E350 CDI OM642 slight temp fluctuation very cold weather

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MrGreedy

MB Enthusiast
Joined
Jan 13, 2020
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2,274
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Here and there
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E350 CDI
Good day all,

I know normally when people's cars aren't getting up to temp, the first place to look is the thermostat being on the way out.
The shift I'm about to explain is relatively minor, and not the 'struggles to get above 65°C' that we often get asked about.
Given that it's freezing outside, I'm clinging to the hope it's that and I don't need to get out and change my thermostat (I have a spare on the shelf, so getting the part isn't an issue).

Normally, the car warms up as expected, and sits at a rock solid 88/89°C once warmed.

However, last night, I needed to take a 1 hour trip, and it took a long time to get up to temperature.
It was late and I was in no rush, so it was a 56mph economy cruise, nearly all motorway, enjoying some music.
I noticed that the temperature only got to about 81-83°C, except for long inclines where it would climb to the expected 88/89°. After levelling out or on decent, it would fall back to 81-83°C.
Fuel economy took an expected hit, and I was only able to hit an average of 53mpg (believe me, I can usually get much more than this on a similar trip).

My first thought was thermostat likely on the way out. But it was -4°C, hitting -6°C in places.
Is this just a freezing temperature related issue, or do we think thermostat?
I've never driven the car for prolonged, easy cruise periods in such cold weather before.

Heat into the cabin wasn't affected, nor did it have an impact on the water temperature e.g. if I put fans on full blast full heat (with the window cracked because it was boiling), it didn't drop the temperature noticeably.


Note: on the '63 engine, PCS suggest some fluctuation might be normal.
 
My C350 doesn’t get above 80 degrees and runs completely different until it is up to that temperature, gears hang onto revs until around 1900rpm before shifting up. MPG is around 30 as Opposed to 45 in warmer weather (on a 12 mile run to work and back on B roads circa 50MPH)
 
I think thermostat, especially if it’s never been changed. Good news it takes about 20 mins to change, and you won’t lose that much coolant!

The only bad news is that the stat is expensive, but you already have it so it’s basically free 😂
 
Does anyone have any 'cheats' to remove the thermostat?

Is it just chuck a load of old towels and blue roll around the area beneath the stat to catch the small amount of coolant spill?

How about the third, longer bolt on the thermostat that's obscured by the turbo pipe? is that just loosen the black intake box further down towards the rad and just remove the turbo pipe fully?

I'm really looking forward to tackling this on a cold day with skinned knuckles tomorrow. Provided Merc have a gasket in stock. I should have planned this better!
 
On my car it doesn't have the protruding bit on the turbo pipe, so there was no need to remove it... not sure it would get in the way anyway.

I think I probably got to the third bolt with a flexible socket adaptor thing... you know the ones: https://www.amazon.co.uk/Sealey-AK7...=flexible+socket+drive&qid=1676073426&sr=8-12

Yes, the amount of spill wasn't a huge deal.
 
Cheers buddy.
Yes, the tools are covered. It's just that extra protruding bit on the turbo pipe. Having watched some rubbish vids on this in German and read around the forum, it looks like once the bolt is undone, it can't be retracted without removing the turbo pipe on the 2011+ 265bhp cars.

Gah, never mind. It will be good practice for when I change the rad and all the aux belt stuff in the summer. I was going to do the thermostat at the same time (over a year later than originally planned) but the slightly lower stat temps are forcing me into that early. I'm just glad it's the relatively easy bit of all of that stuff!

I also bought an eBay special as recommended on another thread on here with a job lot of x10 the seal for the top and x10 the seal for the bottom of the turbo pipe (green and black) for about a fiver, so I don't need to worry about replacing those via the local Mercedes.
 
Does anyone have any 'cheats' to remove the thermostat?

Is it just chuck a load of old towels and blue roll around the area beneath the stat to catch the small amount of coolant spill?

