My car is being pegging at 120 F, all fans including auxiliary turn on and stay on, with or without A/C, I have tried turning the heat up to highest to cool it down but still temperature stays at 120. Outside temperature has been around 82 F

Could this be a bad/stuck thermostat?

S500 97

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Bluerio

It could be a couple of things. The first that come to mind are a bad thermostat, clogged radiator, or bad temp sending unit. Does the needle climb as it warms up, or always at 120? Reason I ask is you say always. With the auxiliary fans going on high, that's a good sign of it running too hot, because mine never come on without A/C running. My '93 runs just over the 80 mark after warm up no matter what, sitting or on the highway. You really don't need to drive it that hot as prolonged overheating will blow headgasket, warp or crack the head, and cook countless electrical parts and wires sitting close or on the engine.

Are you losing coolant? Any water in oil or oil in water? White exhaust?

Thauber,

Initially everything is normal once car is star up the car gradually warms up, but around 87 F, the auxiliary fans kick in, and the temperature continues to go up pretty fast to 120F where it remains as long as the car is moving but if I stop to idle it can go a bit higher. Like I said it does not matter if I turn the A/C on, or the heater, the temp still been climbing to 120F

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Bluerio

I imagine if the thermostat is stuck closed then the radiator will not get too hot. So that might be one test. I have never seen a t-stat get stuck closed but I guess it could happen. I have seen radiators get clogged, and I have heard of water pump impellers wearing away...

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Let's face it, with such a sound logic train to follow who could disagree?

I would lean toward the water pump.

I've also never seen a thermostat get stuck closed - although this is a posibility.

When was the last flush done ? You can feel the radiator and see if it warm all over - there should be no cold spots if it not clogged.

Look around the water pump pulley and see if there is any water leaking this is a classic sign of a bad pump.

Thank you all for your input.

I went ahead on Saturday and changed the thermostat; it was not an easy job due to it being in a very tight spot. I drove the car to run errands on Sunday (outside temp 87F) and so far everything is working normally the car never overheated, the A/C, ran perfectly.

Again thanks

Oscar

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Bluerio

It might have been that it was not full of coolant and that's why changing the thermostat corrected your problem. MB thermostats are designed to fail open.

it seems the thermostat failed in the closed position. No overheating damage so its timely rectification.

clearly the problem is your thermostat...replace it...go to AutoZone or whatever and get one, and a gallon of anitfreeze and replace the thermostat...like $30 out the door...just do it

Reading comprehension.

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1991 300E - 212K and rising fast...