W204 om651 Fluctuating Temperature Gauge

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Judge123

New Member
Joined
Feb 24, 2019
Messages
8
Location
Northern ireland
Car
W204
Hello, Can anyone that maybe experienced the same issue shed some light on this as its driving me mad. 2011 facelift W204 c220 cdi om651. The car gets up to temperature as normal which I believe to be 2 white Mark's above the 80 degrees around 90 degress. Then after some driving the temperature hand starts to drop to just below the 80 and will stay there around 20-30 seconds before it starts to rise back up to the 90 again. It goes up and down a few times with each journey. First photo is off where then gauge starts to drop to after some driving. The second photo shows it rise back up again to normal operating temperature after dropping below the 80.

New Genuine Mercedes parts fitted 3 months ago all at the same time.
New Water pump.
New Thermostat.
New Fuel filter housing.
New mercedes coolant.

It was doing the symptoms before all the parts were changed and I think it's getting more regular as well as at the beginning it only done it now and then over a few days.

Any help would be appreciated, Thanks
 

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Strange one , all I can think is the recent thermostat is faulty , they haven't the best track record .

Is the CTS built into the thermostat ?
 
Yes I believe the cts is on the thermostat housing. I have just discovered there is a switch on the water pump that gives a lot of trouble, it's a solenoid that can turn the water pump off and on. I'm going to look into this but I will maybe get another new thermostat too and test it before I fit it
 
Yes switchable water pump to help warming up and the radiator shutter .

Even with it all working it's not the quickest of engines to warm up .
 
This maybe totally off but do you have heaters on when the tempersture drops. I had a similar issue where it would warm up then the temperature dropped after the heaters had been on. Replaced thermo sorted that. Maybe your new thermo.is faulty
 
Yeah my heaters be on a lot, I'm going to turn them off for a day to see if that stops the problem and if so I will buy a new thermostat again. If oem thermostats are known to be bad does anyone know a better replacement brand? Thanks
 
My first new water pump (metal, upgraded one) was doing that before it started to leak within 6 months after change. Changed within warranty for another new one and it's been fine since.
 
So not only can you get a faulty new thermostat from mercedes you can also get a faulty water pump from them as well? The more I learn about mercedes the less I like. I had a 2012 lexus is250 f sport for 1 year before I got the mercedes and it didn't need 1 part changed in the entire year until someone drove into me and write it off. I then got the mercedes and it was back to the dealer 4 times in the 3 months warranty plus iv lost count of how many parts iv had got changed myself, as soon as you fix one thing you get a few weeks driving before some other sensor or something fails. I can't imagine me owning this Mercedes for much longer and I certainly wont be getting another ever again lol I understand cars need matience but I think iv replaced 6 sensors so far in a year each with their own dramatic effect when they fail. You live and learn lol i think I just expected a better build quality from Mercedes, iv only ever driven Lexus and they are in my eyes superior in every way to mercedes and I'd rather pay petrol mpg prices of lexus anyday. Rant over lol
 
When I had my water pump changed because of the sticking solenoid on the water pump, mine started doing this as well. After a few journeys it stopped doing it so much so I wonder if there was some adaption going in due to the new water pump. It still does it slightly eg warms up to normal, then if I go on the motorway and boot it, the gauge will drop by 2 bars for about 10 seconds before then coming back up to normal . It then stays at normal. Done it for ages now so I'm not concerned. Electrically heated thermostat on mine as well, so not keen to get involved in changing it (E250 petrol)
Interesting things is that the temperature goes down, like a surge of cold water is allowed to circulate, so maybe its a quick opening of the thermostat. Lazy thermostats are more likely to show an increase in temperature before they open?
 
So a bit of a update, I spent yesterday getting the car ready for its mot and I bled the system again to make it has no air locks, checked for vacuum leaks to the y133 sensor. I finished everything and moved it outside and when it was idling I put the heaters to maximum to dry out seats after a clean and as soon as I put the heaters at max I noticed the temperature hand start to drop. So I turned heater of and the hand started to raise again. Put it too maximum again and it dropped again. So that's making me think mercedes did give me a faulty thermostat?
 
When I had my water pump changed because of the sticking solenoid on the water pump, mine started doing this as well. After a few journeys it stopped doing it so much so I wonder if there was some adaption going in due to the new water pump. It still does it slightly eg warms up to normal, then if I go on the motorway and boot it, the gauge will drop by 2 bars for about 10 seconds before then coming back up to normal . It then stays at normal. Done it for ages now so I'm not concerned. Electrically heated thermostat on mine as well, so not keen to get involved in changing it (E250 petrol)
Interesting things is that the temperature goes down, like a surge of cold water is allowed to circulate, so maybe its a quick opening of the thermostat. Lazy thermostats are more likely to show an increase in temperature before they open?
+1 same situation happens on my S204 C250 CDI. Water pump and thermostat both changed too.
 
