AppleFan1

I haven't posted anything for a while here, but I need to at this time. On a trip back home from Akron, OH to Des Moines this past Monday, something very unusual took place with my 2014 S550 transmission. All of a sudden, about 100 miles from home and just after I filled the tank with gas (91 octane), I was not able to accelerate while on the entrance ramp to I-80. It would get stuck on the 4th gear and that was it. The teach would shoot up when I would step on the accelerator, but the speed would not get much higher than about 40-45 mph. This really scared me. Then it occurred to me to manually shift it with the shifter on the steering wheel. Thus, I was able to accelerate to about 65 mph. However, I had to constantly keep pressing the shifter to keep it up on the 6th or 7th gear. Otherwise, it would down shift quickly to 3rd or 4th gear. I did see that the engine light indicator appeared on the driver info screen. The engine temperature was right where it should be at 200 degrees.

About 15 miles (and 15 minutes from home) from home we had the unfortunate experience of getting stuck in a horrible traffic jam (multiple car pileup due to a heavy snow storm) near the I-35/I80 interchange east of Des Moines. We were basically stuck in traffic for an hour and a half. After the mess was cleaned up and we were able to move again, the car seemed to behave normally with the acceleration, but the engine light indicator was still on.

Yesterday morning, I called the dealer and they could not schedule me unto tomorrow morning. So, I have no idea what the problem is with the transmission. It may be a simple as a computer chip going bad. I will report back when I find out tomorrow sometime. I can still drive the car now, but I have not taken it on the Interstate since Monday. Incidentally, the gas mileage for this trip was horrible. I only averaged 23 mpg. I normally get 27-30+ mpg on the highway. I think it had to do with the extremely cold temperatures (5-8 degrees the entire way back from Akron, along with strong cross or head winds for most of the trip. But perhaps the transmission problem may have played a key role as well.

Now for some good news. This car is absolutely amazing on snow with all season tires (RFT's) I might add. I could not believe the amazing control and security I felt passing others at 65-70 mph in the snow. Buying the 4Matic option is a MUST in winter snow climates in my opinion. This was the first real test I had driving the car in snow.

Quadcammer

valvebody or conductor plate.

can't believe mb can't get this right.

alshaikh85

I feel the problem you are having is probably caused by the fuel you used. Do you usually use this type of fuel (91 octane) on your car?

AppleFan1

91 Octane is premium gasoline in my state.

AppleFan1

Can you please elaborate? What do you mean by "valve body or conductor plate"? What does that have to do with the transmission?

alshaikh85

I am suspecting the fuel because of the engine light indicator and the low mpg you suddenly gut (23 mpg).

Just to eliminate this possibility, I suggest once you empty this tank, you refill with the highest premium fuel available.

I live in Saudi and we have 91 and 95 octane. For Mercedes we use 95.

Good luck.

steveb9771

91 octane is the highest available in most states and is approved by Mercedes for all vehicles originally sold in the USA.

hyperion667

I don't think I've ever seen 95 gas, only 93, at any rate; sorry to hear the problem Mr Bean, hope you get it worked out

jrpisha

Me either. 93 is the highest octane I can buy in Michigan - usually only at Sunoco

emilner

It's not the gas. Has nothing to do with gas. If gas was the problem you would have hesitation and sputtering. You have a transmission problem. It's either software or hardware. If it runs fine the next day then that leans towards software but doesn't rule out something wrong with the tranny hardware....

alshaikh85

W221 used to suffer from high failure rate on the transmission, I thought they fixed it by now! Is this "The best or nothing"?!!

I hope it is not a transmission issue.

Let us know once you get it checked.

Streamliner

Call me crazy, but after reading a full page article in "Car & Driver" several years ago, I started using regular unleaded 87 octane and have done so for over ten years now, with no problems and excellent performance. My 2013 S550 ran beautifully on 87. That said, I must admit that I am feeling a bit of roughness when my one week old S550 is at idle. So, I am running the tank down to near empty and will fill with premium to see if that cures the problem.

I know we all love our cars and many here like to think of themselves as Michael Schumacher as they pilot their huge saloons through our metro areas, but the C&D article went into great detail about how so much of that extra octane is going right out the exhaust pipe with no benefit, so why pay for it?

alshaikh85

I sincerely hope it is a small issue and not a transmission problem on your brand new car

looking forward to hear good news from you on this.

06E55

How in the world do you get 27-30+ mpg in this car? Are you keeping it at 55?

AppleFan1

Don't ask me, but I do. I usually drive at speeds of 75-80 mph. Now as I stated above, there is no way I will get 27-30 mpg when it's extremely cold outside along with a cross wind or headwind as I had on this trip back west from Akron. But the transmission problem may have contributed a little as well for the lousy gas mileage for the trip back.

AppleFan1

I am also certain it has something to do with the transmission (software or hardware). I am anxiously waiting at home for the dealer to to hopefully give me good news. It seemed to be running normally this morning when I drove it to the dealer.

Quadcammer

ok, so your 722.9 transmission, like all torque converter automatics, uses fluid to create drive from engine to wheels. The routing of that fluid is controlled by the conductor plate (sits inside the tranny pan and takes signals from the Tranny control unit). What then mechanically routes the fluid is the valve body. Combined, these items control line pressure, shift times, slippage, etc. If there is a glitch somewhere, the car usually goes into limp mode (sounds like your case) and can sometimes lock you out of all the gears. If you do a search for 722.9 valve body on google, you'll see its a very common problem.

silly to do so. Generally when running lower octane, the spark advance is reduced through knock retard, and the pennies you save result in lower fuel economy.

if you drive very very easily and never load the motor up, you might be just fine, but why buy a powerful car only to reduce its power through knock retard?

emilner

^Yup!

Diesel Benz

I live in Europe an we have octane figures as high as 98 (some less commercial gasoline can go higher). But the point is our figures are not for the same measure, not comparable with the US octane figures. See wikipedia to find out which US figures equal with ours or yours.

For Mercedes we use Diesel (mostly).

alshaikh85

Indeed based on Wikipedia, in Europe (Germany for example) you have up to 100, and the mainstream is 95 as it cost more to produce 91 due to low demand.

I think I should switch to diesel too, my car will like it

AppleFan1

Ok, the verdict is in. The problem is caused by a faulty boost pressure sensor. It is not allowing the engine to gain full power. Quadcammer, may have had the right idea on his analysis above, but I'm not sure. The dealer had to order the part and it should arrive on Monday. I guess if I understand this correctly, the problem is not very serious, but I'm obviously disappointed that this happened to my car. Meanwhile ,the car is drivable at least around town.

ajm0623

bean - your degraded mileage experience on this particular trip is probably due to your being stuck in 4th gear the majority of the journey.

AppleFan1

Yes, ajm! As I posted somewhere above, the problem I had likely contributed to the horrible gas mileage on the trip. I was not stuck on 4th gear for the majority of the trip but even so, there was some sort of a problem with the transmission for the past month. I mentioned above that I started noticing a problem on downshifting where it would grab at about 20 mph. That was probably almost a month ago now and come to think of it, my gas mileage was not as good as I was use to even in the city driving lately. Having said all this, I don't want to take away from the main culprit, however: Bitter cold and Arctic cross or head winds.

I look forward to getting this resolved early next week, but the car seems to drive pretty smoothly now after being in the shop yesterday. Maybe they did some sort of tweaking. I don't have the "service engine light" any more.

ajm0623

complicated machines to be sure. good luck my friend!!

AppleFan1

It's actually called "turbo boost pressure sensor". The dealer notified me that it arrived, but I have to wait until Thursday to install it. The service department is very busy.