2010 e350 cabrio, w212 is more active. M272, 7g+.

I'm having this super weird problem that I have not been able to solve. Really need some ideas.

Car runs and idel fine. A while back I started feeling that the car shifted rough and while driving steady at highway speed the rpm starts to fluctuate. When the fluctuation happens the car almost feels like it moves back and forth. Like the car change gears but no gear change is really happening.

Change transfluid and filter. No change in above problem.

Took the car to a "Mercedes specialist". They found codes for nox sensor. Which my scanner could not see for some reason. Replaced sensor. No change in above problem.

Also did an adaptation, rough shifts started to go away. The only time it shifts rough is on upshift when rpm is high, such as after kickdown. It doesn't shift rough every time. Rpm still fluctuating.

Did some inspecting. Found a bad o ring on dipstick causing a vaccum leak, replaced it. Also disconnected battery and cleaned maf. The car runs great, no fluctuation, no hard shifts. Everything is well. For two weeks and about 3 gas tanks. Today, on the highway, the fluctuation started again for no reason. The fluctuation only occurs above 100 kph, varies between a couple of 100 rpm. Only at steady speed. If you drive in any other way the problem won't occur.

I've found a small tear in the new o ring since it was over sized and the only one I had att the time. Replaced with the right one. Disconnected battery, cleaned maf, new adaptation, visually checked for vaccum leaks, no change.

Any suggestions on what can cause rpm fluctuation, disappear for two weeks then come back? It could be an other vaccum leak but feels unlikely?

Car is in good condition, about a 100k km. No codes.

Any suggestions are much appreciated.

 

Possibly torque converter lock up clutches. I don't think it will throw any code until it gets really bad.

Is the problem the same in sport and eco modes?

 

This is prime time for your fuel filter... read your fuel pressure using your scanner:
60psi steady or collapsing?

You can spend a little on new sparkplugs + boots. Coils are ok.

Its interesting how you noted that rebooting helps sanitize this. How about doing battery float charge?

The disturbed tranny is a side effect of your engine acting poorly. It will normalize as soon as engine runs predictably.

By Nox you mean Lambda sensors right?
This help your ECU run well given enough fuel is on tap.

 

Thank you for the response.

Been thinking about lock up clutches as well. But to my understanding the 7g has lockup in all gears? This only happens above 100-110 kph. All modes Inc manual, all gears and all rpm at those speeds.

Checked fuel pressure using Icarsoft yesterday. If I'm not remembering wrong it was around 4,5-5 bar more or less all the time. What is normal value?

Sparkplugs on order.

I notice my battery voltage was a bit low sitting 12v while driving. Shouldn't it be higher?

Nox sensor is O2. That's correct

 

12v while driving is actually a drain below normal resting voltage of 12.5V.

Regardless of O2 sensors vs lambda wide band... what mater is the ECU computed fuel trims. Read Long-term FuelTrim for both banks at idles.

 

Long term fuel trim for bank 1 9,38% and bank 2 sits at 12,5%.

Short term varies between negative and up to around 10% over a run.
Do you make anything of this?

 

that practically shows your engine fuel control and ignition are both working well - That's a great deal to know.

Your fuel delivery pressure is fine at 70Psi.

So that bring you back to "12V while driving". Measure exactly how voltage control goes under load. ALT should be supplying 100% of all chassis current, not battery.

You can setup "Workshops menu" to read that voltage - Anywhere above 12.6V is ok and below is troubles.

 

Hm, what would be the next thing to check? Just order a smoke machine to check for vaccum leaks.

According to your opinion there is nothing wrong with fuel system based on current information?

 

vacuum leaks at 100Kph are unsignificant... PCV vacuum leak only around idle.

Fuel trim and pressure could not be better.

The next thing is your reported low voltage control below 12.5V while driving - Thats a real issue.

 

No problem at idel and low speed.
Will check voltage but don't think that's the issue.

 

if you don't have the voltage your reported as 12V then you can concentrate on hot tranny acting up.

Do you have good/clean MB oil in your tranny ?
No leak or lower level ?

 

I will get back to you on the voltage.

Tranny level all good

 

You can also check the transmission and TC values live while driving just to see if everything is operating as it should while the symptoms are happening...

 

Cant see anything with the Icarsoft scanner. Lock up slipping all the time according to scanner with is not correct.

Any recommendations on a scanner for cheap that lets me see more?

 

search around... better prices are all over

 

Look at the speed difference between turbine and shaft. If the lock-up disk is slipping between that "normal" about 50 rpm and some bigger value, you will see it.

I also have iCarsoft MB II and it says the lock disk is slipping that I don't believe is true. It shows about 50 rpm speed difference between turbine and shaft. If you see this speed difference jump up and back down when car is fluctuating, then there is a problem with the TC locking and you can stop messing with everything else while looking for the problem source.

 

Have you fully completed re-adaptation of torque converter through the scanner tool? It requires to meet the criteria numerous times at multiple torque ranges to see the results. Try it

 

Why do this once manually while the TCU already does this all day long by itself?

 

Will check the lock up clutch speed today. Can I even do an adaptation with icarsoft?

 

Did a long drive today. Had a hard time replicating the problem. Only very slightly rpm fluctuation and surging at highway speed this time.

Anyhow, difference between turbin and shaft were always around 50 rpm +/-20. There were no difference in the values when the problem occured and when everything was normal. No spikes in rpm or similar so I guess no problem there.

Fuel pressure at rail was constant at around 2,836-2,900 psi.

Something interesting was that the tourqe app picked up fault code p1141. Icarsoft did not see the code. Which according to my understanding can be fuel issue or intake runner flaps among other things. My engine does not have the three problematic actuators on front but instead only one. Engine code is 272984 31 728445.

What do you guys make of this?

I'm not sure but I think the problem might be getting worse when I'm low on fuel leading to believe I might have dirt in the gas tank. But at the same time, dirt in the gas tank should give me more problems than rpm fluctuation at highway speed? Pressure looks good as well so there should be no problem with fuel pump and filter?

 

If you give the car the beans, how low do the 2 fuel pressures drop?

It's concerning that your scanner didn't see the code. Perhaps you need to update it or get a better scanner.

 

Will give it the beans tomorrow and we'll see.

I think it's time for a new scanner

Long term fuel trim on this run was around 9% for bank 1 and 10% for bank 2.

I have experience no issues when driving faster. The rpm fluctuation is only at steady throttle and highway speed. The speeds were the problem occurs differs. Some times from 120 kph and above, sometimes from 90 kph and above.

 

can this be about the intake plenum flaps of M272... try exercising them with scanner test.

 

Can't do scanner test. Then I have to go to Mercedes/Indy so let's wait with that. Can the launch scanner previously posted preform such a test?

As stated before my manifold have som later design with only one actuator which seems fine. I can move it by hand and feel the flaps. Even though that doesn't say anything about actual operation.

 

How about Crank Shaft Position Sensor? It is the one that reads engine speed and they are known to get flaky before they fail.

Could randomly bad signal from CPS cause engine speed to fluctuate?