In December 2015, my 2004 C-class wagon overheated (temp light came on, nothing dramatic) for the very first time while stuck in traffic for 30 minutes. I had it towed to a nearby MB dealership (not my regular shop) where they were unable to replicate the problem. For them the electric fan kicked on at 110 degrees C. They threw their hands up in the air, but replaced the thermostat and did a cooling-system flush. For $1200. Car running fine since then.

Well, last week I was stuck in traffic again (for the first time since the December repair) and again it overheated. I had a bad feeling that it might, and it did.

The shop that did the "repair" in December maintains I must have some new problem, which I don't believe.

I am not losing coolant, there is no oil in the coolant or coolant in the oil, or white smoke out the tail pipe. Water pump is new as of 4/15, radiator new as of 7/2011. Original reservoir cap.

What might it be?

EDIT (Additional information):

To answer your question about small-claims court, I am in NY and the shop is in NJ. I am seriously considering taking them to court, depending upon what my garage discovers.

I know for a fact that they did not check the temperature gauge for accuracy, nor did they use an IR thermometer to evaluate the radiator. They did not replace the cap on the radiator or the cap on the overflow tank. Simple things that were not done.

I evaluated the fan myself today. With the A/C on, the fan kicks on at a high speed shortly after I turn on the A/C (within seconds). When I turn off the A/C, the fan stops. When I let the car idle w/o the A/C on, the fan will kick on at ~100 degrees C, and run until the temp goes goes back down to ~90 degrees C. It will repeat this cycle as the car idles. I could not get the car to heat up to the red zone while running, so it has to be an intermittent problem that exists only under extreme conditions.

It did seem that the fan ran at a higher speed when the A/C was on.

  • 1
    Check the coolant mixture with a tester, it should be 50-60% coolant. Its possible you have an intermittent issue with the fan operation. Next time it starts to overheat, pull over and see if the fans are running on high, some cars have 2 fans and both will come on at a high temp if one does not that may be the issue.. If the AC is on a fan should run full time. Hard to believe a dealership cannot properly diagnose the issue.
    – Moab
    Apr 26, 2016 at 23:59
  • I, too, would expect the dealership to diagnose the problem, especially given what I paid them. Distraught female, two hundred miles from home and they cashed in big.
    – MelissaMH
    Apr 27, 2016 at 3:09
  • Thanks for the fantastic level of detail provided. This is a great question.
    – Zaid
    Apr 27, 2016 at 18:41
  • 1
    I need a great answer!
    – MelissaMH
    Apr 27, 2016 at 19:54

3 Answers 3


One important thing here. What exactly do you mean by "the engine overheated"? How do you know that? Did a light come on? Did the gauge go into a warning zone? Did the A/C quit working altogether (that's by design.. ) Did you see steam coming out from under the hood? How do you know its overheating? I'm not saying you are wrong, I'm just asking. Note: if we're talking temperature here, normal coolant temperature for your car should be between 180 deg F and 235 deg F or so. Just because the temperature is greater than 212 def F doesn't mean a thing. Thats totally normal. Remember the coolant system is under pressure @13 psi and water at that pressure won't boil until 245 Deg F or so.

Was the air conditioning running when you were stuck in traffic? How hot was it outside? What are the results of the idle / wall test? the idle wall test, is you park your car 18" from a brick wall / garage door, turn on the A/C, leave the car in idle. (If you were a mechanic hobbyist, I'd ask you to read the refrigerant head pressure at the inlet to the Condenser in front of the radiator while doing this test.)

  • So, there are a few different things that turn on the engine cooling fan. Its quite possible that something is askew here.
  • The set points for the engine cooling fan are totally different if the air conditioning is engaged. The air conditioning is set to run in automatic mode (when asked by the user) or automatically in Defrost or defog mode (often without the user's knowledge.) In some cases the A/C system will turn on the engine cooling fan without any input from any other temperature related sensor. So its possible that one of the sensors is not working or has a bad wire to it.
  • Its also possible that you've got poor sealing between the radiator and the fan shroud that surrounds the cooling fan. Those seals are WAY important when the vehicle is at slow speed (like being stuck in traffic jam.)

If you are not a do-it-yourself person, have your mechanic do the idle / wall test. I'm quite sure the defect will repeat. Note: normally that test takes 30 minutes.

