2006 Hyundai Sonata tries to overheat

I believe you, too, but from what you've written, I'm not convinced it's abnormal. Here's why:

The temperature the coolant must reach for the coolant fan to start running is higher than the temperature for thermostat opening. When you're doing highway driving, you either have high coolant flow (due to engine rpm) or no coolant flow (thermostat shut) and high cool air flow across the radiator. When you stop, your coolant temperature builds to the point where the fan starts to run. If your thermostat was shut at the time, you will have likely built up a significant amount of heat in the engine, and the low coolant flow (due to low engine rpm) will take a little longer to create cooling, possibly allowing the temperature to increase before it actually lowers.

If you see your fan operate, it's doubtful your fan is the issue. Your fan is pulse-width modulated by the PCM, effectively meaning that the PCM controls the fan speed. If the coolant temperature were to continue rising, the PCM would speed up the fan motor as necessary.

Exactly how high does the temp gauge go? I've had three sonata and all three of them the normal temp would be just below the half way mark. When they got hotter than normal in stop /go traffic on a 100 degree day, it would still only go just a little over the half mark. The over heat mark is the last 1/10 of the gauge.

How high does the gauge rise? The temperature will definitely increase while idling in traffic. This is normal. Unless if goes really high, I don't think you have to worry. You should also see it cycle a little as the fans cycle on and off.

Matt

That sure reminds me of a coolant flow problem.

You may have trapped air in the system, which can cause all sorts of weird things to happen. I have seen radiators that look full, but have trapped air. Trapped air can cause mild overheating, false temp gage readings, and the thermostat to not behave exactly as it should.

Have you tried burping it?

(Don't know if your cooling system is subject to trapped air, but I KNOW some are!) -

Bob

Thanks for the input.

This only happens maybe once every few weeks, and when it does the temp gauge will go waaaay higher than at any other time. Many times getting off the freeway does not raise the temp reading hardly at all.

The fan will cycle on and off even when the temp gauge is far about above the normal value (slightly below horizontal). So the computer sees a normal cycle of temp increase, thermostat opens, fan comes on for 45 seconds or so, thermostat closes, fan turns off, all the while the temp guage is high or very near the red mark. I want to think the temp guage is wrong, but this situation will last for 10 or more minutes, before I get nervious and drive off. The temp will then come down to normal and stay that way. If the temp guage is wrong, why is it wrong only some times and why does of 'fix' itself when driving off? I'm not sure what in the act of driving off is helping: the extra air coming into the radiator through the grill, or the 1800 engine rpm (increased coolant flow over idle).

After taking temperature readings of various locations on the engine and around the cooling system, I do think the temp guage is valid in reporting a higher engine temp than normal, at least to some extent.

I agree that near the red is cause for concern. But if the fan is cycling while the gauge remains near red, it does lead me to believe (as you suggest) that your coolant gauge is probably not reporting the correct temperature.

Your coolant temperature sensor actually incorporates two sensors, one for the ECM and the other for your gauge. Presuming that your coolant temperature is normal it's possible that the gauge portion of the sender may be faulty, that there is an intermittent short in the wiring, there is some sort of ground circuit feedback, or the cluster is faulty. Probably the least likely scenario is shorted wiring, since a short to ground would put the gauge in the red.

This is one of those situations where a scan tool is an invaluable aid for diagnosis. With the scan tool, the technician can monitor coolant temperature through the ECM and compare with the gauge while driving the vehicle. The largest issue is that it happens only once every few weeks. Since it hasn't yet been determined whether the coolant is too hot or the gauge is in error, the dealer will need to duplicate the condition to even know what sort of problem they're addressing. If you're not ready to leave the car at the dealer for a tech to drive it to and from work for a few weeks, the chances of arriving at a correct diagnosis are very poor.

For now, I'd mention it every time I took the car in for other work, but realize that there's little the dealer can/will do without duplicating the condition. It's possible you may get lucky while it's there. The good news is that even if the gauge is accurate, you're not overheating if the needle isn't in the red. While you report a gauge position that's cause for concern and indicative of some sort of problem, it doesn't warrant immediate emergency action (such as stopping to let the engine cool), either.

Sounds like your t-stat may be sticking occasionally. If the dealer can't diagnose anything else with some certainty, I'd consider changing the t-stat as they are generally fairly cheap - although I've yet to buy one for my Sonata so maybe Hyundai t-stats aren't cheap...

Matt

Matt, it's almost impossible for a thermostat to close once it's opened. The engine would have to cool down nearly to room temperature to make it close back up.

I guess it IS possible for a thermostat to be opening too slowly, though highly unlikely. I think the OP's laser thermometer would have caught that. -

Bob

Try burping it.

Even a little trapped air in the system can fool the temperature sending and sensing, and cause the water pump to work inefficiently at times.

Odds are low trapped air is the problem, but since your problem is unusual, you may get lucky.

*Let the car cool to ambient.

*Note the coolant level in the radiator.

*If there are significant large burps, and the coolant level drops, there's your problem. Run it until all burping stops for a minute or so.

Probably won't help, but it's a very cheap thing to try.

Good luck! -

Bob

Air/Burping-

The Mercury Villager and Nissan Quest are notorious for this. The dealer recommended practice is insane, and the older units come with an air bleed screw. The newer ones do not. Easy solution- park the car nose up and force the air out of the radiator while filling.

Another side note- as HT mentioned, ensure that the dealer documents on the RO each time you take it in about this issue. CYA my friend...in the event something catastrophic happens, you have it documented...

Steve

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replying to rick007, vella wrote: my car do thr same thing I have av6 hundai sonata run hot the same way