OM651 fluctuating temp gauge,

[Tonygw]

Hi Guys

Can any experts out there tell me if they think I have a problem or not with my OM651 C250cdi.
When I start the car and drive off the temp guage rises as normal with normal driving, after a few miles it reaches around 80C and then after short while longer gets to 90C. Then if I hit a dual carraige way or fast road after a short while it drops back to 80C and then creeps back to 90C. Thuis was with the aircon/heater off in all cases.

At the weekend I went on a 18 mile drive, car did the same but then settled at 90C then I stopped the engine for 5 minutes, started it, drove normally and the temp was at 90C and then it started to drop to 80C once I got some speed up after a good 5 minutes.

I have searched the interent and its full of contasting views, many saying this is normal, many saying the thermostat is bad, some saying the coolant mix is wrong, Some saying that even the merc dealers state this is normal.

I am very confused. LOL

I dont really want to fork out for a new thermostat only to find it makes no difference as some have said. So any help or advise is much appreciated thanks

 

[zipdip]

Well not sure you have a problem the temps you have are all ok,the car is warming up quickly which is not always the case with diesels, so the thermostat is closed and then when the temp goes up it opens and the temp goes down a little,so I would not be too worried,if you want on a day like today when it is hot where I am ,get out and find a traffic jam or drive slowly around a industrial estate and see if the car gets hotter than 90 if so open the bonnet and see if the electro fan is going full bore,but as I say I think you are seeing a problem where there is none

 

[Tonygw]

Thanks for the reply, That was kind of my thinking to but never had a car do this before. It never gets over 90C so will check the fans also when I can. cheers

 

[steveq]

Hi,

My 2011 C220 CDI used to do the same. I watched it for a while and most trips it would go up to 90ish then drop back and slowly creep back up again.

Like you, I thought it was the thermostat failing.

That was about a year ago and it now has stopped doing it.
Now the temp goes up to 90 and stays there. I think it is slower than it should be but certainly not worth doing anything about.
Is that a self healing car?

 

[MrGreedy]

Unless being thrashed to death in very hot weather, my opinion is that the thermostat should keep engine temps rock steady all year long once up to the correct temperate. If it's fluctuating, I think that means a problem. First guesses would be either air in the coolant system, or a thermostat on its way out. I think that's about £150 at an Indy for replacement.

 

[Tonygw]

LOL and here lies my original problem, some say its normal, some say its not..
Does anybody know??

Thanks

 

[OneForTheRoad]

Of all the mercs I've owned once at 90 it stays at 90. I'd guess at your stat is opening just a tad early. My c250 takes a good 15 to 20 minutes to get to 90. 10 minutes if I redline it.

 

[mbzclk]

Thermostat or air lock.

 

[Tonygw]

I appreciate the replies, but can anybody know 100% definite if this is a problem or not.
My audi used to fluctuate sometimes.. even after a thermostat change.hence why I am reluctant to just change it.

Car is on 97K miles at the moment.

 

[OneForTheRoad]

All it will do is change the maf values ever so slightly and you may see a 2 mpg drop in fuel efficiency. Imho.

 

[mersum1es]

your thermostat is electric controlled...

heater activates 'normal' wax actuator and make it opening faster when ecu finds high power output

 

[zipdip]

Well as I posted before I do not think it is a real problem,water temp is important for only two things,if it never gets near 90 then your fuel economy will be bad and you will freeze in the winter, if the temp goes way above 90 then you could easily burst pipes and or the engine,you car is doing neither of those things,if you must try something then try the sensor,it is just possible it is not working right,but I would leave alone,because,ok thermostat fitted at garage £150,it is still the same then where do you go, water pump,thats the next step,and because you change that you need to replace the belt,no just let the car run,and stop looking at the gauge all the time you will have a accident.

 

[MercedesDriver]

Hi Guys

Can any experts out there tell me if they think I have a problem or not with my OM651 C250cdi.
When I start the car and drive off the temp guage rises as normal with normal driving, after a few miles it reaches around 80C and then after short while longer gets to 90C. Then if I hit a dual carraige way or fast road after a short while it drops back to 80C and then creeps back to 90C. Thuis was with the aircon/heater off in all cases.

