Car overheats going up mountains

Hello, 2 months ago my car overheated when driving up a mountain in Spain during the Summer where its between 30-40 Celcius. Since then the car works fine unless you are constantly going up a hill or mountain for a period of time. If you return to a flat surface it cools down and instantly cools down if you turn on the heating system. It also overheats faster and sometimes on flatter surfaces if you put on the AC.

We brought it to 3 different mechancs and they don’'t know what the problem is. They said its not the water pump or radiator, we don’t know who to believe as we are not from this country and neither is the car.
Also im not sure the correct coolant is in the car i put pink in but i fear it should be green instead, maybe its mixed at this stage. Also we had the thermostat changed since it’s cheap but the problem is still there. I have no idea about cars so very happy with any help , thanks.

What model car, what year, and how many kilometers on it? What fuel does it use? This information can help us figure out what the issue is.

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Hyundai i30 diesel 2012. Will report back on kilimeters when i check, thanks.

Sometimes when an engine only overheats going up a steep hill etc it means that the cooling system is inadequate and when turning on your heater helps to keep it cool that is just making the cooling system more adequate… Make sure your coolant is mixed correctly and the correct type for your vehicle… Also make sure you are not overloading the vehicle (to much weight)…

While it is overheating, are you going the speed limit, going faster or slower??
Are you running high gear or downshifting or?? Does it overheat at lower RPM range or higher RPM range??

There are two main causes for engine overheat: The coolant does not flow through the system like it should. There is not enough air flowing through the radiator.

Good thing is being a diesel, that rules out to low a octane rating and the computer pulling to much ignition timing out…


I will just add that it depends on your definition of overheating… does the temp gauge just get slightly elevated or in the red where you have to stop and let it cool down. At any rate it just shows that your cooling system is on th edge and can’t deal with any additional load. I don’t know how they tested the water pump or radiator but they may be not very good. Some common issues would be bugs clogging the radiator, or just an old and inefficient radiator. Also could have a worn or old hose that collapses and reduces coolant flow. Of course a new radiator cap and thermostat would be called for as well as a fluid flush.

Bottom line though the car is old enough and the quality never that good, that the little engine is just too worn for what you are asking it to do. Just in my experience, I’m not a mechanic, but a pressure washer to the radiator did wonders for me. I don’t know if Spain has bugs like the us though.

I use an infra-red thermometer and check the radiator temperature at the upper tank and lower tank while overheating. If the temperature drop is insignificant with the fan running on high it is likely a plugged up radiator core.BTW the hose from the thermostat goes to the upper tank even if it[s on the side.

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As a kid I remember a guy with a new Chrysler about 1962, having an over heating problem pulling their camper. He showed me how he had brazed an additional water tank to the radiator and that solved the problem. A lot easier with brass radiators instead of the current aluminum and plastic ones. Any car I ever ordered though I added the heavy duty cooling option which included a bigger radiator.

It’s possible the first overheating incident created a small leak somewhere, internal (e.g. head gasket), or external (e.g. water pump). Even a small leak will prevent the cooling system from reaching the correct pressure at normal operating temperature, adversely affecting cooling system performance.

Are you needing to add coolant from time to time to keep it topped off? I had a similar problem on my Corolla, initial overheating incident (seemed minor at the time) caused by faulty radiator fan coolant-temp-switch. After fixing the fan problem, then noticed cooling system wasn’t regulating the coolant temp like it had before, but no signs of external leaks. Until a day or two later noticed a tiny puddle of coolant under the car the next AM. Tried and tried to figure out where it was leaking, but never could figure it out, could never get it to leak when I was looking for a leak. Having to add about a cup of coolant every 3 months (3,000 miles). Eventually I was able to narrow the leak down to the water pump area, but the water pump seemed fine, could never see a leak from its weep hole or anywhere else on the pump at all. Finally just decided to bite the bullet & replace the water pump, 200K miles on original pump, so it was due anyway. Replacing water pump is a big job, requires removing a lot of stuff, timing belt covers etc, and once all that was out of the way, sure enough, I could see the source of the leak, water pump. New water pump installed, cooling system immediately back to proper temperature regulation.

If it is overheating only when you have the ac or defrost off, then something might not be turning you cooling fan on.

As has already been questioned, is the engine really overheating or merely responding the increased work of having to climb a mountain on a very HOT Day, and assuming the car is not overloaded, If the temperature gauge merely higher or is it in the HOT Range…

Besides having the correct type of coolant and proper mixture, Straight or 50/50, and do not dilute it with extra water, make sure there is enough coolant. Sometimes, the hose between the radiator and the overflow bottle gets disconnected or broken and instead of pushing antifreeze into the bottle, it gets spilled onto the road…

When the engine is cooled off and the radiator is NOT hot, open the radiator cap and ensure the radiator is full to the top, then fill the overflow bottle to about midrange, half way to full.

Again, use the correct coolant, do not mix types, DO NOT do it when the radiator is HOT!

Good idea. The cooling system can appear to be full by viewing the coolant level in the overflow bottle, but actually the coolant level in the radiator is still low. This seems unlikely in OP’s case, but easy enough for shop to check.

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thanks Dave I believe i have the wrong coolant or mixed coolant so i must get it flushed and replaced. The car is not overloaded. The car can overheat even in 6th gear with low RPM its just when consistantly going up hills for a time without a flat surface break.When it happens I’m pushing the gas more often so the car is working harder and since its Summer in Spain it is always very hot the climate.

Hi bing, when it overheats it goes past 90c and goes as far as 110c but I never let it go to the red zone. We reduce it asap by either putting hot air on in the car or by stopping or if the car reaches a flat slope or starts going down hill. Also as soon as we start going down a hill that may last for a few minutes driving I am not needing to press the gas and the temperature comes straight down.

The wrong mix could indeed cause overheating, esp when cooling system is stressed. The wrong coolant could cause add’l problems, b/c the coolant chemicals must be 100% compatible with the engine’s internal gasket material, otherwise gaskets degrade & leaks. Suggest for the sake of your wallet to take care of this issue asap. Hopefully the problem is just the wrong mix.

There’s your answer. You’re overloading the engine by running it uphill in too high a gear. Downshift a gear or two.

It is a diesel, the max torque is 191.77 lb.-ft. @ 1900-2750 rpm, HP is 128 @ 4000 rpm…

Diesels engines by their very nature run hotter than gasoline engines, the diesel engine not only crates heat when the fuel/air mixture burns, the Diesel has to compress the intake air high enough to combust the injected fuel…

It is imperative that the cooling systems be maintained properly.

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The front of the radiator should be inspected for debris. On some vehicles it is impossible to see between the A/C condenser and radiator without some disassembly. The front of the A/C condenser may be clean but that doesn’t assure that the radiator hasn’t accumulated a great amount of dirt.

Below is the radiator from a 2016 Lexus, more than 80% airflow blockage.


If it’s running uphill in high gear it won’t matter if it runs on dilithium crystals, it’ll overheat.


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I love the scene in Star Trek (2009) when the Enterprise is leaving Space Port and Captain Pike tell Sulu to “Punch It” and nothing happens and Pike asks Sulu if he released the “Parking Brake,” and Spock says it’s the Inertia Dampeners…

So, I guess if the OP has released the Parking Brake and disabled the Inertia Dampeners, it goes right back to a coolant issue with the OP lugging it in High Gear…

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