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Hi everyone i have a 17 elantra se with 114k i bought it new and its given no issues until very recent its coolant is full i only get heat when running highway speeds and it will start to run hot slowly if idling with no ac and even faster with ac on and at highway speeds it runs normal temp any help will be appreciated
 

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2018 Hyundai Elantra SE 2.0L 6MT
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Hi everyone i have a 17 elantra se with 114k i bought it new and its given no issues until very recent its coolant is full i only get heat when running highway speeds and it will start to run hot slowly if idling with no ac and even faster with ac on and at highway speeds it runs normal temp any help will be appreciated
Check your thermostat. Sounds like that could be the problem
 

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Hi everyone i have a 17 elantra se with 114k i bought it new and its given no issues until very recent its coolant is full i only get heat when running highway speeds and it will start to run hot slowly if idling with no ac and even faster with ac on and at highway speeds it runs normal temp any help will be appreciated
Check your thermostat. Sounds like that could be the problem

Yea, definitely would say it is more than likely a thermostat or radiator that is malfunctioning. The way to test this is to either rent or purchase a coolant system and cap pressure tester. Check the cap first, then check the system. If both pass - it's more than likely an issue with the water pump.

Have you checked under the hood - as far as heater hoses? They may be leaking, although not as common as a cap or thermostat (at least for Hyundai's that is...) It could also be just a pocket of air in the cooling system. Have you recently flushed the coolant? It's very common to get air bubbles in it and if not flushed properly, they can cause the exact issue you're talking about if the cap and system pass the pressure test.

For the pressure test - just increase the pressure up to the rating on the cap and leave it sit for about 10-15 minutes, check it then and if no signs of a leak, leave it be for another 15-20 minutes for a total of 30 minutes. If the gauge still reads the same: the cap and system are ok, but the thermostat may still be stuck closed - so you would need to pick up a gasket for it, unbolt the housing, check to make sure it has the right amount of spring tension (should be able to compress it in your hands, not too easily and not too hard).

If all that seems to check out, pickup a coolant funnel attachment kit and connect it to the radiator (keep in mind you will need to lift the front of the vehicle in order to raise the radiator above the heater core) with the car running you can hold RPMs at or around 1500-2000 and also idle, you will push more air out with the higher revs. Then be sure to top it off.

If you have any further questions @FL_Dustin87 - don't hesitate to reach out.


Source: Master Mechanic for nearly a decade, worked at local Hyundai dealer for a third of my time spent in the field.
 
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