Cooling system troubleshooting fans were only coming on with ac?

2003 HYUNDAI ELANTRA

90,000 MILES • 2.0L • AUTOMATIC

FUNKEDUPFLOYD

MEMBER
4 POSTS

So my fans were only coming on with ac so I replaced the ect sensor and now they come on as they are supposed to but as soon as I replaced it my temp gauge started rising way above normal while I was driving so I figured I had air in the system and while trying to burp it I realized my lower radiator hose was not getting hot at all so because it's cheap I replaced the thermostat (in the correct direction with jiggle valve up) and I also purchased a vacuum airlift coolant refill kit to refill the system. Still cold hose still running hot then I replaced the water pump/ timing belt because my water pump pulley had always made a quiet cyclical squeak and this did not fix my issue either. I poured water through the radiator and it seems to be coming out the bottom pretty well so I'm unsure if it has any blockage but here are my exact symptoms; when I start the car completely cold the temperature gauge ,within seconds, rises up to the operating temp level(even though engine is still cold) everything runs fine and after about 5 minutes or so the low speed fans kick on and by this time the upper radiator hose is fairly hot and heater blows fine. Then the fans will kick on to the high speed. Lower radiator hose still cold( thermostat housing gets hot) just not the hose. I then will rev the engine and get the rpms up to about 2500 and after holding it there for about 30 seconds the temperature gauge will slowly creep up to about 1/4 of the way between the center mark and the maximum hot if I then let off of the gas it will pretty quickly drop back down to right below center mark where it has always normally stayed at. The fans never turn back off, and if I turn the cabin heat off the temp gauge creeps up to the center mark and I'm sure even further if I left it run but I usually shut it off around this time to not do any engine damage. There are no symptoms of major overheating no boiling over coolant, no steam, but the lower radiator hose doesn't seem to ever warm up and also the radiator fins from left (passenger side) to the right (driver side) goes from warm to very cold and the top is very hot and not at all on the bottom.

Image & Video (Click to enlarge)

Jul 6, 2026 at 10:08 AM

STRAILER

✓ASE Certified Mechanic
54,832 POSTS

Wow, you are not kidding the temperature gauge goes up quickly while revving the engine, thanks for the video. The fact that the lower hose is not heating up is the issue. So this leaves the thermostat or the water pump. I have seen the water pump impeller come off the main shaft but I don't think it can happens twice so I would remove the thermostat for now to see if that fixes it, and if so purchase an OEM unit and install it. Also, I have seen a loose connection at the coolant temperature sensor cause the gauge to malfunction so please heck the as well. Let me know how it goes

Jul 6, 2026 at 9:24 PM

FUNKEDUPFLOYD

MEMBER
4 POSTS

Wow thank you I have searched on every corner of the Internet and nothing had suggested test without thermostat, and you were right. with the thermostat removed the car did not overheat, and both hoses became hot. So does this mean that the thermostat that I bought was DOA? IM almost 99.9 percent sure it was installed correctly.. and even if this is the case what would have caused the first one to fail immediately after I replaced the engine coolant sensor? maybe the engine had been overheating for a while and I didn't know because of the faulty sensor? Lcould something else still be the issue and removing the thermostat just keeps it colder and counteracts against the real cause of the overheating? There was definitely like a coating of beige colored sludge inside the thermostat housing that I never thought to check for when I replaced it the first time could it still be a slight blockage in my radiator? Sorry for the million questions. Thank you for your help

Jul 7, 2026 at 4:21 AM

STRAILER

✓ASE Certified Mechanic
54,832 POSTS

No worries, yes the engine might have a very slight blown head gasket which will cause the thermostat not to open sometimes, I would run this test to confirm and if so here have great luck with this product.

https://www.youtube.com/shorts/0cCln2hgjKA

and

https://www.youtube.com/watch?v=Xi41RFMP8Ow

Here is the product.

https://amzn.to/4pfSlxW

Let me know how it goes

Jul 7, 2026 at 2:35 PM

FUNKEDUPFLOYD

MEMBER
4 POSTS

Hello so I ended up doing a combustion leak test with leak detection fluid and the car passed. I have also removed my radiator and ran the hose through it and some dishwasher detergent to try and see if there was a blockage and attempt to break it loose. I did forget that when replacing my alternator recently I dinged up the radiator on one side I'm not sure how badly that would have to be for it to cause circulation issues. So I put it back together and then reinstalled a new OEM thermostat to see if either of those things fixed my issue. And nope, lower radiator hose still way cooler to the touch and still overheating. On top of all of that it's been idling extremely rough noticably since doing all this work to it I'm not sure if the cause. I got one of the scanners that reads live pcm data and google says it's all good numbers but I don't know for sure. I've attached a video of my radiator flow and some of data to see if you think I need a new radiator or if it's something else causing the circulation issue. Also the pcm data is all during which I was driving without the thermostat installed and no overheating was occuring and one other thing I forgot to mention is that on very hot days,and while driving on the highway up the mountain the temperature slightly overheated twice even with thermostat removed but only to about 225° -230°F...

Images & Video (Click to enlarge)

Jul 21, 2026 at 2:11 AM

STRAILER

✓ASE Certified Mechanic
54,832 POSTS

Yep, the radiator is not the problem, it seem like the water pump is the issue, if the impeller is corroded or lose on the shaft it will cause this issue but you said you replaced the water pump correct? Here is how the thermostat should be installed.

Image (Click to enlarge)

Professional Diagram for 2CarPros Members
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Jul 21, 2026 at 3:46 PM

FUNKEDUPFLOYD

MEMBER
4 POSTS

Yes I replaced the water pump, and I have the OEM thermostat installed correctly. I am at a total loss at this point for what could be the issue. I really thought that replacing the thermostat with an OEM one would fix it. It drives perfectly fine with the thermostat uninstalled. But as soon as I install it and try to drive it overheats. After putting the new OEM thermostat in the lower radiator hose even got a bit of warmth to it nothing near how warm I think it's supposed to be but noticably warm compared to how cool it was staying with the other thermostat.

Jul 27, 2026 at 7:48 PM

STRAILER

✓ASE Certified Mechanic
54,832 POSTS

The only other thing I can think of is that the head gasket test was not accurate, let try running the engine and look inside the radiator or reservoir to see if there are tiny bubbles being produced.

Jul 28, 2026 at 12:42 AM

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