zrheath

My 2013 C300 Sport (w204 M276) has been overheating for months and I haven't been able to pinpoint exactly why. Parts are pretty pricey so I have been trying to figure out whats wrong before throwing parts on it. Well, I broke down and paid the dealer $120 to diagnose the problem for me so I didn't waste any money. They called today and said they couldn't say for sure. No codes, no error messages, no warnings. They weren't even able to get it to overheat after letting it run in the shop for 30 minutes. I will note that I havent seen it overheat for a few weeks since the weather has been cold, but it was very regular before that. They told its likely the thermostat or water pump. GEE THANKS! I KNEW THAT!

So I am here posting again hoping to get some more insight before I lose my mind!

Symtoms:

-Overheats ONLY at idle, usually when I have been driving and come to a stop more then 30-60 seconds.
-Much worse when the weather is warm and the a/c is running.
-I will stop the car, after 30-60 seconds the fan will start ramping up faster and faster, then the temp gauge will start rising.
-Note: If I start moving again, the temp ALWAYS immediately drops back down. It has NEVER overheated while moving.
-Since the temps have dropped, I haven't seen it overheat. I HAVE noticed that in the morning while warming up the car in the driveway, the motor will usually heat up to normal temps but the HVAC system will not pump out warm or hot air untilI I start moving. I blows fine, just cool air while its sitting in the driveway. As soon as I start moving, it heats up very quickly.

My thoughts: I am really leaning towards the water pump for a few reasons. I believe if the water pump isn't working properly, then while idle when its not pumping as it should, it would cause overheating and also explain why the HVAC isn't pumping hot air at idle. As soon as I give it more RPMs, the pump works harder and everything works as it should. It his flawed thinking?

Any helps appreciated!

TL;DR Car is overheating only at idle, always drops down when I start moving. In colder weather car will heat up at idle but won't pump hot air into the cabin until get moving. Dealer says its likely there thermostat or water pump... duh.

95viper

zrheath

That is exactly what a few people have told, that it's possible the water pump impeller shaft is starting to slip, therefore its not operating enough at idle but at higher RPMs, its doing fine. Unfortunately the cost of a water pump is twice as much as the thermostat so I'm trying to be sure before choosing to go that direction. The service manager told me without taking the parts off they can't determine for sure. I am going to talk to them more when I pick it up tomorrow.

Adi-Benz

My car looks the same - pile of snow lol.

I guarantee it's not the thermostat.

zrheath

What makes you say that? I am thinking the same thing but I would think the damn dealer would agree and they're still saying it could be.

Adi-Benz

Our thermostats fail safe open and the car stays about 10C cooler consistently compared to normal operating temp, no deviation.

W204Motorsports

It's the radiator fan. A thermostat would not cause these symptoms on our cars and if it was the water pump, it would do it all the time. If the impeller was damaged, well that means it's never going to work properly. A mechanical part can't just work on and off, it either does or doesn't.

I would look into the radiator fan and see if it's working properly. Could be the resistor not working when getting hot or the fan itself failing.

tommiett

AGREE... If the cooling system works OK when moving, the only part not in play are the fans. The thermostat and water pump are working when moving and in many cases, the fans are not needed. Water pumps works or does not. Never heard of one that works part-time......

95viper

95viper

catmandoob

This is a tough one because the dealer is kind of right. Circumstances dependent of course, it is typical of a water pump failing to not have enough pressure at low RPM and yet at higher RPM it can build up enough to get things cool. Also if the thermostat did go out and possibly it didn't open fully and sat only 1/3rd of the way at high RPM it can allow enough coolant to pass but at low RPM there isn't enough pressure to push past enough coolant. I wouldn't think it was the electric fan only because OP describes the fan ramping up and you can tell on our cars when the fan really ramps up cause it's loud, if anyone has programmed their ECU or anything like that then you know how loud it is.

Easy test for the most part is to feel the hose coming from the pump when vehicle is at operating temp, you should feel some pressure as you try to squeeze it and kind of a surge when you let go at idle. If there is no pressure feel at all that tends to point towards water pump. I'm assuming it's not loss of seals or anything since no coolant leaking from weep hole. All it takes is some cavitation to occur in the pump at low RPM.

