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Discussion Starter #1
Ok, I have a weird one here. My 2000 SLK 230 overheats when the heater is not on. In other words, If I turn the dial to air conditioning it slowly starts to overheat. The Fans are working as they should and are on full speed during this condition. I have changed the water pump, I have not changed the Auxiliary water pump.

When the car is overheating if I turn the heater dial to heat, it almost immediately returns back to normal operating temperature.

Any ideas?

Also what is the purpose of the Auxiliary water pump? Is it also a bypass when the air is on?
 

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Premium Member 2014 SLK55 AMG
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I'd start by looking at the thermostat.

The pump is so that you can get warm air for about 30 mins after the engine is off.
 

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Normal Operating Temperature ?

Greetings all,

At the suggestion of the local MB dealer, I recently replaced the electronic cooling fan located at the radiator. My car had always operated at just a hair over 80c in all conditions. It would reach this temperature in a reasonable amount of time (5 minutes), with normal cabin heat prior to reaching 80c.

The dealer diagnosed with SDS using percentage control, finding that fan was inoperable until 30% fan request is sent. Recommended that fan be replaced. I did so myself. It was an easy job, and saved on the part ($235.00 OEM) as well as labor (dealer wanted $350.00 for the fan; and $270.00 labor).

Along with the change in warmer weather, my car seems to want to run at a higher operating temperature, and did so prior to the new fan. This would be 90c to 95c.

Have not been able to identify or locate an absolute temperature or range that is normal for this SLK 230/170.

I have read all of the threads I can locate regarding this subject, with many findings for the 171's.

Any suggestions or recommendations? This is such a great forum filled with knowledgeable and experienced owners. I have already learned a lot, and will be returning to the dealer tomorrow for another diagnostic analysis, and will report the findings.

Thank you,
Kent

p.s., recently purchased some wheels and tires and am waiting for the fan issue to be resolved before changing
 

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Discussion Starter #4
Ok so I replaced the thermostat even though the symptoms didn't indicate that's the problem, same condition exists. another thing I notice is that when I am driving on flat roads I have no problem, but a slight hill will cause the temp gauge to start to climb. Coolant levels are full. Turning on the heater will rectify the problem. Now if the car was truly overheating the heater should not solve the apparent overheating so quickly. I am wondering if my sending unit is playing tricks...
 

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Premium Member 2002 SLK230K
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It almost sounds like your system is air locked. When you changed the pump did you bleed all the air out of the cooling system.?
 

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After checking to assure that you don't have a air bubble trapped in the cooling system as mentioned above; I would have the radiator cap and cooling system pressure checked - to assure that you aren't losing pressure somewhere.

If all that checks out -

I would pull the radiator out - take it to a good radiator service shop and have it boiled out, rodded out and pressure checked. It may be time to replace the radiator core.

Turning the heater on - adds a quart or so of additional fluid - plus the heater itself is nothing but a small radiator with another small fan blowing air though it. So it helps cool the fluid in the system.

Turning the heater on - seeing the temp go down - usually indicates that the radiator isn't operating at the efficiency level that it should be.

FWIW,
Carl B.
 

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With regard to cooling systems and temperatures, I have gotten very hands on with the cooling system of this car. Having experienced similar issues with various aluminum and alloy based engines of all types, I decided to take the obvious route and address the cooling system rather than sensors, etc.

Number one...I flushed the system, draining the radiator, expansion tank and block. Over the years, my car had evidently "accumulated" various types of coolant. I purchased the appropriate MB coolant, and yes it was 3 times more expensive than the most expensive "off the shelf" brand. However, I have learned through their oil filters alone...they produce a superior product.

I have always run aluminum based or alloyed engines with a good coolant (previously Advance, due to low levels of silicone*), and distilled water w/o sodium. No need to explain.

Furthermore, the percentage of coolant to water mixture, will additionally have an effect upon your operating temperature. If your mixture is more than 50% coolant, you can count on a higher operating temperature. Any coolant will disperse heat slower than a more purified form of water. A mixture of 40% coolant to 60% distilled water works best for my vehicle. The gallon of MB lifetime coolant I purchased (as well as my operator's manual), recommends no more than a 45% coolant to 55% water mixture.

In addition to these other factors, my expansion tank float had become defective, due to the rubber seal at the top of the "float/pellet" becoming dislodged. This made "sealing" the tank (radiator), cap when the coolant level was filled...impossible. New tank with float...53 bucks, eyes closed replacement.

