Oslo

One thing that you guys that have diesel engine must be aware of. The diesel oil is circulated, so that the oil that the Common Rail Pump can’t take care of is circulated back to the tank. Actually, the pump normally uses only 1/3 of what is fed into the pump. The rest is returned to the tank. Now, the Common Rail Pump operating temperature is very high (up to 70 degrees centigrade). That means that after some time of driving, the temperature in the tank will reach almost the same temperature as the pump. (The less in the tank, the higher the temperature will get)

But if the diesel that you put into the tank is cold, and you only have a few gallons left, the sudden change of temperature can destroy the Common Rail Pump. Some cars (even the VW Golf (or Rabbit), Audi and Opel) have a cooling device that will cool down the circulated oil. But that is not so on the Mercedes. The CDI 3 (third generation of CDI engines. Don't know when that was introduced) should have a valve that regulate the return flow, so that less oil would be returned to the tank. (Something for your diesel guys to check out.)

Therefore, a good advice is to fill up the tank when its ¼ left if it’s cold outside. Some people also say that this is a good advice on the Mercedes even if it’s not cold due to fact that the effect of lubrication is reduced when the oil is hot.

ENEM

This is my first diesel car and your advice is welcome, but quite worrying (if I have understood what you are saying correctly).

I am used to just filling up my car when I want, maybe if I pass a filling station, when it is nearly empty or sometimes just before a long journey. I pay no heed to the outside air temperature, the temperature of the fuel in the tank or the temperature of the fuel at the filling station!

I see nothing like this in the manual (and heard nothing from the dealer) and wonder how likely it is that the common rail pump will be broken simply by filling the car with fuel! It sounds like a pretty poor design if this is even a remote possibility.

Should I really worry about this problem in practice?

Has anyone actually experienced the problem?

glojo

As usual Oslo is quite correct, BUT let's not loose sight of reality. The fuel I believe gets heated due to being compressed, no doubt on its return back through the system it will loose some of this heat.

This is my fourth CDI, third long term, and I have read Oslo's post with great interest, and now I will read further threads with just the same interest. Worry? No, Concerned, No.

Look at all the modern Sprinter vans on the road, most, perhaps 99% have CDI engines, how many have you seen broken down at the roadside????

Be more concerned about all those speed camera's out there and just enjoy your new CDI.

Regards,
John
Beautiful morning in Torquay

Barry45RPM

Also remember, (In the USA) you are used to filling up whenever you want because every gas station sells gasoline. However, every gas station does not sell diesel. (Not that its not around, but when you need it, cruising around looking for it might be worrysome) My dad usually kept his diesel topped off.

glojo

What a very good point. I can only talk about the South-West of England and here every garage that sells fuel, sells both diesel and petrol. Certain garages also have disposable gloves sited on the side of the diesel pump. In fairness the pumps are usually very clean and it is very rare to see any spillage of fuel.

There has also been a very slight increase in garages selling LPG for vehicles.

Regards,
John
Overcast with sunny spells in Torquay

johna1

I imagine that 'cold' in Norway could be quite extreme. I am sceptical that MB would release a car you could break by refilling with fuel under normal conditions.

Oslo

You’re right. Cold weather up here is -20 or less (-4 Fahrenheit). But the information was taken from a well informed German board. The temperature is normally not that excessive down there. But they have the Autobahn, and are known for their long trips. It’s actually some MB mechanics that gave the above mentioned advices. They said they had several cases that can be tracked back to this issue.