That's a tricky one for sure but there is only so many things to check, radiator, coolant, fans, thermostat, water pump and heater core. If no leaks it has to be one of those causing a flow issue unless your engines waterjacket is blocked etc.

zrheath

Ok everyone, I picked the car up today and talked to them more. They did a pressure test with no leaks. Fans are working properly. They were also able to successfully open and close the thermostat through their scanning tool. Scan showed no previous faults relating to this and they weren't able to get it to overheat. On my way home I started thinking and I kicked on the a/c. As I got closer to home I heard the fans start to speed up. When I got in the driveway, I parked and sat with the a/c running. Surely enough I got it to overheat on video. I kicked the heart on before it hit 120 and it brought the temps right back down. And to answer previous questions, when I rev the RPMs up while its overheating, it DOES bring the temps down.

I called the dealer again (which has been cool as hell through this) and told her what happened. She is leaving toward the water pump now, but she's going to talk to the tech in the morning. The plan is from to take it for a drive in the morning and then stop back over there and try to get it to overheat for them, and she said they would do another scan free of charge. Wish me luck!

zrheath

Here is the video for anyone interested. Skip in a little bit.

View My Video

95viper

zrheath

I took it back to the dealer today after I got it to overheat. The master tech looked at it again and watched it overheat. He still says it's the thermostat! They had a 2014 c300 in recently that had similar symptoms. It would stop blowing heat anytime it stopped. They replaced the thermostat and it worked. I'mnot sure what I am going to do yet, but I found the thermostat new for $130, so I'm probably going to start there. I'd rather risk wasting $130 than over $400 for there water pump.

Kelvin Bellrose

https://www.fcpeuro.com/Mercedes~Benz-parts/C350/Engine-Water-Pump/?year=2012&m=850&e=2010


Water pump is 80 bucks

Kelvin Bellrose

https://www.fcpeuro.com/Mercedes~Benz-parts/C350/Engine-Thermostat/?year=2012&m=850&e=2010

Thermostat is 180 bucks

Kelvin Bellrose

You can get a foam acidic radiator cleaner called ren o coil that you spray in the rad, let it sit then back flush from the inside. If you live in a buggy area this could be a thing.

95viper

MB_FanAddict

The OP has a 2013 with the M276, so the parts are different. The thermostat is slightly cheaper, but the water pump is higher (at least for OEM parts).

In any event, the fact that turning on the A/C triggers the engine to overheat may be a pretty big clue. I wouldn't think turning on the A/C during the middle of winter would add much load to the cooling system and if the radiator was plugged or if it was the water pump (I don't think it's the thermostat, even though you may be due for one ), I would think the car would overheat all the time.

The next time it's warm enough to engage the A/C (or take it back to the dealer), after the car has been driven and is fully warmed up, open the hood and observe the serpentine belt/pulleys while someone turns the A/C on. Do you notice any changes in the belts rotational speed or excessive movement ect. Is the belt routed correctly, facing the right direction or in alignment? It's all I could come up with (with the info you've provided so far).

Kelvin Bellrose

The parts I quoted are for M276

What about the auxiliary water pump?

MB_FanAddict

Well, the thermostat is the correct part number, but the water pump you linked to on the FCP Euro site is for the M272. Apparently, they don't carry the M276 pump (only the gasket).

The auxiliary water pump is not meant assist in engine cooling.

Kelvin Bellrose

Yes, the 2012 looks like it has either the m276 or m272 engines but 2013 is just m276

My 2012 has the m276 engine so the water pump they said would work on my car obviously would not fit. Good to know.

'11W204

^^^Yeah, we had this happen on a W210 diesel. Took FOREVER to figure out the cause. Crazy.

zrheath

I went ahead and ordered a thermostat against my gut feeling, but also tasking the dealer's advice. It's the cheaper of the two options considering the part is cheaper and I can install it myself. Certain dealers sell it for $130, but I found one for $55 that someone had bought and didn't use. For that price it's worth a try. If that doesn't fix it, water pump it must be.