For years on and off now. I wondered just why there was a milage discrepancy reported from owners. Some that even purchased them new stated they never got more than 25 miles per gallon. While some averaged around thirty. Both claims where on their highway milage. What was so strange was the difference quoted appeared almost so constant. A least to me.

In a way it should have been variations on the reporting and that would account for driving habits. But there were basically few if any. It was almost always 25mpg or 30 mpg usually within a mile per gallon.. Usually missing where 26,27,28,and 29 miles per gallon.

As 240d owners are aware the oil drag when engine is cold is really noticeable. Combined with most manufacturers engine oil coolers probably are not thermostatically controlled. The natural aspired Mercedes engines oil coolers are thermostatically controlled. As are the turbo versions.

So the possibility entered my mind. If the thermostatic control was always open. It could be keeping the oil cool enough on the highway there remains enough of an oil drag factor that could create the loss of the five miles per gallon. The oil viscosity is simply kept thicker in comparison to a thirty miles per gallon example. Because it is colder. The thermostaic control is there to prevent it. It would also have an effect on the initial engine warm up period. With the fan pulling air through the oil cooler.

At that time I think Mercedes had two suppliers for most outsourced parts. It was an item that may not have been checked for quality control. I have never had one in my hand so have no knowledge of if they fail open or closed. yet it is in the realm of possibilities that given a choice the engineers may have specified open.

You never expect a thermostat to last this long in the main cooling system. Yet another one lurks in the oil cooler circuit.

This should be considered just speculation at this time. On the other hand I was unable to find anything else over time that was a decent fit. I can think of a few tests. Someone might come up with a really simple one.

I only post this because it should be both considered and perhaps a test made on a lower fuel milage example.

I am not claiming this is factual. Just that it is worth both some thought and some owner checking it out perhaps. It to me is almost a shot in the dark. Yet I have never found what was the actual cause of that milage discrepancy. I even got tired of looking. Yet there had to be something. I just do not have the time available right now. Plus I am not using a 240d either at this time.


The natural aspired engines are not the tremendous heat generators in reality. That the turbo engines may be. I took a 617 natural aspired engine on a trip years ago. The bridging between the radiator tubes had rotted out over large areas of the radiator. Except on hills I did not have to run the heater to keep the temperature down. It also was a hot day on that trip.

This also displaced the thought that the cooling systems on these engines where only marginal. There were so many posting with running hot in those days. It helped me decide that if everything was relatively normal on these cars with the 617 and 616 engines. The cooling system was more than adequate.

Comments and considerations of others welcomed. I run pretty close to the edge at times. A test once refined and accurate should be applied to the 617n/a and turbo engines as well. I think it can be very easy.

I at least suspect I have also made a little headway on why in general the 617 n/a may be so poor on fuel milage. With everything in good condition. Where the 617 turbo one is much better. I do not believe it is all in the rear axle ratios being different. Although reasonably they are part of it. This too has taken too long.

There also may be a way to increase the fuel milage and power on the 616 and 617n/a engines. As well as the turbo engines but not for as much gain on them. Why people have missed this is understandable. I also have real concerns that Mercedes did what they did. It is a fair amount of work to deal with the design deficiency. They built aircraft engines at one time so they had to know better.

Anyways I will finish with a thought on a possible quick check of the oil cooler thermostat. A colder day would be better perhaps. A magnetic oil pan heater may be the best way.. Or any safe type of oil pan heater. I do not think the block heater would do . Read the temperature of the hotter oil pan. Read the temperature of the oil cooler. Run the engine for perhaps a minute or less. I think the oil temperature increased to 150 degrees would be enough for the test. The oil cooler temperature should not rise. You check it just before you start the engine and just at minute intervals that allow the hotter oil in there if present time to heat it up. After you have shut the engine down.