Hi everyone,
I'm currently with a cls500 that has that sweet round M278 engine but with a tiny issue of "disappearing coolant" which lead me to bring it to 2 garages for various tests (no answer on where it's going so far, reassuring on head gasket with no CO/CO2 detection in expansion tank).
The second garage I brought her to was very scary on the effects of ethanol and had quite scary stories to tell on it and how I shouldn't run her on it. Effects being possible knock, spark plug degradation (and sometimes breaking) and cylinder scoring.

Has anyone of you run one of these engine on up to 58% ethanol for a long run? any hindsight on the engine capability to run with that fuel ?
Thanks to anyone reading.

Why are you running E85 in your car, unless it is designated Flex Fuel? My 2014 C350 can run E85 because it was built that way, but any car not engineered to run ethanol at those high concentrations can damage the fuel system and other engine components. In any case, I don't see how losing coolant would be caused by running ethanol, though it may.

Did either shop run a block test for exhaust gases in the coolant tank? It sound like the first may have. But you can always test it yourself. If there is a head leak, from either the gasket or the head itself, it will show up.

They did and it came back negative so it seems it's not the head gasket or a crack.

I run E85 because it has a reprog to run it, I did not just put it in without any kind of prep. Though if it's detrimental to the cylinder treatment (not nikasil but can't remember the name) I'll stop right away.

Supposedly, running E85 (or anything above 10%) requires more than programming, but also switching out hardware components, such as fuel pumps and fuel injectors, to ensure they continue to function properly. You may have already done this, but here is a helpful video.

FWIW, I found running E85 to be no less expensive than premium gasoline because the mpg with E85 is about 70% of what I get with gasoline. When I compare the cost per mile, gasoline is better.

Thanks for your input, I'll check the various elements of this kit and video.

Here it's interesting as E85 is around half the price of regular gasoline (0.994 E85 / 1.939 E10 / 2.039 premium in €/l), unless it's detrimental to engine then I'd change it ASAP as said before.I know my M112 is running perfectly fine with it after I changed injectors, I need to do the proper calculation for the injectors but they should be appropriately sized. As for fuel pump, I'll check again but it should be OK.

E85 is not cheap enough here for me to use it, though my C350 is configured to run it.

I run ~54% Ethanol. I run out of fuel pump if I go much more, but I also dump in a lot more fuel than stock. I have no doubt it would run straight E85 if stock. Probably even E100.
Not sure how many miles I've used it but I suppose 15k at various %. I started ~25% and worked my way up. You don't need to tune for it, technically, or I should say I don't need to.
The ECU will adapt to it, which only takes a few seconds, but it can also fail to adapt if done wrong. Idle is where it may have a hard time adapting and fail.
Basically I'd just add maybe 15% at a time and it should adapt without issue.

If you add too much Eth, with the engine Off, that's where it might not like it. With the car Off it won't have a clue that Eth is in there until the O2 sensors warm up. Once they do your STFT will shoot up, and LTFT will follow shortly after, especially if get out of idle mode. If your STFT shoots up too high, 25 to be exact, it'll fault out the closed loop system. I don't believe it'll tell you it faulted, mine won't, so you have watch your STFT and Lambda. ST will go to 25 for a few seconds, then when it faults it'll go to 0 and Lambda will go to 1.00. Simply restart the car and closed loop will work again, after the O2's kick back on that is.
Cars will vary, but mine I think 25% Eth was about the point where STFT had issue. I suppose you could also add Eth while idling.The gas pump should add it slow enough for the car to adapt.

I've heard stories about fuel sys damage, almost always older cars. For my car just hearsay and no positive yes or no, so I figured I'd run it and find out. I have been adding Lucas fuel inj cleaner, at ~1/2oz per gal ethanol, as a little insurance.

Thanks a lot for all these info.
I'm currently fighting a coolant leak that no one has found so far, my next appointment is with a trusted dealership and I hope they can find the issue.

So far, I haven't really seen any changes between 100% E85 and <5% ethanol fuel except startup and exhaust smoke (E85 tends to smoke for longer and a bit more but I read it's to be expected).

I may take a look at what it takes to change the fuel filter and do it at some point if it's easy enough as turning to E85 cleans the tank and sends all of the impurities over there, which is often a case for issues.

I recently went to buy spark plugs for a change in my M276 3.5L NA engine and found FCPeuro listed two different part numbers. One part was cheaper than the other by a couple of dollars. When asking why there were two different part numbers, I was told that one was for FlexFuel and the other was for normal gasoline. So, you may need to change out spark plugs, as well.

Is the only evidence of a leak "disappearing coolant"? Nothing dripping? No coolant in the oil? No oil in the coolant? No exhaust gases in the expansion tank?

Did either shop run a block test for exhaust gases in the coolant tank? It sounds like the first may have. But you can always test it yourself. If there is a head leak, from either the gasket or the head itself, it will show up.

Yes, I'll soon change the spark plugs, I need to do the same on my M112, they're getting old anyways but are quite a pain to remove.

We have found no outside evidence of leak but coolant is disappearing quite fast even when the engine isn't running (about 70ml/2.3ounces per day). I do not have any evidence that it goes in oil as dipstick shows only oil and not milkshake and oilcap isn't quite milkshaky when removed after running. And no oil is going back to the expansion tank. I have a little bit of smoke but nothing alarming especially with the cold temperatures and high humidity we currently have.

Tests that were done are:
- visual inspection (two different garages), nothing to see and I didn't pick anything on carboard I put under the car in the driveway either
- pressuring circuit and visual inspection (once), no pressure drop and no apparent leak
- chemical test at expansion tank to check for combustion gases (once), came back negative.

I have now stopped looking with independant garages, the second one basically was very expensive for how little he did and did not warn me he didn't have all the equipment needed.
Next one is a Mercedes dealership I trust (they do cars and trucks) in a couple of weeks, they're expensive but I expect a definitve answer. From what I understand they will redo the same tests and other that would be conducted would go with bore scope and other means of detection if needed).

We'll see how it goes.

This is what needs to be done to make sure there is nothing wrong with the head or block.