Silicone Dropout. A local "radiator" guy wrote an article for Car & Driver years ago about the high levels of silicone in "aftermarket" coolants. Even the cleanest radiator core, with perfectly clean tubes can have silicone dropout. I have experienced this with a 2600 Alfa Romeo. After cleaning, flushing, sonic cleaning...the only answer was silicone dropout, and a new core. In this case, with the same mixture of distilled water, and Advance coolant, the engine ran at it's "normal" operating temperature.

With this in mind, the suggestion of a re-core would not be unreasonable. However, the lowering of operating temperature from engaging the heater is not in and of itself an indication of a faulty cooling system. Any engine with a properly operating system will do the same. In most cases you are simply increasing the number of radiator cooling fins by 1/4. It is partly why there are single; double; and multiple "rows' of cooling fins within various radiators.

With regard to my SLK 230...after these "corrections" and learning the thermostat temperature for this "model" is 87C, my car runs at 82C on the open road...and 90C - 95C in stop and go traffic. As it should. The local dealer confirmed this. The cooling fan comes on at various "percentages" depending upon what the engine is "calling" for.

I found all of my sensors including the temperature gauge (which was checked with an internal thermometer), to be absolutely correct. This also included the coolant level sensor.

Best,
KS
 

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Great posts! Lots of good insights and you have also put my mind at ease, (at least on coolant temps!) as I've not had my slk for long and am still learning about it.

I also have a 2000 slk230, it has 137,000 miles on it. My temperature is around 82 degrees on an open road doing around 60mph and 88/89 degrees in heavy traffic. I can't provide details on my coolant mix as this was last checked by MB dealer after I bought it seven months ago and Im not sure if they even had to top it up at that time. There was no additional cost on the bill but I was paying them 250 for a service / check of the vehicle so maybe any minor top up would have been included.

All the best
 

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some good advice on here- I would def see about maybe a radiator/water system flushing product, (bars leak) and flush the system out first and first ensure you have replaced coolant/antifreeze to correct OEM spec.. maybe your service book might give you an idea when coolant was last replaced..
 

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2000 SLK running temperatures

TO: Gee23,

From what I have learned from my own investigations, it would seem that your temps are fine. Along with modern engines running at higher temperatures, I have learned that these cars normally operate at the temperatures you describe. My car is very clean and newish with 67k, and runs like a top. However, when I really started to observe the temperature gauge, I became concerned due to my previous experience with similar engines. I have owned many cars with heads; block or other things that were aluminum, or aluminum alloy.

I had believed that my car always ran at 82C. I have learned it does vary. But within 15 degrees C.

If I find myself in very hot, stop-and-go-traffic, my car wants to creep up to 100C. For me...my Maserati 3500; Alfa Romeo 2600 Zagato Coupe; E-Jags; or 1967 Z-28...that's freaking boiling, and WAY too hot!!!

Even the fellow who wrote the article for C&D, recommended a 195 degree thermostat for every engine. This was because at 195...the radiator had ample time to cool the coolant, and keep the engine running at a typically lower operating temperature.

So...with our SLK's having a 87 degree C thermostat, guess what they're running at? About 190 Fahrenheit. In other words...spot-on for any actively running motor with this type of performance.

My first or simplest recommendation might be to completely flush your cooling system. Though I have been pegged as OCD, I chose to flush my car with distilled water. It's less that a buck a gallon, and is really what these engines were made to accept....along with the OEM MB coolant. Get the crap Prestone, Peak, or whatever is in there..out of your engine!!!

Purchase the MB OEM product, and regulate your coolant mixture 40-45 % coolant to distilled water.

At least with these improvements, when your engine runs at a temperature that seems to be high, you can be assured it is within manufacturer specs. For example, the service manager at my local dealership informed me that every MB...from new SLS, to Mclaren; to SLK...runs at the same temperature mine does. 80 - to 100 degrees C.

This service manager at Zimbrick European, Jody Nelson, said that if I get to temperatures of 120 -140 degrees C...turn my heater on, and come see him!

With every good wish,
KS
 

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Thanks!

Thanks for sharing your struggles with this! I recently have noticed the same issue with my 98 SLK230 that I have had for 6 months. Knowing the average temps and performance of the car will keep me from spending more time at the Mercedes repair shop!
 

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I believe slightly above 80 in all weather is normal. This has been the case with mine over the past 2.5 years. anything over 90 might be running hot in my experience. (1999 SLK 230)
 

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Hoses get old and will collapse under pressure. Check hoses under running condition. I had one that looked good when I was under the hood but when the car started it would suck in flat.
 
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