temps with the heat off is a very bizarre combination. You'll want to check the diagnostic trouble codes like I've already mentioned, and you'll want to do a good assessment of the cooling systems functionality.
With the car cold, remove the radiatior cap and check the coolant level. If it's low, check for any leaks, repair them, and top off the coolant. With the cap off, start the car and allow it to reach operating temperature. As it reaches operating temperature, the coolant in the radiator should begin to flow. If you're unable to tell by looking in the filler neck whether the coolant is flowing, both radiator hoses should be about the same temperature when the coolant begins flowing. Prior to that, one will be much cooler than the other. A little while after the coolant begins to flow, the radiator fan should begin to cycle on and off to manage the temperature of the coolant in the radiator. If the fan blows cold or cool air off the radiator, you don't have proper coolant flow. If the coolant begins to boil out of the radiator before the fan turns on, you should suspect there may be an issue with the fan circuit such as a blown fuse or defective fan motor. If the fans cycle about two or three times without anything abnormal occurring, that's an indication that everything is working normally at the moment.
