W204 OM651 slow warmup, temperature fluctuations

[megatr0n]

Hello, i am not from UK so in advance i apologise for my english

I had my car for about a year and 2 mounths now, bet working perfectly, passed 20000km since i got it.
I noticed my coolant temperature fluctuate between 80 and 70 for about 20km before reaching operating temp of 90 (outside temperature >20C), but in cold mornings its less apparent (temperature < 15C). I canged thermostat for non OEM one (brand ERA), but problem seems even slightly worse than before, but i also noticed when the fluctuations start happening if i stop and just turn engine off and on again, it gets to 90C very fast - this i dont know if worked on old stat, never tryed. When car hits 90 it stays there, i was going on 500km trips and no problems, only once on highway when i was doing 200kph and suddenly let go the gas medal temp droped to 80 but got back to 90 very fast (i assume stat was being open to cool the engine because of the high load and stayed opet for a couple of ceconds after i let go of gas pedal.

No leaks of colant, nothing, temperature never got past 90, no overheating, fuel consuptions is very good, i dont drive pedal to the metal (about 5l/100km, mixed driving, mainly open road around 90kph)

Could it be that new stat i got is defective also?
I found threads on this forum describing the same problems, but no solutions, someone says its the stat, somene says changing the stat didnt help.

Regards and thank you for your help

 

[AMGeed]

I'd replace the thermostat with a part from the main dealer.
You can then rule that out if the temperature fluctuations continue.

But before doing that, just check the connecting plug is firmly attached to the temperature sensor which should be located close to the water pump on the front of the engine.

Welcome to the forum too, and your English is fine. In fact, its better than some English speaking members

 

[megatr0n]

I'd replace the thermostat with a part from the main dealer.
You can then rule that out if the temperature fluctuations continue.

But before doing that, just check the connecting plug is firmly attached to the temperature sensor which should be located close to the water pump on the front of the engine.

Welcome to the forum too, and your English is fine. In fact, its better than some English speaking members

Thank you
I will try looking for the sensor tomorrow, i will also look the temps as weather gets warmer next week it will be around 25-30C here in Croatia.
Looking forward forward for more opinions also, maybe someone else has got the same problem

Regards

 

[mersum1es]

You may have electrically controlled thermostat. It should drop temps pretty fast if you press pedal to floor, but if it makes very fast (under a second) up/down, probably temp sensor itself is going to fail.

 

[megatr0n]

You may have electrically controlled thermostat. It should drop temps pretty fast if you press pedal to floor, but if it makes very fast (under a second) up/down, probably temp sensor itself is going to fail.

Hi, thank you
Yes it is electric, it has 2 wire plugs in it. You mean main water temp sensor or the one in the thermostat housing which comes included?

 

[megatr0n]

update: temp sensor is firmly attached, all good
Asked the dealer today, they said that its completly normal, car adapts temperature depending on outside temperature to compensate for emmisions, also adapts to driving style (which in my case is relaxed driving with fast driving highway sometimes, very little city). Also being very economic, very little fuel goes in to the heat. Today with 26-27C outside and little harder driving (acceleration) reached 90c in about 8km, fluctuations occur between 15 and 25 C, thts when you notice it, bellow 15 and above 25 very little (i am told cooling has 2 different cycles, small and big and each one is used depending on ambient temp, cabin heating or ac..). My car is in the garage at home, so it takes more time when im going from home to work than from wor to home since its sits in the sun and coolant probably get little warm even then. Complex system indeed.

 

[mersum1es]

Sorry I missed you question.... how fast temp is fluctuating? In second peak to peak or several seconds?

In addition you themostat still may be failing it should not open at so low temps at all. Here pic os similar case where something broken inside thermostat and keeping it slightly open Mersuforum.net • Informaatio

But as far as temp is rising to 90 I wouldnt be very much concerning...

 

[megatr0n]

Sorry I missed you question.... how fast temp is fluctuating? In second peak to peak or several seconds?

