Vehicle: 2004 Hyundai Sonata 4 cylinder vehicle.\

The thermostat monitor on the dash goes up to hot very quickly, especially when the air conditioning is running. When you turn on the heat the temperature indicator becomes normal. When the car ignition is turned off, it automatically cools down to normal. In fact when the dash indicates it is hot, actually the car is at a normal temperature. It is never actually hot.

The first thing replaced was the thermostat. That did not cure the problem.

The next thing replaced was the "sender unit," and that did not cure the problem.

Do you know of a third thing to try?

Have you flushed the radiator/engine? Did you use a thermometer to check the temprature vs what your dash gauge says? Its also possible that your radiator is clogged on the inside.

Engine Overheating

Most engine overheating problems are directly attributable to poor preventive maintenance or improper coolant filling procedures. Attention to detail makes a big difference. It's best to change coolant every 3 years with a 50/50 mixture of auto maker recommended antifreeze and distilled water. It's good practice to remove engine drain plugs during this process. To refill Hondas, it's important to open the bleeder bolt, add coolant until it comes out of the bleeder bolt, tighten the bleeder bolt, turn the heater control to high, turn the fan off, run the engine until the thermostatically controlled radiator fan comes on, add additional coolant until the radiator is full, and rinse out and refill the coolant reservoir. On all cars, it's important to purge air from the cooling system, since it can cause overheating or no heat in the system.

Today, it's important to only use the antifreeze recommended by the car's manufacturer. There are huge differences in the types of antifreeze and using the wrong one can have disastrous consequences. Don't take a chance and use the wrong antifreeze. Honda, for instance, recommends using an anti-freeze that doesn't contain silicates or borates. Silicates are abrasive and cause premature water pump bearing seal wear, which greatly shortens the life of the bearing. Once antifreeze gets into the bearing, it will not last long. Other manufacturers have differing requirements and it's not wise to experiment with the chemistry.

It's also important to use distilled water, because tap, well, and softened water can cause unforeseen problems. Tap and well water are loaded with minerals that can build-up and block radiator and heater cores. Minerals found in tap water tend to combine with coolant additives. When this happens, they form a chemical complex called “phosphate scale,” which coats the heat-transfer surfaces of radiators, heater cores, and heads, resulting in decreased heat-transfer efficiency. When this occurs, your vehicle will overheat and run much hotter than it was designed to. This scale can also lead to the destruction of water pump bearings. When this happens, the radiator or heater core must be removed and roded or replaced. Soft water is known to weaken solder joints and cause corrosion, due to salts in the water. Keep in mind that chemical reactions increase with temperature. With the high heat of modern engines, it doesn't take long for caustic steam and heat to corrode, overheat, and damage an aluminum block engine not properly maintained.

It's also important to regularly change radiator and heater hoses. Most experts recommend changing all hoses every 4 to 7 years. Inspect hoses regularly for softness, bulges, and deterioration. Pay particular attention to the top radiator hose. Internal deterioration may be almost impossible to detect. Therefore, don't be "penny wise and pound foolish" and try pushing things too far. Many aluminum heads and valve trains have had to be replaced, due to hose failure. The repair bill can easily be $2,300 or more. All it takes is allowing an aluminum head engine to overheat once. Never drive a car that is overheating--have it towed.

Common causes of overheating:

1. Insufficient coolant and/or air in the system. When the system is cool, check the radiator and overflow tank coolant level. If the level is low, start looking for leaks. Check for deteriorated or damaged radiator and heater hoses; e.g. cracks, pin holes, and clamps that leak under pressure. Also, look for bulges and collapsed hoses. Attach a cooling system pressure tester to the radiator, if necessary, to help locate leaks. Pressurize the system to 16 psi. Learn how to properly purge your cooling system of air--this can be crucial in some vehicles. On many Hondas, for instance, fill the radiator with coolant, until it comes out of the opened bleeder bolt. It's important to follow the exact procedure for draining and filling the cooling system in the Owner's Manual--they do vary and it's very important. Failure to do so can result in a warped head, blown head gasket, and the engine seriously overheating.

2. Air flow problems--leaves, bugs, dirt, feathers, or plastic bags blocking the flow of air through the radiator. Gently clean the radiator with a garden hose.

3. Radiator fan not operating properly--check fuses; Engine Coolant Temperature (ECT) Switch, not the ECT Sensor; and radiator fan relay. The cooling fan not operating properly is one of the most common reasons engines overheat. If the coolant is low and/or there's air in the system, the ECT Switch may not turn the radiator fan on.

4. Radiator shroud does not seal tightly enough for the fan to draw air through the radiator. This can cause the engine to overheat, while the vehicle is stopped.

5. Faulty radiator cap--doesn't maintain proper pressure or one or more valves are bad. Test cap relief pressure with a radiator cap tester (KD 3700 or Stant 12270). On non-expansion tank systems, which are most systems besides Ford's, there are two valves inside the cap that can fail. One allows hot coolant to flow into the overflow tank, the other allows coolant to flow from the overflow tank to the radiator, as the engine cools. If the coolant can't flow back into the engine, there won't be enough coolant and the engine can overheat. It's best to use a high quality, preferably OEM, cap.

6. Faulty thermostat--sticking closed. Improperly installed thermostats are one of the leading causes of Hondas overheating.

7. Radiator core blocked--mineral build-up (phosphate scale) and corrosion are the most common causes.

8. Inoperative water pump--pinch the top radiator hose closed with your hand, while the engine is idling, then release it. You should be able to feel a surge of coolant, if the pump is working properly. Change the water pump every time you change the timing belt.

9. Improper grade of engine oil--higher viscosity oil generates more internal friction and heat. Synthetic engine oils have higher coefficients of heat than conventional oils, which allows bearings and engines to run cooler--use them.


In an emergency, set the heater to maximum heat, blower to high, and open the windows. The heater will act as another radiator and help lower the engine temperature.
8. Antifreeze: Automaker Approved Chemistry

Using the wrong antifreeze in your vehicle can be a costly mistake. No "one size fits all" antifreeze is approved for use in all vehicles. Play it safe and consult your Owner's Manual. Use a 50/50 mixture of approved antifreeze and distilled water, in order to minimize deposit buildup (cheaper and better). Otherwise, buy pre-diluted ready-to-use coolant, which is a 50/50 mixture of antifreeze and demineralized water (more expensive, but convenient).