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.

The difference is probably related to how the mileage is calculated. I do it by filling it, shaking the car to let the bubbles out of the tank and repeating that process until the bubbles cease to release.

I've taken trips though the NE Georgia mountains and gotten 30 mpg but I was an old man even back then. I drive like that on purpose, my hearing and vision is fine.

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84 300SD
85 380SE
83 528e
95 318ic

on a good running 240d I always got 20 around town and nearly 30 highway. stick

Automatic would be maybe 5 less.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Had a 115 and 123 240D both manual transmission. The 115 could do about 30mpg pure highway. The 123 could do 34.5 pure highway. Both conditions were zero city driving. I thought some of difference could be attributed to the difference in aerodynamics.

Also believe some of the differences reported can be odometer inaccuracy, like to verify them via interstate mm's. Odometer variation wouldn't explain the either or though, I'd expect some numbers in the middle.

I never thought about the oil cooler thermostat, interesting hypothesis.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

That's pretty consistent to what I've experienced.

My 240D is an automatic. My driving is about 25% free-flowing freeway (average speed of 60-65mph), 75% slow driving (whether stop-and-go traffic on the freeway or surface streets).

Over the 2,500 miles driven that I've tracked with Fuelly, I've averaged 19.6 mpg.

I usually get somewhere between 24-25 city or highway. That said, I’m doing highway at 75mph for hours at a time. Since my last fill up, I’ve been experimenting with driving more reasonably, shifting earlier, etc. Should find out soon whether it’s made much difference.
I run diesel Kleen on each tank. While it’s quieted down the clatter, it’s done nothing for economy.

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1981 240D 4sp manual. Ivory White.

The slower you go the less fuel you'll use.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

On car A when purchased it had auto, I got solid 27 in mixed highway (7omph) and city. When the IP took a dump it never got over 25. Since motor/IP rebuild and manual trans it now is 30-32. Thats a daily driver, at times 4000 miles/month.

Car B similar use and auto. Was 25-27 until that IP took a dump, then never over 25. Pump rebuild and injectors, is 25-26mpg now at 4000mi/mo.

I look for ways to possibly improve these engines. I think the flow passages in the head for both exhaust and intake runners are not the best design. Plus there is a lot of material that can be removed. Someone should offer this service.


I believe they are very rough passages as well. There was no effort at port matching either.


I found a 617 turbo. That a guy was doing and the amount of material he removes was really substantial. Using an ultrasonic tool to determine the thickness of walls to leave. There is a nasty 90 degree turn that is also reduced in diameter at the same time present in them.


These engines may be semi strangled for air intake and exhaust extraction. It just might hurt fuel milage as well. I did notice that adding a turbo to the 616 does not really gain as much as one should expect. The 617 natural aspired engine of course is weaker but should the milage suffer as much as it does? I do not think the rear end ration being greater makes the difference.


Anyone know much about this area? The head casting for the 617na and turbo are reputed to be different. I wonder if the same casting cores are used for both? Other than the valves. They may not be as the valves are also larger I think.

How exactly did it take a dump? What happened?

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1981 240D 4sp manual. Ivory White.

An obvious change was the decrease in fuel mileage with no change in use. Then mostly poor power delivery, full throttle on highway just to get 65mph. Rough idle, one car had a "dead cylindar" according to a motor shop. Initially replaced injectors but that didnt much improve things.

On Car A (125000mi, 79) in fact had black fuel in the translucent filter and the tank (via return). Cleaned tank, did the purge, no fix. Eventually realized the black was oil via the IP. This IP had been serviced prior to my purchase.

Car B (160000mi, 79) was used as DD while A was being rebuilt and had a noticeable decrease in running and performance over a few months despite all the simple fixes. When A was done we downed B for the IP rebuild, much improved.

I've said it before; these things are timing out, not wearing out. I now include IP rebuild in all expected maintenence/repair plans.

I would be surprised if porting the heads would yield much improvement in power or fuel mileage.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Me as well but looking at the pictures of them they are pretty sad. We will never know until someone does one.

You may gain nothing for example. Although there are head flow testing machines that may indicate there is good or really bad flow issues or not. In relation to the engines needs used.

They may be adequate as is. When I was looking at a ninety degree turn in the head. That was also seriously restrictive in area and very rough as well. Plus the massive amount of metal that guy took out on the 617 turbo head. I even wondered if it might also weaken the head structurally.

Perhaps this issue might be better in the increasing the power portion of the site. Chances are they may already have dealt with it. I will go have a look in the next few days.

Again what triggered this is adding a turbo to the 616 I remember people mentioning did not do that much. Or at least not what I would have thought. Although it could have been not enough fuel.