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Car Engine Overheating - Why It’s Happening and How to Fix It

October 27, 2017

An overheating engine can be a nerve racking experience. In the best cases it can cause decreased fuel mileage or simply light up a dashboard warning for engine temperature. In the worst cases, it can cause smoke or even complete engine failure.

As soon as you think you might be experiencing an overheating engine, it's important to pull over and take stock of what's wrong. If you cannot pull over safely turn off the air conditioning and turn the heater on to maximum.

Overheating car

How to know if your engine is overheating

An overheating vehicle may display one or more of the following symptoms:

  • A temperature gauge that is reading higher than normal and into the red
  • An illuminated check engine light and/or engine temperature warning light
  • White or black smoke from the exhaust pipe when accelerating
  • The air conditioning stops working
  • Poor fuel mileage may be experienced
  • Contaminants in the coolant reservoir
  • Coolant escaping from the coolant reservoir due to boiling over

Common reasons for an overheating car engine

There are many reasons your vehicle could be overheating, but the most common include a coolant leak, a bad radiator fan, or a failed thermostat. These factors can contribute to an overheating in different ways.

Have a professional take a look

Note: In an emergency, to keep your vehicle operating until you're able to reach a nearby repair shop, you can turn the heater on to the highest setting to keep the engine from overheating while driving. The heater core can act as a secondary radiator, but temperatures inside the vehicle may become unbearable.

Coolant leak

Your vehicle cooling system's objective is to maintain an optimal operating temperature for the engine. It achieves this by circulating coolant through the engine where it absorbs heat, then the coolant is delivered to the radiator where the heat is removed from the coolant. When the system has a leak and the coolant level drops too low, it will be unable to remove heat from the engine, and it will start to overheat.

Solution: There are some ways to identify an obvious coolant leak issue - mainly you will see a message or warning light on in the dash. You may also notice a smell of coolant in and around the car, or observing fluid dripping onto the ground or other parts of the vehicle. However for less obvious leaks, the best option is to have a mechanic inspect your vehicle for a coolant leak diagnosis.

Bad radiator fan

The radiator fan sits in front of the engine and helps cool hot coolant in the radiator before it returns to the engine to help keep it at a safe temperature while operating. When this fan stops working correctly, the radiator will lose its ability to cool the engine and the vehicle can quickly overheat. This will tend to occur more often while driving in traffic and not at higher speeds.

Solution: In most cases, the solution to this problem will require a radiator fan motor replacement or to replace the radiator fan itself.

Failed thermostat

A engine coolant thermostat controls engine temperature by blocking coolant flow to the radiator until the engine reaches a predetermined temperature. Because of this, the engine can first warm up efficiently, and then begin to maintain a safe operating temperature once the thermostat opens and allows coolant to circulate.

When a thermostat fails, it can either stick open, or closed. In both cases you may find that the heater doesn't blow warm air into the passenger compartment. A thermostat that is stuck open makes it difficult for the engine to warm up and can cause it to run colder than normal. On some vehicles this will also illuminate the check engine light and set the diagnostic trouble code P0128. On the flip side, a thermostat that is stuck closed will cause the vehicle to overheat since it will block coolant from circulating and doing its job to keep the engine at a safe temperature.

Solution: The most likely solution in this case is to replace the thermostat.

More information on engine overheating

About the Author

John Gower is an automotive expert at RepairPal, the leading online source of auto repair resources and estimates. With many ASE Master certified mechanics on staff who have decades of experience, RepairPal knows all the fine points of car repair.

3 User Comments

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By , April 30, 2018
What is the cost for replacing be Engine of Mercedes 420 SEL 1991. It is
By , June 01, 2018
If you’re facing car over heating again and again then I would really suggest you to book a car service today at Gomechanic for the seasonal maintenance and other over heating issue.
By , June 13, 2018
ff my 2004 Honda Civic EX with no prior mechanical issues 112000 miles and now 117,000 miles has shown hot by temperature gauge,( sometimes above H) but it happens very infrequently. I replaced timing belt and water pump before this, so I do not think it is that. I replaced spark plugs after it started happening as well. I brought it to a mechanic immediately 1 year ago when I first noticed this happening who drove it for a week and couldn't reproduce it. Fluid wasn't missing and he replaced thermostat since he had no other ideas. Ever since it has happened sporadically and intermittently. Sometimes months between instances. My driving is predominantly long trips of 2+ hours weekly at least and it runs fine and rarely shows hot. It just happened again today after a long time not happening(months) and after a 2 hour trip. I was in line at my inspection station/emissions testing when it happened. I put heat and defroster on but wasn't really helping. I was in a line of cars so I tried turning car off for maybe 5 minutes if that and it stayed cool duRing emissions testing but elevated quickly after leaving station. It seems like both engine fans are coming on, I tried stressing the engine some today by aggressive acceleration when gauge was hot but not at H and the elevated gauge didn't seem to go higher because of it. I idled the car and revved engine hard and this didn't seem to make it run any hotter either. I usually put heat and defrost on when I see gauge running hot but I'm not sure it helps gauge lower any it seems to slowly rise and fall of its own accord whether AC or heat or nothing but I can't be sure it doesn't help. I had noticed in past, when gauge running hot with heat on I sometimes feel it isn't blowing as hot as I'd expect. I'm not sure if this always is the case. Also today when it happened and I accelerated hard to see if gauge would get hotter and it didn't, the gauge actually went to normal shortly afterwards for a period instead of showing hotter as I expected. I almost thought maybe I cleared something out somehow until it returned. The car sat some 7hours and I just drove it 2+ hours with no issue. I'm at my wits end, I hate to bring it to a mechanic when it isn't happening and ask to spend hundreds or thousands and maybe not get a fix, but don't wish to get stuck either. Any help would be appreciated.