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Hello everyone, i have a 2011 4matic c300 sport with 113k miles. For the past month or so i have noticed that my temperature gauge in the dash of my car has the tendency to decrease at different times. My car is able to reach operating temperature (90 C) and does have heat. But i will discuss two instances where my temperature gauge needle drops. I live in NY where it is cold, and after a cold start i will begin driving to my destination like i normally do, and without the heat on or anything, i will come to a full stop at a light or due to traffic, and i will notice that my temperature needle drops, and it will continue to rise after i get driving again. A second instance is when the car has reached operating temperature, but as soon as i turn the heat on the temperature gauge needle will begin to drop once again, i have provided pictures. Picture A is when the car is at operating temperature and picture B is after i turn the heat on. After i turn the heat on and temperature goes down a little, it will then continue to rise to operating temperature as i drive the car.
I suspected that this might be a faulty thermostat, but now that this problem occurs when i turn the heat on, i feel like somethings wrong.
When the car runs without the heat, and reaches operating temperature it will usually stay at operating temperature, but as soon as i turn the heat on, it will drop.
P.S. all my fluid levels are normal, and i do not have any engine lights/codes.
PLEASE give me input!!
PICTURE A PICTURE B
Last edited by merc5555; Jan 1, 2019 at 01:27 AM.
Reason: needed to add information
I don't think you have anything to worry about. When the car idles it is more susceptible to the air temperature. A slight drop when you run the heat does not seem like a major problem. When my thermostat went bad it was stuck in the open position and the temp stayed low. My mechanic said not to worry and it was safe to drive. Overheating is a bigger issue!
I suspected that this might be a faulty thermostat, but now that this problem occurs when i turn the heat on, i feel like somethings wrong.
P.S. all my fluid levels are normal, and i do not have any engine lights/codes.
You just answered the question. If it was a faulty thermostat your car would set off the P0128 code OR your car would be overheating due to a sticking thermostat. The temperature change is your thermostat doing its job of opening and closing the coolant loop.
I sure hope so. But would you consider it normal or abnormal behavior? For example like a couple months back, or right after purchase (July) this never happened... turning on the heat didn't cause the gauge to decrease, i'm sure if i try to sell it in the near future and that problem still persists, it might scare off the buyer. But regardless, thank you for your input!
Normal, mine's been doing it for years, I did keep an eye on it when I first got it, until I realized it was normal.
Interesting... this never happened when i first purchased the car around July, I've really only noticed it this past month.... but thank you for your feedback, i can put my mind at ease and maybe keep my eyes on the road lol
Last edited by merc5555; Dec 31, 2018 at 08:51 PM.
Reason: wording reasons
I had similar issue recently.
I ended up replacing the thermostat and new coolant. It's back to normal.
If the t-stat is going bad or stuck open, you will see these symptoms.
I asked my mechanic whay there's code yet and he told me the code P0128 will eventually come on, sometimes it does not set right away. It takes time before it comes on.
But anyway, I bought the t-stat for $122.00 and wanted to tackle it myself but never had the time. I ended up paying an indy benz mechanic $150.00 and I bought 2 gals of coolant.
I had similar issue recently.
I ended up replacing the thermostat and new coolant. It's back to normal.
If the t-stat is going bad or stuck open, you will see these symptoms.
I asked my mechanic whay there's code yet and he told me the code P0128 will eventually come on, sometimes it does not set right away. It takes time before it comes on.
But anyway, I bought the t-stat for $122.00 and wanted to tackle it myself but never had the time. I ended up paying an indy benz mechanic $150.00 and I bought 2 gals of coolant.
If it goes under 80 Deg really fast, then I would say the thermostat will be due. From what I see, it drops 3-5 degrees. Mine also varies from 85-90 degrees. It has been doing it since I put in a new thermostat over 100 000km ago so I would not worry unless it starts going under 80 degrees or a check engine light.
The original thermostat from from 120 000km to 160 000km did the same until it got stuck open resulting in the temperature never going over 75 C . I think some people just don't notice it.