Hello, Can anyone that maybe experienced the same issue shed some light on this as its driving me mad. 2011 facelift W204 c220 cdi om651. The car gets up to temperature as normal which I believe to be 2 white Mark's above the 80 degrees around 90 degress. Then after some driving the temperature hand starts to drop to just below the 80 and will stay there around 20-30 seconds before it starts to rise back up to the 90 again. It goes up and down a few times with each journey. First photo is off where then gauge starts to drop to after some driving. The second photo shows it rise back up again to normal operating temperature after dropping below the 80.

New Genuine Mercedes parts fitted 3 months ago all at the same time.
New Water pump.
New Thermostat.
New Fuel filter housing.
New mercedes coolant.

It was doing the symptoms before all the parts were changed and I think it's getting more regular as well as at the beginning it only done it now and then over a few days.

Any help would be appreciated, Thanks

Hello
just come across this & wondered whether you had found a solution?
I have an Infiniti Q50 which I believe uses the same Merc Diesel engine, and started suffering the same symptoms last summer, ie from start up the temp gauge will rise, before dropping back down. This will continue a few times until the engine is completely warm / up to temperature, and on average around 12 to 15 miles from cold. I’ve suspected thermostat but garage are adamant that car is running fine.
Appreciate any thoughts or ideas
Regards.
 
Hello
just come across this & wondered whether you had found a solution?
I have an Infiniti Q50 which I believe uses the same Merc Diesel engine, and started suffering the same symptoms last summer, ie from start up the temp gauge will rise, before dropping back down. This will continue a few times until the engine is completely warm / up to temperature, and on average around 12 to 15 miles from cold. I’ve suspected thermostat but garage are adamant that car is running fine.
Appreciate any thoughts or ideas
Regards.
Yes it was the new genuine mercedes thermostat that was faulty. Not uncommon from what I have heard.
 
Yes it was the new genuine mercedes thermostat that was faulty. Not uncommon from what I have heard.

Thank you for the quick response. Glad you got sorted. I’ll press ahead & look to get mine changed.
 
When the cars water temp starts to go down again,stop open the bonnet and see if the fan speed is going very fast as though the car is overheating,that could be why the temp is going up and down.
 
When the cars water temp starts to go down again,stop open the bonnet and see if the fan speed is going very fast as though the car is overheating,that could be why the temp is going up and down.
👍
 
Hello, Can anyone that maybe experienced the same issue shed some light on this as its driving me mad. 2011 facelift W204 c220 cdi om651. The car gets up to temperature as normal which I believe to be 2 white Mark's above the 80 degrees around 90 degress. Then after some driving the temperature hand starts to drop to just below the 80 and will stay there around 20-30 seconds before it starts to rise back up to the 90 again. It goes up and down a few times with each journey. First photo is off where then gauge starts to drop to after some driving. The second photo shows it rise back up again to normal operating temperature after dropping below the 80.

New Genuine Mercedes parts fitted 3 months ago all at the same time.
New Water pump.
New Thermostat.
New Fuel filter housing.
New mercedes coolant.

It was doing the symptoms before all the parts were changed and I think it's getting more regular as well as at the beginning it only done it now and then over a few days.

Any help would be appreciated, Thanks
Hi, I also got a similar problem, I have changed water pump and thermostat housing with OEM parts done by the dealer, all systems checks were done and all are correct. Temperature run at 90 degrees however on the freeway at 120km per hour, under heavy acceleration, the temperature gauge drops to 80 degrees. Under normal acceleration, the temperature returns to 90 degrees. Car is going back to dealership next week for further diagnosing. Any ideas what the possible problem/s maybe. TIA
 
Hi All Mine does this. I think its perfectly normal.

Modern engines like this have a "mapped" thermostat, in other words the engine management system can control its opening and closing.
Under heavy acceleration it cools the engine for max power and better performance. Under light loads it heats it for better economy

If I run mine 100miles down the motorway and then slow down on the A roads it starts to cool down but never drops below 80. I have used my OBD live data to see what the actual temp of the coolant is and it tracks the needle perfectly. It really does cool down to 80.

I don't think this is anything wrong. Mines been doing this for a long time. Probably since I bought the car, its a design feature of this engine.
 
The only time I have seen my Mercedes temperature fluctuate is when the coolant didn't have the proper mixture.
I have ONLY seen this happen with Mercedes. I drained the coolant and put in a fresh batch and the problem was solved in my case which really surprised me.

The only time I have seen this happen with my other German cars is when there has been air in the system or their was a problem with the thermostat.
 

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