In addition you themostat still may be failing it should not open at so low temps at all. Here pic os similar case where something broken inside thermostat and keeping it slightly open Mersuforum.net • Informaatio

But as far as temp is rising to 90 I wouldnt be very much concerning...

fluctuating slowly, just around 70-80 mostly and then rising to 90 and stying there all the time, but just when outside temp is 15-25, at 10 it goes straight to 90 for example

 

[Tonygw]

This does sound VERY similar to mine S204 OM651 engine. I was wondering the same thing but its been like this for some time now.
Today I did a school run of about 6-8 miles round trip, heater on about 22C and it got to temp very slowly but stayed there, yesterday I didnt have the heater on and on a ast run after about 4 miles it dropped to 80 and then crept back up. It seems to vary. My car is on 96K and recently had a new water pump, but was liek this before the pump was fitted. I always wondered if this was normal or not but nobody can give me confrmation it is or isnt. So I'm still not sure. Mines been like ths a while though and seems fine. Once up to temp on a long run it stays there.

 

[megatr0n]

Will be testing tomorrow with start stop enabled vs disabled, as turning engine off amd on solves this fluctuation

 

[Tonygw]

So this morning, no heaters on do 4 miles temp nearly to top, stop for 5 mins engine running, then onwards, engine got up to temp, got onto the dual carraige way, temnp dropped back down to around 80, then crept back up after another 2-3 miles. At which point I parked up. not sure if this is normal?

 

[mersum1es]

yep it should drop temperature when it detects hevier acceleration

 

[Tonygw]

ah right, well I didnt know that. I thought there might be something wrong but its been like this for a long time. So its working properly, thats good to know

 

[Tonygw]

SO I researched this a little more, it seems modern cars mercs VW, BMW have "map controlled" thermostats that will vary the flow of coolant depending on driving conditions. It gives max flow under heavy acceleration to cool the engine and improve power delivery, under light driving it stays constant to improve fuel economy.

This is what I am experiencing. The car gets up to temp but if I put my foot down the coolant temp drops. I watched this on my OBD2 app monitoring coolant temp live. The instrument guage is pretty accurate.

So for anyone else with this issue, it appears to be working and is normal

 

[megatr0n]

Fluctuations are still there, very constant, car runs perfect, fuel economy is great, with short journies (around 12km one way) i get around 4,7l/100km, i suppose its normal then, as it never drops below 70, declared normal temperature range for om651 is 70-100c, it usualy sits around 75-80 for 15-23km before reaching 90 and stays there when reached. When ambiant temp is bellow 15-16c it goes straight to 90 (around 10km to reach in those conditions with light driving).

 

[Rokyluk]

Hello, i am not from UK so in advance i apologise for my english

I had my car for about a year and 2 mounths now, bet working perfectly, passed 20000km since i got it.
I noticed my coolant temperature fluctuate between 80 and 70 for about 20km before reaching operating temp of 90 (outside temperature >20C), but in cold mornings its less apparent (temperature < 15C). I canged thermostat for non OEM one (brand ERA), but problem seems even slightly worse than before, but i also noticed when the fluctuations start happening if i stop and just turn engine off and on again, it gets to 90C very fast - this i dont know if worked on old stat, never tryed. When car hits 90 it stays there, i was going on 500km trips and no problems, only once on highway when i was doing 200kph and suddenly let go the gas medal temp droped to 80 but got back to 90 very fast (i assume stat was being open to cool the engine because of the high load and stayed opet for a couple of ceconds after i let go of gas pedal.

No leaks of colant, nothing, temperature never got past 90, no overheating, fuel consuptions is very good, i dont drive pedal to the metal (about 5l/100km, mixed driving, mainly open road around 90kph)

Could it be that new stat i got is defective also?
I found threads on this forum describing the same problems, but no solutions, someone says its the stat, somene says changing the stat didnt help.

Regards and thank you for your help

Hello, did you resolve this issue.

 

[m80]

Hello, did you resolve this issue.

Tbh I'm not sure this is an issue.
The engine is designed to have running temps between 70 - 100 C.

 

[Rokyluk]

It coul

Tbh I'm not sure this is an issue.
The engine is designed to have running temps between 70 - 100 C.

It could be some software upadate need it. I dont know how to come with this to the dealer. They dont know nothing anywhere..they have testers but dont have konwledge

 

[m80]

Mine is worse at reaching temperature, but we are colder here, and on warmer days it gets there faster.
I could be tempted to unplug the thermostat heater element, that is controlled from the ECU. But wouldn't admit having done such a thing as someone would feed my balls through a mangle.

 

[Tonygw]

Im in my 5th year of ownership of my car, currently sitting on 120K miles, My car has been like this since i got it, nothing has changed, in all the time I have owned it. Its perfectly normal.
Enjoy..