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Old 10-02-2003, 07:34 PM
Registered User
Join Date: Sep 2001
Location: Santa Clara, CA
Posts: 18
Question 1997 C36, W202 Chassis, overheating, or normal?

Hi All,

I am a longtime lurker, but have not posted much. I have learned a great deal about my car here, and don't recall this particular question raised, so i hope someone can help me determine the answer.

I have a 1997 C36, with 102,000miles. Recently my engine appears to be running warmer than usual.

Now I know there is debate on what is considered 'normal" engine temp for a MB, so i suppose this is based upon personal experience for the car.

For my car, i recal the hottest it ran is not more than 90 degrees. Typically 80-82 degrees in city driving, with a slight rise to 84...but would soon cool back down to 81 or 82. Basically the needle would not move past the half-way mark between 80 and 100. Usually at the middle or between middle and 80.

On the freeway, the engine usually ran no hotter than 82 degrees. When stopped from a half-hour drive on the freeway, the car would rise to about 85-ish, then drop down to 82.

At idle, the engine would slowly fluctuate from it appeared to be the result of the cooling system working.

Lately I notice the engine staying more towards 85 and 90 degrees at idle.

On the freeway the needle stays around 85- 83degrees.

Recently, the needle shot up to 120 once I stopped after a 1 hour drive from the freeway (drove around 80-90mph). During that time the needle was at 100 degrees while driving....

When i got out of the car with the engine running, the fans were on full blast. I turned off the engine at 120 and let it cool. I checked the engine bay and the coolant resivor was full. Oil was ok.

Fans were screeming when I turned the engine back on 15 mins later. Engine temp was 115.

Towed the car back to my mechanic and he found the thermostat faulty, so he replaced that. He did not find any other cooling system problems.

Now the car is not overheating, but is still running around 91- 85 consistently.

I would like to know if this is "normal" or a symptom of other issues I need to investigate.

Because my engine ran cooler before, i know the car is not running as it used to.

Rgearding the last time I had my coolant flushed, it was approx 4 months ago from the dealership. I do remember the car running a tad warmer than usual afterwards, but did not think about that much until these recent overheating issues.

I read this thread: Thermostat

And thought it was very informative, but I thought it was odd for a w202 to run 90-100 degrees and consider it "normal"!

So that's why i'm not sure what is "normal" for my car.

If this is still within normal operating temperatures, is there anything else I can do to cool the engine further? I read biasing the ratio in favor of more water than coolant helps for example...

Anything else that can be done?

I suppose I consider "normal" to be no hotter than 85 degrees on a hot summer day with AC, or after driving the car hard. Otherwise the car was almost even with 80 during light driving or on a cool day on the freeway. My car used to be this way.

Any advice on how to troubleshoot this is much appreciated. Also, any feedback on what is "normal" engine temperature for my car is also appreciated.

Thank you!

Last edited by kameraguy; 10-02-2003 at 07:41 PM.
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Old 10-02-2003, 11:04 PM
Registered User
Join Date: Mar 2003
Location: Southern California
Posts: 2,010
The OEM themostats are designed to BEGIN opening at about 87C, but there is several degrees tolerance to this and they can drift over time. My '88 190E 2.6 used to run 90 on the freeway in nearly all weather. Now it runs 80 at up to about 70F ambient and 90 if it gets up to 90. I not worried about this behavior.

The purpose of the thermostat is to regulate the lowest operating temperature. The highest is controlled by the fan algorithms.

In traffic in hot weather it can get up to 100 or more. After my last emission test, I started up the car the auxillary fans engaged at high speed and the engine driven fan was roaring. It quickly cooled once I started driving. If you shut down a hot engine at, say 100, and start it up a few minutes later it can be 105 or higher. This is due to heat soak. The residual heat in the engine will continue to heat the coolant after the engine is shut down, but soon after you start the engine the water pump, fans, and radiator will quickly reduce the coolant temperature.

Engine temperatures between about 80-110 are normal depending on ambient temperature and how fast you are driving. When the auxillary fans engage at about 105C the engine should radidly cool unless it is very hot, like summer in Phoenix.

Radiators lose heat transfer capability as they age. The best way to keep this degradation to a minimum is to use MB Antifreeze, which is available commercially as Zerex G-05, or use Dexcool. Green antifreeze will cause radiator deposit buildup and more quickly reduce the heat transfer efficiency of the radiator.

As cars age operating temperatures may differ under the same conditions as when the car was new. This can be due to "drift" of themostats, temperature sensors, the gage inself, and the slow degradation of the radiator. No doubt the alignment of the planets in the Zodiac is also a partial determining factor.

Normal operating temperature is 80-110. Freeway driving in cool weather will keep the temp near the bottom of this range. Low speed driving in hot weather may cause it to go to the upper end of the range. If the car consistently operates at the high end even in conditions were it should probably be lower then one should verify coolant level, radiator cap pressure holding capability, and fan function, and if these all check out the radiator is suspect.

It sounds like replacing the thermostat solved your problem.

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Old 10-03-2003, 03:25 AM
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Join Date: Jan 2003
Location: USA
Posts: 699
Dear kameraguy:

85-91 range is perfectly normal for your car. The new thermostat actually did fix your overheating problem.

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Old 10-03-2003, 12:19 PM
Registered User
Join Date: Sep 2001
Location: Santa Clara, CA
Posts: 18
Thanks very much for the replies guys. I appreciate the prompt response.

Because these cars cost a pretty penny to repair, i'm overly cautious when it comes to these things. After suffering trhe infamous M104 headgasket leak and 1997 5-speed transmission failure, the last thing I need is a damaged engine!!!

I'll consider checking/replacing the coolant cap, and re-flushing the system using Mb Antifreeze as Duke explained. I question if the dealership properly did this, so i better do it myself this time.

If Radiators aren't to expensive to replace, I may also consider this as well. Might not be necessary, but can't hurt on a car with over 100K on the clock.

I've never seen my car go over 90 degrees prior to the t-stat issue, so maybe i was lucky my car ran no hotter than this. But as I learn more about MB, it seems 100 and above is also normal. I personally find this disconcerting! But I suppose i'll have to accept this as a truth with MB cars.

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Old 10-03-2003, 04:17 PM
Ali Al-Chalabi's Avatar
Senior Member
Join Date: Nov 2000
Location: Knoxville, TN
Posts: 1,837
Perfectly normal as stated.

Thermostat is supposed to begin to open at 87 C and be fully open by 102 C.
Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record
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Old 10-03-2003, 05:51 PM
Registered User
Join Date: Sep 2001
Location: Santa Clara, CA
Posts: 18
Are there thermostats which open at a lower temperatures, say 82 degrees, which can also work in a w202?

Or are there any other ways to reduce the engine temp further?
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Old 10-03-2003, 06:03 PM
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Join Date: Jun 2003
Posts: 104
As most guys say, your temp is normal. In the past I have had engines running more over than under 100 and that was normal as well, but then it was all steel and no aluminium in the machines.
Anyway, my 36 runs between 85 and 95 and always has.
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Old 10-03-2003, 06:49 PM
Registered User
Join Date: Sep 2001
Location: Santa Clara, CA
Posts: 18
Thanks for the feedback AMG. I guess I should count my blessings I have not hit over 95 degrees driving normally, or even hard.
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