One question merc5555, what is the mileage on your car? Thermostats on these cars typically last to about 150 000- 180 000kms.
Last edited by W204Motorsports; Dec 31, 2018 at 09:24 PM.
If it goes under 80 Deg really fast, then I would say the thermostat will be due. From what I see, it drops 3-5 degrees. Mine also varies from 85-90 degrees. It has been doing it since I put in a new thermostat over 100 000km ago so I would not worry unless it starts going under 80 degrees or a check engine light.
The original thermostat from from 120 000km to 160 000km did the same until it got stuck open resulting in the temperature never going over 75 C . I think some people just don't notice it.
One question merc5555, what is the mileage on your car? Thermostats on these cars typically last to about 150 000- 180 000kms.
The mileage on my car is 113miles or 181,000 kilometers, from google conversion calculator. This degree drop doesn't just happen once the car is at operating temperature for example, when the car is cold and the temperature is still rising below 80C, it might drop slightly while waiting at traffic or a light, but then will continue to rise as i drive. Once it reaches operation temp (90C) it usually stays there, unless ofc i turn on the heat then itll drop a little, then rise up.
The mileage on my car is 113miles or 181,000 kilometers, from google conversion calculator. This degree drop doesn't just happen once the car is at operating temperature for example, when the car is cold and the temperature is still rising below 80C, it might drop slightly while waiting at traffic or a light, but then will continue to rise as i drive. Once it reaches operation temp (90C) it usually stays there, unless ofc i turn on the heat then itll drop a little, then rise up.
if its dropping while sitting in traffic on a cold start in addition to the mileage and the heat symptoms, I would say keep an eye on it. Buy a thermostat online for the best price right now so you have it at hand, get some coolant (Mercedes coolant or equivalent) and as soon as you see it getting worse, change the the thermostat.
Thats not normal your car just doesnt get cooler all of a sudden at a traffic light. You dont want anything faulty with how your car reads temperature. You might be over heating and you dont even know it because your gauge is off.
Thats not normal your car just doesnt get cooler all of a sudden at a traffic light. You dont want anything faulty with how your car reads temperature. You might be over heating and you dont even know it because your gauge is off.
I wouldn't necessarily say that my gauge is off, because my car gets to operating temperature (90C), the gauge deffinitely reads it. I've never had it go above that temperature/overheat, im sure if i was overheating, i would get warnings, engine lights, sounds, my car would shut off, etc. At least thats what my 2004 jetta did when i blew the water pump.
I wouldn't necessarily say that my gauge is off, because my car gets to operating temperature (90C), the gauge deffinitely reads it. I've never had it go above that temperature/overheat, im sure if i was overheating, i would get warnings, engine lights, sounds, my car would shut off, etc. At least thats what my 2004 jetta did when i blew the water pump.
Your gauge is reading just fine.. You thermostat is just on its way out.
Your gauge is reading just fine.. You thermostat is just on its way out.
hmmmm.... yea thats also something im thinking and will have to prepare for, like you noted. Whats your opinion on turning on my heat causing temperature drop? Do you think this has anything to do with my heater core, i heard its a very expensive repair. Sometimes i hear a hissing noise when i turn my heater/A/C on, but i also think that my freon could be low? Thanks again, i appreciate you
hmmmm.... yea thats also something im thinking and will have to prepare for, like you noted. Whats your opinion on turning on my heat causing temperature drop? Do you think this has anything to do with my heater core, i heard its a very expensive repair. Sometimes i hear a hissing noise when i turn my heater/A/C on, but i also think that my freon could be low? Thanks again, i appreciate you
Well, the thermostat is probably not closing properly so when you put the heating on, their is more volume of coolant to cool the engine, the heater core acts like an extra radiator, providing more cooling for the engine. This on a properly functioning cooling system would not be visible on the temperature gauge since the thermostat would be closed until the engine has reached operating temperatures.
