Car: Hyundai Sonata Hybrid

Yesterday, before going into work I forgot to shut my car off. That was at 9am. Around 1pm, I received and email that my car was running so I went out to shut it off. Once I got outside, there was a large puddle of fluid under the car, gas tank empty, and battery dead.

I had a service come to jump the car and put fuel into it. As I went to drive home, I noticed the AC was not cold and the temperature gauge was all the way up. I pulled over and let the engine cool before continuing home. No weird noises or any red flags.

Upon getting home, I let the car cool down and popped the hood. First bad sign was my coolant reserve tank was empty. Secondly, there appeared to be coolant residue near the small hole in the coolant reservoir. Finally, I decided to open the radiator cap and it was bone dry.

I will note, the temperatures where I live we’re around 90 and the car was in the sun... extremely hot.

What exactly could’ve caused the car to leak all the coolant fluid?

  • 1
    Did you get to the bottom of why the battery was dead? Nov 8, 2018 at 11:16

2 Answers 2


Coolant can leak just about anywhere it travels through the cooling system.

For example:

  1. It can leak externally at the hoses, water pump, gaskets, and/or heater core to name a few.

  2. It could also be that the cooling fan was not operating and that caused the engine to overheat while sitting, pushing coolant out of the resevoir.

  3. The coolant could also have been consumed and went out the exhaust via a bad head gasket, crack in the head, or block.

In your situation, since it obviously ran hot for a long time, I think the best plan of action is to 1st pressure test the system to look for external leaks, and then have a chemical block test performed to see if their are any combustion gasses present in the cooling system.

You will want to verify if the worst case scenario has happened (cylinder head or gasket is bad), before fixing any other major repairs. Depending on what it may be, some or all of the work may overlap.

  • Would I be able to put coolant back into the reservoir and see if everything goes back to normal? Assuming I put the same one in as before.
    – Nick
    Jul 11, 2018 at 19:14
  • Sure you can. I just wouldn't fill it up with coolant, drive 10 minutes and then determine everything is going to be okay. Start with shorter trips and go a little further each time. Without testing anything, it leaves that dark cloud of uncertainty that rides shotgun with you for a while.
    – Milison
    Jul 11, 2018 at 23:48

Ok here is what happened (or at least my theory). When car idle for long period of time in hot weather it overheats because the cooling fan can't keep the car cool enough. Car is not moving so not enough air go through radiator and car overheats. When car overheated your coolant went boiling and started to leak from coolant reservoir or some hole in radiator or coolant hoses, I put my money on reservoir. When car overheated your cylinder head gets bent from the heat, this result in bad contact between head gasket and cylinder head which cause the coolant to be leaking outside of the engine or inside it and burning(going out of exhaust) making you lose coolant. Or there could be a hole in your cooling system and your coolant just leaking from there. In both cases after you find the leak. you should check your cylinder head. Sometimes when car overheats pistons expand from heat and start dragging inside the cylinders damaging both cylinders and pistons and ruining your engine completely, you are kind of lucky your car still can run.

  • 1
    A well-maintained car made this century should be able to idle in the heat without problems. Unless the humidity is astoundingly high and even then there would have to be some other issue; bad thermostat, bad hoses, incorrect coolant, timing issues, misfire.... I live in the Phoenix, AZ area and when the thermometer showed 121 F on the dash, the coolant temp was 196 F, in stop and go traffic. I am extremely skeptical that a well-maintained car would overheat to that extent without other circumstances. There has to be something more going on, IMO.
    – Tim Nevins
    Nov 8, 2018 at 17:19
  • I agree with you, but we can't see the car so we can only guess.
    – AsenM
    Nov 8, 2018 at 17:37

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .