Coolant temperature going down

[Bellow]

Because the engine is running on a rich mixture until it reaches operating temperature, so using more fuel than it should be.

It's a diesel - lean/rich definitions are for SI.

Why will 70C not improve efficiency?

Because it increases the temperature differential between the hot gas and the cylinder wall/combustion chamber increasing heat loss.

 

[ALFAitalia]

A thermostat is called a thermostat because it's designed to keep the the temperature (relatively) constant . . . thermo +stat

Once up to temperature you should only see a variation of more than a few degrees under exceptional circumstances.

Actually a stat only has any control over the minimum running temp.....it had no effect on the maximum....thats the fans job. Ones the stat is fully open it can have no control over the max temp the coolant can reach.

 

[alabbasi]

When it drops, try to look at the hood and see where the coolant level is. The temp sensor is typically on the thermostat housing and may not be reading accurately if it's not submersed in coolant due to an air pocket.

 

[Tonygw]

It's a diesel - lean/rich definitions are for SI.


Because it increases the temperature differential between the hot gas and the cylinder wall/combustion chamber increasing heat loss.

Erm isn't that the idea? Cooling system is designed to remove heat? A bigger differential means more heat can be transferred to the cooling system and removed by airflow through the radiator??

 

[ALFAitalia]

An engine running as hot as possible but without overheating will be the most efficient .Second Law of thermo dymamics and all that. The Second Law of Thermodynamics suggests that higher engine temperatures can lead to higher efficiency. Of course the AIT needs to be as low as possible though.

 

[tode]

Yes it often goes to 70C and fluctuates but mostly sits around 85C, only under certain circumstances will it change. Since my driving is pretty consistent I can usually predict where this would be based on my driving style. And can prevent it if I drive differently.

As I said its normal.

Just because your car often drops to 70° doesn't mean it's normal . . .
Even 85° is a tad low.

 

[Bellow]

Erm isn't that the idea? Cooling system is designed to remove heat? A bigger differential means more heat can be transferred to the cooling system and removed by airflow through the radiator??

What's the objective of the diesel engine - to provide mechanical propulsion or heat water?

 

[Pontoneer]

Just because your car often drops to 70° doesn't mean it's normal . . .
Even 85° is a tad low.

Most of my cars have always just shown a hairline above 80 , which is the temp indicated on the gauge . If it was meant to be 90 there would be a line at 90 , not 80 .

 

[Pontoneer]

What's the objective of the diesel engine - to provide mechanical propulsion or heat water?

That is why Diesels always have poor heaters

 

[boxer 01]

Yes it often goes to 70C and fluctuates but mostly sits around 85C, only under certain circumstances will it change. Since my driving is pretty consistent I can usually predict where this would be based on my driving style. And can prevent it if I drive differently.

As I said its normal.

Maybe normal but whose to say your stat might be faulty too. Fluctuations in temps within limits is acceptable, but these fluctuations in op car are too wide and I personally think that mb needs to sort it out. By the way my old w211's temp needle is always rock solid no fluctuations that I can detect.

 

[Fayiz]

interesting thread...

something I must say, I've also been curious in regards to temp

as some of you may know, I replaced the engine on my e350 cdi coupe, one thing I've noticed with the current engine is what some have mentioned above in regards to fluctuations, it doesn't always sit solid around that 90 mark like my old engine did, I've even seen it drop to around 70 ish at times, so is this normal or not? if not, I'll consider a thermostat replacement

although one difference I recall between the current engine and the old engine, that one had the older blue coolant, current engine has the newer pink / red coolant after a full cooling system flush, could that cause something as such?

I also recall a friend's old e350 cdi 2010 saloon doing the exact same thing in terms of the temps, never sat at a solid 90 as my old engine (with blue coolant) and I always wondered why

so now the question is, was my friend's thermostat bad and mine is also bad? lol... or could it be down to the newer pink / red coolant which is a common factor here?

 

[MrGreedy]

My personal view (including the OM642 diesel) is that the thermostat maintains a steady 88-89°C once up to temperature.
If it is not doing this (for the OM642) it is most likely a thermostat fault. For this particular engine:
(i) breakdown of a rubber gasket seal within the thermostat can get trapped in the mating surface when the thermostat closes (hence leaving it open a bit and therefore creating a running cool situation), or
(ii) scale building up around the pre-heater, can cause the thermostat opening mechanism to stick.

As per one of the previous posts in this threat Mercs usually run around 90°C, and the modern thermostats (like the one I have on my OM642) use the pre-heater to pre-open the thermostat by simulating (fractionally early) hot coolant, ready for a rush of hot coolant under heavy load situations (acceleration etc).

My limited experience tells me that for most normal road cars, around mid 1980's or later, to around 2018 or earlier, thermostats should maintain a steady temperature.
I can't speak outside of this range or for special technological cases; just that for every car I've had experience of in this range, the operating temperature has been steady once at that temperature.
I've probably only driven around 100 different make/models, so my experience is certainly not touching the sides of exhaustive, but not one of those cars exhibited temperature fluctuation once up to operating temp.
The only time I've had rapid temperature fluctuations is due to inadequate coolant bleeding and airlocks after working on the coolant system.
I've never had rapid and significant temperature variations in use with a thermostat fault - thermostat faults tend to exhibit persistent failure to reach desired temperature or steady creep to over-temperature.