At the weekend I went on a 18 mile drive, car did the same but then settled at 90C then I stopped the engine for 5 minutes, started it, drove normally and the temp was at 90C and then it started to drop to 80C once I got some speed up after a good 5 minutes.

I have searched the interent and its full of contasting views, many saying this is normal, many saying the thermostat is bad, some saying the coolant mix is wrong, Some saying that even the merc dealers state this is normal.

I am very confused. LOL

I dont really want to fork out for a new thermostat only to find it makes no difference as some have said. So any help or advise is much appreciated thanks

Approximately at 100k miles I had to change mine on the same engine. It started to play at first like yours, going up to 90 then all of a sudden falling to 80 and even below just to go up to 90 some minutes after. Fast forward few months and it couldn't hit the 90 except in a really heavy traffic. I'm not saying yours is a faulty one, it can be that it only opens too fast which makes the coolant temperature to drop.

 

[Tonygw]

Right As mersum1es has mentioned. it would seem as though most modern German cars have "map controlled" thermostats. Its a new one on me, guess I am old school.
Anyway I researched it and it appears to be perfectly normal operation.

Under heavy acceleration the thermosstat is electronically "told" to be forced open for max coolant flow and at speed this lowers the engine temp for improved power, Under normal driving the temp is held constant for max fuel efficiency. After some experiments of my driving style and reading the "actual" coolant temp from my OBD2 live data, This is exactly whats happening.

Under certain conditions I can see from the OBD2 live data the point at which the thermostat is forced open. The car performs normally and around town and at steady speed or acceleration the temp is held steady give or take a few degrees. Give it some gas and the thermostat opens and things cool down.

So if anybody else is wondering or seeing similar to me, its normal.

 

[AndyBike]

Right As mersum1es has mentioned. it would seem as though most modern German cars have "map controlled" thermostats. Its a new one on me, guess I am old school.
Anyway I researched it and it appears to be perfectly normal operation.

Under heavy acceleration the thermosstat is electronically "told" to be forced open for max coolant flow and at speed this lowers the engine temp for improved power, Under normal driving the temp is held constant for max fuel efficiency. After some experiments of my driving style and reading the "actual" coolant temp from my OBD2 live data, This is exactly whats happening.

Under certain conditions I can see from the OBD2 live data the point at which the thermostat is forced open. The car performs normally and around town and at steady speed or acceleration the temp is held steady give or take a few degrees. Give it some gas and the thermostat opens and things cool down.

So if anybody else is wondering or seeing similar to me, its normal.

Hi, newby here but have been lurking in the background sifting through great info from you guys...

I have a 2010 s212 350 CDI with 117k i bought 3 weeks ago, 1 owner full MB history. Drives great for its age except even at "Driving Miss Daisy" style I have been struggling to get past 28 mpg on long runs and while checking the display I noticed temperature was very low even after an hours driving. I plugged in my diagnostics to OBD2 and screen recorded the temperature while driving.

Car takes around 40 minutes driving from cold to get to 72 degrees and only makes it to 82.5 when in traffic but on the motorway or at speed the temp goes down to an icy 57 fluctuating up to 62 degrees. Car is under warranty so called the indie dealer and got it booked in. I mentioned to them that if its already warm and left to idle the temp will make it to sub 90 and stay there but will plummet when driving, i mentioned this as I could see they may get it warm, look at the gauge without driving it and then say they can't see a problem and I was right. Just had a call from them and they can't see a problem but however, they will replace the thermostat for me!?

Haven't got the car back yet to test but now im left wondering (especially reading the quoted info above) if there actually wasn't any issue and if so, thats blown my whole theory on the low MPG. Surely, a diesel shouldn't drop down to mid 50's early 60's? I suppose the only way to tell is when the new thermostat is installed so will update in due course.

 

[Sweetooth]

I don't know if this helps ,a previous car I owned with a DSG had two thermostats,it took 20 mls+ to reach 90 deg .I had the DSG one replaced,driving 5 miles from cold it reached 90 degrees.Your mpg does seem low,I average 35 to 40,and my car is heavier than yours.I hope you get it sorted.