How about the third, longer bolt on the thermostat that's obscured by the turbo pipe? is that just loosen the black intake box further down towards the rad and just remove the turbo pipe fully?

I'm really looking forward to tackling this on a cold day with skinned knuckles tomorrow. Provided Merc have a gasket in stock. I should have planned this better!
Did mine last week. I had to take the long boost pipe off to get the bolt out. loosen the bolt the goes into the plastic resonator at the bottom of the pipe so there is enough clearance to pull way from turbo outlet. remove bolts that hold down the fuel pipes, you can then move them out of the way with the aid of a long screwdriver to remove long bolt. Lube up seals on assembly.
 
Did mine last week. I had to take the long boost pipe off to get the bolt out. loosen the bolt the goes into the plastic resonator at the bottom of the pipe so there is enough clearance to pull way from turbo outlet. remove bolts that hold down the fuel pipes, you can then move them out of the way with the aid of a long screwdriver to remove long bolt. Lube up seals on assembly.
Ah, thanks. Yes, resonator box was the word I was looking for 👍

Not black intake box ☺️
 
Got to here:

I thought it was all going too well.

The long bolt at the back holding the thermostat in seems very stuck.
The inlet manifolds were swapped for very clean low mileage ones just over a year ago, and the original thermostat was put back on.

Unfortunately, I'm very worries about the long bolt. I've been gentle rocking backwards and forwards with a breaker bar, and it's uncracked the thread, but it will only easily turn about 5 degrees. I'm now very worried that if I continue to turn it, the bolt will snap. This worked a treat on my w203 petrol when I was replacing the original spark plugs at 90k miles, but less hopeful here.

The other option is try a low torque impact driver to see if that helps free it off.

Screenshot_20230211_141627_Gallery.jpg
 
I thought my luck was in because I went to get a new thermostat gasket (now gone up to £22!!), and there are 3 in the country. 1 at Liverpool Merc, 1 at Oxford Merc, and 1 at my local dealer, now in my possession. I thought it was going to be my lucky day.
 
Got to here:

I thought it was all going too well.

The long bolt at the back holding the thermostat in seems very stuck.
The inlet manifolds were swapped for very clean low mileage ones just over a year ago, and the original thermostat was put back on.

Unfortunately, I'm very worries about the long bolt. I've been gentle rocking backwards and forwards with a breaker bar, and it's uncracked the thread, but it will only easily turn about 5 degrees. I'm now very worried that if I continue to turn it, the bolt will snap. This worked a treat on my w203 petrol when I was replacing the original spark plugs at 90k miles, but less hopeful here.

The other option is try a low torque impact driver to see if that helps free it off.

View attachment 136886
Maybe get some decent lube onto the thread and keep gently working backwards and forward ?
 
Maybe get some decent lube onto the thread and keep gently working backwards and forward ?
The binding bit of the thread is (was) 10cm under the thermostat.

I was very carefully with the low torque impact driver, backwards and forwards. It eventually fully cracked open. Thank the Lord!

I'll have to think about what lube to put on it for future removals!
 
Obviously I can't drive to buy something.
These are my options. Any thoughts?

I also have spray lithium grease, and cheap all purpose lithium grease.

I've done a bit of reading, and there are some reports of copper grease causing more corrosion, so I'm going to go with the LUK lithium clutch grease. Clutches get quite warm buried under the engine, so it seems like a sensible guess 😬

Screenshot_20230211_145422_Gallery.jpg
 
Last edited:
The binding bit of the thread is (was) 10cm under the thermostat.

I was very carefully with the low torque impact driver, backwards and forwards. It eventually fully cracked open. Thank the Lord!

I'll have to think about what lube to put on it for future removals!
Bit of copper grease ?
Edit , just seen the pic ….either one
 
Hi,

Think I need to do this on a 265bhp car. Did you end up buying a full thermostat from MB or did you get the spring and seal?

I’m guessing just the spring as the full stat comes with the seal, no?
 

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