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Hello, I just replaced a radiator and thermostat on my sister in laws 2004 Hyundai Elantra sedan. The car had a leaking radiator and while replacing it I also replaced the thermostat. The car has less than 70k on it. It was owned by her dad and given to her instead of being ripped off on a trade in. Garage kept and very good shape other than the radiator issue has never had any mechanical issues before. Never been hot either as when he noticed the leak he parked it and purchased another car.
So after replacing the parts and putting in water to test for leaks I noticed that though the top radiator hose got warm the bottom never did. Even after running it for twenty minutes with the AC and with the heater on. The temp gauge indicates normal operating temp and there are no visible leaks.
The bottom hose stayed use cold. The water in the radiator stayed ice cold. The fans kicked on after ten minutes or so but the thermostat never opened up to let the water from the motor exchange with the water that's been sitting in the radiator.
I even ran it to the store and checked the hose after getting there and still had not opened up. The top hose hot and the bottom ice cold. The gauge would go to a little more than half way which I'm told is about normal. It even dropped once after hitting the 5/8 position down to about the 3/8 point telling me that the thermostat finally opened up but checked the bottom hose and still cold as ****.
Took the thermostat back out. Purchased another one and put it back in and ran same type of test run with just water and after thirty minutes of idling with heater on the top hose hot as **** the bottom cold as ice. I see no air valve to release pressure while adding coolant and am worried that I'm missing something here. Please if any one has any advice for me on this I would appreciate it as this car has been well maintained and would hate to blow the motor when there is obviously so much life left on it.
 

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I'm new to the forum and recently had a similar problem with my 2006 Elantra. I also noticed that no one replied to this question, so here is what I did to solve the problem in hopes someone else may be experiencing the same issue.

I replaced a cracked radiator and while doing the job, I also replaced the upper and lower hoses as well as the thermostat.

I also filled the radiator with 50/50 Prestone mix instead of water. If you have any leaks, you'll see them, usually from the hose connections. If you have an automatic transmission, also check the transmission lines because they could leak as well.

I had a small antifreeze leak at the bleed-off hose at the top of the radiator. I cut about one inch off this hose and pushed it back on and the leak stopped.

To ensure you get all the trapped air out of the radiator, you must run the engine until warmed up with the radiator cap off. You'll see air bubbling to the top as the engine warms up and water begins circulating. A little trapped air won't be a big deal as it will eventually make its way out through the bleed-off hose and into the reservoir.

I suspect the original poster had an air pocket within the block that caused a restriction in flow, however, the bottom hose will be much cooler than the top hose because this is where the cooler water is sucked into the engine after having heat transferred by the radiator. The top hose is the return line, so it will (should) be much hotter.

Also, if you have a different (higher) temperature thermostat, that could cause the engine to heat up more slowly than usual. As long as the temperature gauge is in the normal range and there is no air in the system, you are good-to-go.

My car ran perfectly and at 142,000 miles, I am happy with its performance.
 
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