My car temperature keeps reaching 120 degrees celsius

Tiny
SCARFAZE
  • MEMBER
  • 1987 MERCEDES BENZ 300E
  • 120,000 MILES
Temp gauge is close to the red zone. What can be the problem and the solution? I have replaced the radiator, fan clutch, thermostat. My auxiliary fan comes on at around 105 degrees Celsius but my temperature still doesn't drop. I was told that the car has to run at 90-95 degrees Celsius. When I accelerate the car when stopped, the temperature drops a little bit. The temperature increases the most when it is in idle.
Thursday, September 29th, 2011 AT 3:50 PM

29 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
You might try a new radiator cap as they tend to go bad. You could have an internal engineprobalem as well causing this. You canalso get some coolant additive that will allow theengine torun coooler, it works pretty well. You also might have a water pump problem.
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Thursday, September 29th, 2011 AT 6:50 PM
Tiny
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I don't think its the cap because there is no leakage of coolant through the cap and the water does not boil. Also I put special Mercedes Benz antifreeze I think its g-05 and there is absolutely no difference. I already removed the water pump once and I noticed that the impeller was like brown in rust but the bearing is good and the impeller would move with no problem. When the car is running I touch the return hose and you can feel the water pressure and how hot the water is. The odd thing is that it doesn't overheat right away it takes a while, and it mostly happens when i'm idle. P.S: Thanks for the info. I really appreciate it.
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Thursday, September 29th, 2011 AT 7:14 PM
Tiny
HMAC300
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Radiator caps go bad because they won't hold the pressure that they are required to. Not because they leak. Any mechanic with a pressure tester or maybe an auto parts store can test to see if it is holding the correct pressure. If a cap doesnt' hold the correct pressure it causes a system to start to overheat or run hot.
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Thursday, September 29th, 2011 AT 8:20 PM
Tiny
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Alright. I will check it. Let me ask you one more question. When a car overheats will the water boil or are there cases that it wont? Because I noticed when the temperature in the car reaches a high level the water doesn't boil.
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Thursday, September 29th, 2011 AT 9:17 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
That is why there is a radiator cap, it pressurizes the system so as the temp goes up it has to go higher due to pressurization. With normal water boiling at 212 with a 15# cap it woldn't boil until about 245 degreesF. Ha this car been checked for a possible head gasket leak?
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Thursday, September 29th, 2011 AT 11:25 PM
Tiny
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Yes. There is no head gasket leak because there is no oil in coolant. And coolant smells normal. And if there was a head gasket leak my car would overheat a lot faster due to the fact that the oil is hot it would just heat up my engine even more. The overheating is not that serious because while I rev and drive the car the temp goes down. Ive repaired the whole cooling system myself. The only thing I havent replaced was the water pump because I saw the bearing was fine and the impeller was spinning well. I would have to test the cap but I checked it and it looked fine. The gasket was perfect and I would squeeze the spring and when I would let go it would return back to the original position.
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Thursday, September 29th, 2011 AT 11:57 PM
Tiny
MHPAUTOS
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I would consider having the radiator core tubes cleaned, a back flush is not good enough, the radiator core is made up of rows of tubes and they are all joined by the top and bottom tanks, so if say 20% of the tubes are blocked you will still get flow through the radiator but at a reduced rate and this can cause over heating, I would look at this.
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Friday, September 30th, 2011 AT 8:12 AM
Tiny
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It's a brand new radiator. I got it last week actually. I will have to look into the radiator cap and the water pump. Thanks for all your help I really do appreciate it.
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Friday, September 30th, 2011 AT 9:02 PM
Tiny
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I replaced the radiator cap and there is no difference. I did a self inspection of the engine when the temp gauge reads hot and I noticed that the car wasn't overheating. I actually touched the radiator cap with my bare hands and it wasn't hot it was warm. I also got a second opinion from a mechanic and he said that the water isnt boiling and the engine isnt overheating, but yet the temp gauge on the dash reads that the engine is overheating. Any ideas?
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Monday, October 3rd, 2011 AT 4:27 PM
Tiny
HMAC300
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First does this have the silicone filled clutch fan or is it a flex fan. The clutch fan may be bad and not working at idle an dlower speeds when it is supposed to. The flex fan should flatten out when you reve it. You could also try replacing the coolant sender, pic inclosed.
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Monday, October 3rd, 2011 AT 4:35 PM
Tiny
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This car has the original silicon clutch fan. The fan is working because when I stick my head to look at coolant level I feel the fan giving a lot of air. I also changed the fan clutch and when I rev the engine I can hear the fan gaining speed. I replaced the temperature sender for the auxiliary fan but not for the gauge. When a temperature gauge goes bad will it say that the car is overheating when its not or does it just go limp?
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Monday, October 3rd, 2011 AT 5:00 PM
Tiny
HMAC300
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It gives the wrong reading
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Monday, October 3rd, 2011 AT 5:22 PM
Tiny
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Yeah I would say that would be the problem because I have done everything I can do to keep that car from overheating. Do you know what is the operating temp for this vehicle? Because if I am using the AC when the temperature gauge reaches 115 Celsius it will cut off my ac and it will stop blowing cold air.
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Monday, October 3rd, 2011 AT 5:50 PM
Tiny
KHLOW2008
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You mentioned the engine is not overheating, you could be having a problem with the sensing unit or its wiring. Have they been checked?
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Sunday, October 9th, 2011 AT 5:08 AM
Tiny
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I believe its the sensing unit. Because I havent replaced it and the other temp switch that I changed was bad so im guessing the same thing happen to the gauge sensor
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Wednesday, October 19th, 2011 AT 10:58 PM
Tiny
LALLYGAGGER
  • MEMBER
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Have you checked the radiator with a touch of your hand on the fins to see if they are worm. Im having the same problem with my 300e and ive changed the pump and thermostat and changed rad. With a used one and still having problem. I just dont understand why mercedes has both hoses coming from the same tank of the radiator. Is there a block in there to stop it cycling within the tank to flow thru the fins
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Friday, November 11th, 2011 AT 7:14 PM
Tiny
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I changed the sensing unit and it still is getting hot the other day I noticed that the water started to bubble and my expansion tank gained a lot of water. I really dont know what is wrong with this car everything seems to work in order but it still overheats
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Monday, November 14th, 2011 AT 1:24 AM
Tiny
KHLOW2008
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The cooling system does not seem to be working correctly and if it overheats only when at idle, it is either the radiator cooling efficiently is below par or the coolant movement are restricted.

Apart from those you have done, check for clogging in the pipes, especially iron pipes which might get reduced in size due to accumulation of rust etc.

Is the radiator fan shroud in good condition? Have you checked for air restriction due to clogged fins of the AC condenser?
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Monday, November 14th, 2011 AT 12:18 PM
Tiny
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I have checked it looks fine and the pipes are clean. And everything seems clean. Like I said the radiator is brand new and I dont use the ac because it overheats my car even faster
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Monday, November 14th, 2011 AT 7:46 PM
Tiny
KHLOW2008
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Is the radiator fan shroud in good condition, or is it still there?
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Monday, November 14th, 2011 AT 7:59 PM

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