 

[ALFAitalia]

Yep....my 350 CDI is rock solid at 90.....its clearly not correct to accept 60 or 70 to 90 degree swings as normal on any car....it aint! If it was than everyone with that model would experience the same thing, which they aren't.

My wife's A Class is the same...but only since the new stat.... was dropping to 60 before that.

 

[Bellow]

interesting thread...

something I must say, I've also been curious in regards to temp

as some of you may know, I replaced the engine on my e350 cdi coupe, one thing I've noticed with the current engine is what some have mentioned above in regards to fluctuations, it doesn't always sit solid around that 90 mark like my old engine did, I've even seen it drop to around 70 ish at times, so is this normal or not? if not, I'll consider a thermostat replacement

although one difference I recall between the current engine and the old engine, that one had the older blue coolant, current engine has the newer pink / red coolant after a full cooling system flush, could that cause something as such?

I also recall a friend's old e350 cdi 2010 saloon doing the exact same thing in terms of the temps, never sat at a solid 90 as my old engine (with blue coolant) and I always wondered why

so now the question is, was my friend's thermostat bad and mine is also bad? lol... or could it be down to the newer pink / red coolant which is a common factor here?

Interesting angle...
But I don't think it makes that much of a difference though I can't be certain. My previous car ran with blue AF and had a coarsely calibrated temp gauge (4 blobs). The gauge never moved and I can't recall if I felt temp changes from the heater. Current car runs with red AF and under duress the temp gauge will climb a touch and I can detect warmer air from the heater. As soon as the heavy load is reduced the temp returns to normal. But it never moves in the other direction ie, going colder from normal running temp. Maybe, the different AFs do have different thermal properties. In my current car it seems very capable of absorbing and shedding heat over very short timescales. More than I recall with blue AF (though that is a litte subjective).

 

[JP79]

Just briefly read all posts so apologies if someone already suggested this.
I reckon is a thermostat fault.

if you think about it, thermostats are closed. and rated to open at a certain temperature, let's say 90C
you start the engine, water in the engine is getting warmed up and not going to the rad.
Once the thermostat/sensor picks water temp at 90C, then it opens up and sends all the hot water to your radiator... to cool it down, and throws it back inside the engine.
once it picks the water has gone too cold, below a certain temp...it closes. Water warms up... opens.. and so on. so it maintains a constant 90C.

if coolant temp is always cold - or more likely below normal operating temperature -, means the thermostat is permanently open. there's no time for coolant/water to heat up in the engine because there's a constant flow of water/coolant going to the rad and being cooled down.
If coolant temp gets too hot (and eventually cooks your head gasket) means the thermostat is not working and is permanently closed.

now, I might not have much experience with all these new cars full of gizmos and sensors, but what I explained before is how it works on cars since ever.
(admittedly my experience is on american classics)

 

[Fayiz]

Interesting angle...
But I don't think it makes that much of a difference though I can't be certain. My previous car ran with blue AF and had a coarsely calibrated temp gauge (4 blobs). The gauge never moved and I can't recall if I felt temp changes from the heater. Current car runs with red AF and under duress the temp gauge will climb a touch and I can detect warmer air from the heater. As soon as the heavy load is reduced the temp returns to normal. But it never moves in the other direction ie, going colder from normal running temp. Maybe, the different AFs do have different thermal properties. In my current car it seems very capable of absorbing and shedding heat over very short timescales. More than I recall with blue AF (though that is a litte subjective).

the first time it caught my eye was when I drove a friend's e350 cdi saloon, same engine, but back then I had my previous engine with the blue coolant and that was one key thing I noticed, the max temps were the same, as ALFAitalia has kindly shown, however my car sat at a solid 90 at all times, whereas my friend's car didn't, and the only apparent difference was the coolant, he had the newer pink / red coolant and my current engine has the same

so as mentioned, either both thermostats are to be considered bad or it's more so the coolant difference

more of an observing drive today, my car does sit at 90 at times for sure, depends on conditions too, especially if I'm driving around town / in traffic lets say, it'll get up to and stick to 90 for the most part, but yeah there are times where the temperature dips based upon driving style / location, and as said, I'm not 100% certain whether this is coolant related or thermostat related.

would be interesting to know which coolant the other members that have commented are using

 

[MrGreedy]

The coolant itself won't have an impact.
I suspect they are both predominantly the same fluid mix (95% or more), with only some additive differences.

Red is the new blue from Merc. They don't sell blue anymore.

 

[ALFAitalia]

I cant believe the coolant will affect it enough to see.....I ran mine on just water for a few days fitted fitting the new rad to test for leaks.......no difference in running temp...even on my torque app readings.

 

[Sweetooth]

Many years ago I had a mk 2 viva, it was running below it's normal temperature fitting a new thermostat fixed it.

 

[Tonygw]

You do know some temp gauges in cars are not true temp gauges right? they are dummy gauges controlled by ECU to indicate the coolant is up to temp and when it overheats. So when it sits rock solid, have you checked live data on to see what's really happening?