Edit: It's also possible your radiator isn't cooling as well as it should. It could be clogged internally. One key here is when you add coolant it should be a 50% / 50% mix of antifreeze and store bought distilled water. You should never use water from the tap. That's just bad, way bad. Did the guys inspect the core inside the radiator for contamination and blockage (I'm assuming with a camera endoscope. I gotta believe the Mercedes dealership has one. They're not crazy expensive.)

Again to summarize: Low vehicle speed cooling is totally controlled by the fan pulling air thru the radiator. That fan could be awry (coming on too late, or too slow, because of bad controller, or a faulty temperature sensor reading incorrectly). You may not be getting the outside air from in front of the radiator to flow thru the radiator (which is why the seals around and in front of the radiator are so important. You could have blockage, A/C condenser in front of the radiator full of dead bugs, or aftermarket fog lamps in the way or a missing ground effects shield below the engine.) And finally your radiator may not be cooling things off well (plugged radiator, coolant flow issues in engine or hoses, bubble in system not bled off correctly, etc..) I will say for $1200 I'd expect a better analysis from the Mercedes Dealer. And with the wall idle test ANYBODY can easily and quickly repeat the failure.

Where are you located? Is small claims court an option?

Edit #2, Apologies:

NY, huh. I figured when you mentioned degrees in C that you were in Europe or Canada.

I did a bunch of research and there is a whole lot of youtube postings and more on this topic. You've definitely got an electric fan and it definitely has a variable speed, pulsed width modulated (PWM) fan controller.

So it seems there is a high temperature warning light and a temperature gauge in your instrument panel cluster. You gave me numbers of the fan coming on at 110 deg C for when the fan came on (that's 230F and a bit high for my liking...) I would have thought it would come on at least at slower speeds at temperatures below that. I'm thinking you've got a faulty temperature sender in the engine somewhere.

So the A/C on = fast fan speed makes sense. I know the guys in my design group did things that way. High speed fan regardless of engine temperature. They're trading off great air conditioning performance at the risk of a little bit of fuel economy.

Again, it sure looks like the engine temp sensor is putting out the wrong temp.

You know this is something you could totally test for yourself without a whole lot of effort.

  • You will need to spend $30 or less on a service manual for your car. You want to buy a paper copy, not an online subscription. I'd recommend Chiltons or Haynes or a factory service manual if you can find one used on Ebay or something.
  • I think they may be big $$ if you order an official Mercedes manual from the dealer. You might check on that cost. $300 wouldn't surprise me if they even offer it.
  • You are also going to need a Volt Ohm Meter (also called a multimeter) You can "borrow" one for Free at Autozone (they come with the fuel injection repair kit) or purchase one nearly anywhere. Cost between $15 and $50, useful for testing dead flashlight and remote control batteries when you get home.
  • Basically you are going to read the service book on cooling fan for your car and how it works.
  • They will tell you which sending units measure temperature and where those little senders are located on the engine.
  • With the engine off you can go to that sensor, and use the volt ohm meter to read the resistance. The book will tell you the correct value.
  • If the value is right it's good.
  • If the value you read doesn't match the range in the book for that given temperature that sensor will need to be replaced.
  • You will need a couple of large towels to lay on the fender on both side of the engine. No need to scratch up that beautiful car.
  • Now here's the bad news. I don't know where the sensor on you car is located. It's possible that you have to remove a few parts just to get to it. And removing parts often requires tools. I wouldn't expect anything crazy, normally wrenches, metric sockets and a screwdriver or two.
  • Additionally you are going to have to remove the wiring electrical connector from the sensor, and they normally snap into place. There should be a release lever molded into the plastic. The connector is just made of plastic and it's going be way fragile.
  • DO NOT JUST PULL ON THE WIRE HARNESS to remove the connector. You DO NOT want to break the retainer system on the plastic electrical connector. You are going to have to figure out which tiny part to push to release the connector. Its going to be a tiny fragile plastic lever molded into the connector. With any luck 'push' will be molded on it.
  • If the temp sensor is bad a new one should cost around $20 or so. (I just checked at Autozone)
  • Note: taking them out could be tricky, and messy so you may not want to do that yourself. You have to drain the engine coolant so when you remove the sensor coolant doesn't go all over the place. And you are going to need tools for that.
  • But at least you'll know, exactly how much the repair should cost. I doubt this would take a shop more then 35 to 40 minutes from start to finish, if even that.

