My 92 400SE oerheated once last year, I replaced the cap, refilled the radiator, and it run ok until now, it was about 90 degrees out and I was running the AC, and it overheated again, what am I to do?Why is it overheating? It only has 120,000 miles, and it is all well maintained.
What should I look for?Ideas please, this is really bugging me!!!!!!!

Thanks to you all

a 92 vehicle, I believe you have a W140.

I had the same problem before and it turned out to be a faulty water pump.

The next I had after the pump replacement, I meant many months after the replacement, was the fan clutch. As the car ages, the clutch ages and had to be replaced to restore the effeciency of the clutch operated fan.

Other things you should also consider:

1) your thermostat. Has it been replaced last time you flush the system?
2) Your aux. fan. Are they working when the car reaches 100C? or intermetly when the air/con is turned on?
3) Is you collant the right mix?

Perhalps you should describe a bit more about your problem, i.e. time vs temperature rise before we can say anything. Any one of the above or all of the above affect the cooling system.

cheers

The car overheats when I am driving on a hot day when the outside temp is over 90
and I have the AC on, I checked the temp gauge and from cold it goes up to approx 100 and then it drops to around 80. That is why I did not think it could be the thermostat, I have not replaced the fan clutch, or the water pump. Should I start from replacing the thermostat? and then fan clutch, or the pump first. What is the proper mixture for the coolant for this car?

Your car is functioning properly. It is not overheating.

Hitting 100 degrees is not close to overheating.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

I think the part of the problem is the large temp swing that I also experience swinging from 100C at stop to ~ 85 C while at highway speed. So far I went through 3 thermostats. I have also changed the radiator with Behr not nissen. The fan clutch is very pricy to just swap.

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2010 ML350 Bluetec
2012 Mustang Convertible

I was driving on the freeway with the AC on and the red ligth came on, as I opened the hood the radiator fluid was coming out of the container, and boiling over. I waited for a long while added almost 1 1/2 gallons of coolant and drove away, this time with the AC off, and it worked fine. But I have not driven it since, until I do further checking

On my 190 the engine driven fan clutch tightens at about 100C and the two auxillary electric fans engage at high speed at about 105C, so temps up to 105C are normal - very hot weather and low speed like traffic congestion.

If you are boiling over before 125C the pressure cap may be defective. It should hold at least 15 psi. Any auto parts store should have a tester to test its pressure holding capacity.

If you have been using green antifreeze it's possible that the radiator is clogged up and its heat transfer efficiency has gone to the dogs.

The best antifreeze to use is Zerex G-05, which is the same as MB antifreeze. Dexcool is a good alternative. Green stuff can ruin your radiator in less than ten years due to the salts and silicates preciptating out and clogging the tubes.

If it's overheating on the freeway at cruise speed the thermostat may be defective.

One test you can do is to let the car idle with the AC on max and sunroof/windows open. When it gets up to 100 and you rev the engine you should hear the engine driven fan roar. At about 105 the electric fan should come on. Also, monitoring the coolant outlet with an IR gun and comparing to the gage temp reading will tell you if the sending unit is defective.

Duke

I will start from replacing the thermostat and using the proper coolant, I do have a new cap, but still overheated. I am using the green coolant, that is what it had when I purchased the car 6 years ago, it may also be the fan clutch, but I will start from the cheaper part first,

How often have you been changing the coolant?

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

I have replaced it only once last year when it overheated the first time.

replace the coolant every three years as a precautionary measure

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currently just 1989 300SE 185k miles

Exactly.

Keep in mind that your car's maintenance schedule requires that you replace your brake fluid every two (2) years and radiator coolant every three (3) years.

If you have been lax on your coolant replacement schedule, as well as running the green stuff, your radiator could be prematurely plugged up.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

seriously, its normal for the car to operate @100. MB spec is that it should operate over 80 degrees and the aux fans DO NOT come in to cool the car until it reaches 100. Even at 100, the aux fan is operating at LOW and stops when the car cools to 95.

I think its higher like 106 and than 11? the fan will spin at higher speed and when its a real concern.

from the sound of it, your car is normal with city traffic at 100 and highway at slightly over 80.

It seems its the cap not holding the pressure, remeber boiling point changes as pressure changes, or your radidatoy is really stuffed... While you have the system flushed, change the T-stat anyway... its cheap and might as well do it item.

the fan clutch, just hand push your fan, with engine off of course, and see how many cycle it turns before it stops. if it swing freely, u are most definitely requiring it to be changed. if its not many cycle, like a 1/4 or 1/2 turn, than you are fine. obviously this depends on how much force you push it with.
BTW, you know which is the clutch fan and which is the aux. fan. Right?

Yes I know what to do, I will try the different options I have, this weekend, I hope I get to the problem. I will run the different tests suggested, flush the radiator
change the thermostat, check the fans etc...Thanks for all your advise, I really appreciate it.