If your heat works and you don't smell coolant from the vents, there's nothing to worry about. These cars rarely have issues with heater cores. The hissing you hear however I couldn't say. If your freon was low, the ac wouldn't be as cold as it should. Could be something as simple as debris in the one of the vents inside the dash. I would recommend changing the cabin air filter, its an easy DIY job which you lungs will help thank you for. Maybe you'll find surprises in the cabin filter and find your hissing is just a dirty stock cabin filter that was never changed.
To the OP. I watched my gauge today and noticed fluctuations between 80 and 90. I think this is normal and just not something most people notice.
My advice is to not look for issues. Focus on diligent maintenance (by the book or better) and address issues when they actually occur. Over time you will get to know the car and recognize true warning signs.
To the OP. I watched my gauge today and noticed fluctuations between 80 and 90. I think this is normal and just not something most people notice.
My advice is to not look for issues. Focus on diligent maintenance (by the book or better) and address issues when they actually occur. Over time you will get to know the car and recognize true warning signs.
Thanks a lot for that personal experience and tips on how to keep my mind at ease when it comes to taking care of my car. Just being a car guy and dealing with these types of car makes me go nuts and be proactive, mainly because i don't want to throw thousands of dollars into a fairly old car. Id like YOUR input on something; today when i started my car, i decided to turn on the heat at cold start because it was cold (around 30F), having the heat on really kept the coolant temperature from rising, it was 0 for a while until it started rising as i drove, and then it hovered around 80. That is until i turned the heat off and it continued to rise to operating temperature, any input on this?
I know you said to focus on diligent maintenance, but i appreciate any answers i can get lol.
Thanks a lot for that personal experience and tips on how to keep my mind at ease when it comes to taking care of my car. Just being a car guy and dealing with these types of car makes me go nuts and be proactive, mainly because i don't want to throw thousands of dollars into a fairly old car. Id like YOUR input on something; today when i started my car, i decided to turn on the heat at cold start because it was cold (around 30F), having the heat on really kept the coolant temperature from rising, it was 0 for a while until it started rising as i drove, and then it hovered around 80. That is until i turned the heat off and it continued to rise to operating temperature, any input on this?
I know you said to focus on diligent maintenance, but i appreciate any answers i can get lol.
Diligence overall is important, if it gets worse, you'll need to change the thermostat. Putting the heat on should not affect the engine temperature . It should get to 90C and stay there. Order a thermostat, at over 100k miles it is due for replacement.
When did a thermostat on our cars failed in a closed position? MB told me that if our thermostat fails it will always fail on a open position to minimize overheating. That happened to me then my car went to limp mode.
just had my thermostat changed for the exact same reason, temp getting to 90 and then dropping to 80 and then back to 90, the dealership said they would not do anything due to no codes, i showed them a video of it doing it and then they changed their mind,. new thermostat. driving home it did the same thing, I'm think maybe the fan controller
Diligence overall is important, if it gets worse, you'll need to change the thermostat. Putting the heat on should not affect the engine temperature . It should get to 90C and stay there. Order a thermostat, at over 100k miles it is due for replacement.
You're right... i will purchase a thermostat asap, and replace it soon. I'm looking at a thermostat from FCP Euro.
just had my thermostat changed for the exact same reason, temp getting to 90 and then dropping to 80 and then back to 90, the dealership said they would not do anything due to no codes, i showed them a video of it doing it and then they changed their mind,. new thermostat. driving home it did the same thing, I'm think maybe the fan controller
Oh man, that's no good. I'm guessing your benz is out of warranty? How much did the dealership charge for the thermostat replacement, if you don't mind me asking?
You're right... i will purchase a thermostat asap, and replace it soon. I'm looking at a thermostat from FCP Euro.
I want to sell you something, I truly do but your car seems to be operating normally. These fluctuations are normal, additionally if you are stopped in traffic and turn on the heat please keep in mind that you are literally extracting the heat from the coolant to warm the cabin and this is why the temp drops slightly. This is similar to the approach you would take if your car was overheating and you needed to limp it home, you would turn on the heat using your heater core to pull heat out of the coolant and into the cabin.
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Can i drive with engine overcooling?