Oh, yeah, and I'm still waiting for you to tell me that the under engine air shroud is in place and that you've checked all the rubber seals and plastic and foam seals around the radiator and air conditioning condenser (in front of the radiator) and that nothing is blocking air flow. Yes you will have to kneel on the ground and look with a flashlight. It should take around 7 minutes max. Do this with the engine off (for safety). Remember we up against a $1200 repair bill so 7 minutes of work is a lot cheaper than that. Add 15 minutes to test the temp sensor = 22 minutes of work to save you money and hassle later.

Knowledge is power. And on that note, if you decide to do nothing else, please do buy the service manual / book. It's a great way to understand how your car works.

This is a long posting. But hey, good thing I'm getting paid by the word count here. Awesome.

  • Here is some additional info for you: both times, the warning light came on, which comes on at about 115-120 degrees C in my car. There was no steam, boiling over, blown hose, coolant leak. The first time (December), the heat was on, and the ambient temp was ~30 degrees F. The second time (April), the A/C was running, and the ambient temp was ~70 degrees F. The dealership let the car idle (not near a wall) and the electric fan kicked in at ~110 degrees C. They threw up their hands and replaced the t-stat for and did a cooling system flush. For $1200.
    – MelissaMH
    Apr 27, 2016 at 3:05
  • What should I look for in the idle/wall test?
    – MelissaMH
    Apr 27, 2016 at 3:07
  • The idle wall test is just a repeatable test on low speed vehicle engine cooling. It allows someone to do a test and verify cooling fan speed at different temperatures. Question. Did this car come with an under engine plastic air flow shield? They are used to improve airflow and fuel economy. I've seen the shields knocked off when the car is driven around when there are three inches of snow on the ground. Possible you had the shield and it's gone now?
    – zipzit
    Apr 27, 2016 at 14:51
  • Do you own a service manual for this car? I'd like to see the specs on cooling fan output. $1200 repair is that at the Mercedes dealer? If you'd already paid that challenge the supervisor that the mechanic didn't check the spec page on cooling fan output.
    – zipzit
    Apr 27, 2016 at 14:57
  • It was, in fact, an MB dealership that examined the car. I have their detailed notes, and, IMHO, it was pretty cursory. I do not have a service manual, and there is nothing in their notes indicating that they looked at the specs on the cooling fan output. I've spoken with them a number of times, and all they're willing to do is to comp me for a new thermostat if the one that they installed proves to be faulty, which I highly doubt. I don't think the original t-stat was bad to begin with.
    – MelissaMH
    Apr 27, 2016 at 16:58

Clearly, there is an issue with heat rejection when the car is stationary.

There are several possibilities that could be contributing to this. Here are my thoughts:

  • lack of air flow

    A fan that doesn't turn on when it should is the obvious go-to reason. Run the car up to temperature at idle with the AC running to replicate the overheating scenario and make sure that the fan(s) is/are running by seeing/hearing it.

    • if it runs, I would want to rule out a fan relay that stops working when hot. If that checks out OK, I'd say that the fan is doing what it's supposed to do and the problem lies elsewhere
  • lack of coolant flow

    Your water pump is new and the radiator is slightly old. It may be that you have a collapsed radiator hose that is restricting coolant flow. A clogged radiator is also possible, but not probable.

  • excellent point. Water pumps suck (er... they "pull" coolant from the bottom of the radiator), so a soft inlet hose to the water pump could definitely be an issue, and that's in an area we can easily overlook! Good catch.
    – zipzit
    Apr 27, 2016 at 19:17

So, I thought that I would give everyone an update, in case you're interested.

For the past month, I've been driving the car around town w/o incident; the temp always at ~85 degrees C. But I don't have NYC traffic where I live, which means that the car would be forced to idle for no more than three minutes.

Today it went into the shop. For them, the fan did not kick on as the car was idled. The SDS did not throw a code of any sort. They decided to replace the coolant temp sensor (before the fan) given that I have seen the fan working under operation. This is the cheapest/simplest repair to do at this time, given everything else that has already been done (much cheaper than replacing the fan).

I'll keep you all posted!

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