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C350 CDI om642 correct engine temperature?

philnewmerc

MB Enthusiast
Joined
Dec 11, 2008
Messages
1,096
Location
Hampshire
Car
C350 CDI
Hi all. My new to me v6 diesel has an engine temp around 80 in town and about 67 on motorways. Is that normal?
My first fill up of 70% modest motorway driving gave 34 mpg, which is a long way from 47 it is supposed to - yes I know it wouldn't do that but I thought 40 would be realistic. maybe not? Or is this a temp issue?
Thanks!
P
 
I've now spoke to 3 well-respected MB specialists and the main dealer and they all say running at 65 deg on the motorway is normal for this engine and if the heater still works then its fine.... So am I going slightly doolally and this is in fact normal ? Any om642 owners who can confirm please? 👍

Can see the attraction of an EV now 🤫

Every car I've owned (about 30 and 5 Mercs) has sat at about 85 deg no matter if in traffic or on the motorway... no sure why this engine should be different unless the temp gauge on other cars is fudged to read rock steady and the OM642 has an 'accurate' gauge that shows it as it is?
 
It is not normal, I promise. Once warmed up it should be sat at about 89C, just under 90 and should stay there regardless of the situation.

I went through this too, people saying this and that. Changed it and realised that it was all nonsense and should be ~90 rock solid and stay there once warmed up.
 
It is not normal, I promise. Once warmed up it should be sat at about 89C, just under 90 and should stay there regardless of the situation.

I went through this too, people saying this and that. Changed it and realised that it was all nonsense and should be ~90 rock solid and stay there once warmed up.

Thanks, I thought so too. But when 3 specialists (all highly recommended on here) and a dealer service tech all say the same I do have to consider the possibility that I am wrong!
 
Same engine in my E350 CDI.....sits just below 90.....and warms up (as does the heater) faster than any other derv Ive driven....which is nice at this time of year!🥶
 
Doesn't sound right, mine gets up to about 89/90 and sits there no matter what type of driving.

Previously I had seen the issue you were experiencing and thermostat required replacing.
 
OK I am convinced I'm not going mad. Thanks. The iCarsoft reader does not pick up any codes. Is there any test/measurement I can do to convince the garage this is not normal?

Oh and the heater output seems fine - this seems to convince techs that the thermostat is fine.
 
Same engine in my E350 CDI.....sits just below 90.....and warms up (as does the heater) faster than any other derv Ive driven....which is nice at this time of year!🥶
Yaah mine heats up quickly too, which as you say is nice, but this is the point at which all the techs I've spoken to say "well if you have heat coming out of the vents and it warms up quick its fine then as if the thermostat was the problem it would not heat the cabin" even though it only goes to 60/70...
 
The diesels have an electric heater element to get hot air out of the vents quickly after a cold start, so heat out of the vents proves nothing. As everyone else here has said, your thermostat is definitely faulty. Can't explain why all of the techs that you've asked have said otherwise.
 
No me neither... all immediately said 65 deg on motorway (if heat out of vents fine and engine heats up quickly) is fine. Maybe its another fault and there's diesel fumes in the cabin and I was hallucinating 🙃
 
Opening temperature is actually 92 , whoever is telling you 65 shouldn't be allowed to trade . Even fully open and running ,with the heaters on max, it's around 90 on the dashboard.






An open stat will still see heat into the engine on tick over upto 90 ,which will take alot longer, but as soon as you drive at over 50mph say , the wind rush over the radiator will knock the readings back down dramatically, your mpg economy will also take a hammering.
 
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I just don't know. You all sound correct, my instinct says I'm correct, the opening temp on 92 deg suggests a running temp around that... but 3 specialists and 1 main dealer say 65 deg on the motorway is fine. Unless I have got the wrong end of the stick... or they have not understood my explanation. No idea. Its booked in (against their will!) with the main dealer for a diagnostic. Not sure what to say though if after 200 quid it comes back as no problem here sir...
 
Just found this post with exactly the same issue of cool running on motorway but heater works fine:

Everyone including a MB specialist reply says thermostat...

Am wondering if I should monitor oil temperature and record on icarsoft? (as proof for the technician)
 
Just found this post with exactly the same issue of cool running on motorway but heater works fine:

Everyone including a MB specialist reply says thermostat...

Am wondering if I should monitor oil temperature and record on icarsoft? (as proof for the technician)
Id already said it will still get heat into the cab , even with a broken stat . But a good stat that won't open till 92 will retain heat in the engine coolant , and maintain 89/90 thereafter . A car with a broken stat might open at 65 ,but the coolant won't retain any heat . It's like a kettle switching off halfway through a boil , your tea or coffee will be cold to drink.

Icarsoft will tell you the actual coolant temperatures . There is a chance the stat is opening at 92 but the sensor to the dash that sits inside the stat housing has failed.

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Cheers.
I don't suppose you know if it can be tested, e.g. how many ohms it should be etc?
 
Just take it to another garage and tell them to change the thermostat for you... get one from MB - unfortunately it's not the cheapest thing in the world. Expect to be charged an hour and a half to two of labour, so probably over £300-£350 all in
 
I just don't know. You all sound correct, my instinct says I'm correct, the opening temp on 92 deg suggests a running temp around that... but 3 specialists and 1 main dealer say 65 deg on the motorway is fine. Unless I have got the wrong end of the stick... or they have not understood my explanation. No idea. Its booked in (against their will!) with the main dealer for a diagnostic. Not sure what to say though if after 200 quid it comes back as no problem here sir...
Check AUTODOC for a thermostat price .
If the main dealer is going to do a diagnostic on the thermostat surely that price isn’t far off the cost of a new thermostat?? (Or at least a long way into it )
The dash gauge should show around 90 and sit unmoving regardless of anything .
Thermostats don’t last forever so should be considered as a consumable .
 

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