Ali123456789

Hi!!! Sorry if this has been covered before or is in the wrong section.

I have a 2011 E350 s212 which has developed a confusing MAF sensor fault code.

The fault has been ongoing for months but it’s now coming round to mot time so I need to get to the bottom of it quickly

for some time now, there has been a code coming up relating to MAF sensor - hot film sensor upper limit reached. The fault has not caused any change in performance and nothing noticeable other than the MIL light illuminated.

I have taken the car in to the local stealers who have at first diagnosed it with an EGR fault, and since then changed their mind and said the EGR is not at fault. (Repeated tests with their star system) They have also replace the green O ring on the metal charge pipe from the turbo saying this caused the fault and it was sorted now… (MIL still on) charging lots of money as they go. The fault has appeared not long after they previously carried out work on the inlet swirl flap motor.

I have since replaced the intake and MAF sensors and this has not helped. I have taken various readings from sensors -

MAF - engine off = 2.8g/s
MAF - idle. = 15.83 g/s
MAF - 2.5k revs. = 72.22 g/s

MAP - engine off = 100kpa
MAP - idle. = 104kpa

Lambda ER B1S1 - engine off (warm) = 1.016
Lambda ER B1S1 - 2.5k revs. = 1.016
Lambda ER B1S1 - idle. = 1.028

would anyone be able to help interpret these figures?

I had a feeling the MAF reading was high for when engine was off?
and Also that the lambda reading was high/ rich too?
I’m pretty stumped now as other than the lambda I couldn’t think of anything that could have such an effect on the MAF readings and it feels that when I take it to the dealers they would prefer to just throw parts at it and hope for the best.

Thanks in advance


Engine off readings

2.5k revs readings

Idle readings

S-Prihadi

Your MAP ( manifold absolute pressure sensor ), check it properly. Is the MAP really located at Manifold ?
At idle no way it would read 104kpa or 1 BAR, it should be 0.4 BAR ( absolute ) because manifold is under suction when engine is running.

Engine OFF MAP being 100 Kpa is like your location is above sea level a bit because it is 0.98 ATM, this is okey okey.

Ali123456789

Thanks for your reply, I was doubting the map so I have just replaced it as it could have been a cheap fix but I am getting the same readings. As far is I understand it, would the 100kpa be right for atmospheric pressure, and then approx 104kpa on idle as it would be an increase in vacuum. The readings go up with the revs to approx 145kpa at 2.5k revs which suggests to me they are correct, as the revs increase so does the vacuum etc or is this incorrect?

The map is located in the right hand air box I believe so it will only pick up the negative pressure?

The part I can’t understand with the fault code is when the codes are erased with ign on and engine off, the code reappears straight away. Surely if it was caused by a dodgy reading from a sensor or an air leak when running, it would only reappear when the engine is running etc?

thanks

S-Prihadi

I think its best first to accurately name the sensors.

Pressure sensors :
01. MAP or Manifold absolute pressure. This sensor has to be after the thottle body and after MAF ( mass airflow sensor ) and inside the intale manifold.
E350 engine in a 212 models can mean M272 or M276 of which both are not turbo or supercharged engines.
A non turbo engine can never have a MAP sensor reading higher than ambient atmospheric pressure at idle or WOT.
With a turbo or supercharger, we then get forced induction and air pressure at intake manifold can then be increased above atmospheric.

Since a MAP sensor reads absolute pressure, it then has to read ambient air pressure and whatever forced induction extra pressure exist and it can read vacuum too.
So at engine OFF and assumed sea level, weather is normal, it will read 1 BAR / 100Kpa ( for easy sake of 1 ATM ). Use this value as imaginary ZERO.
When your engine is idling the intake manifold is under vacuum because the pistons are sucking air, the MAP must read below 100kpa...vacuum is NEGATIVE pressure in respect to ambient/atmospheric pressure around us.
If car is turbocharged, when turbo spooled up and give positive pressure say a 1 BAR turbo, at the peak of the turbo boost, your MAP will read 1 BAR of atmospheric plus 1 BAR of boost = 2 BAR.
There is nothing wrong for a normally aspirated engine to never have MAP sensor reading above its ambient atmospheric pressure at idle or engine running, it is normal....it is supposed to do that.

Other pressure sensor/s if not specified as ABSOLUTE, it is then Gauge pressure, which means it is reading zero psi/bar/kpa in our atmosphere, example is tire pressure gauge when left on the table.






NOTE : I do not know anything about M272 engine.

MAF sensor : In M276 literature, a MAF sensor is only used if the engine is the unique DES type, stratified combustion. Common one is the DEH homogenous combustion.
Read this : https://mbworld.org/forums/attachmen...ice-manual.pdf










New MAP sensor does not mean it is 100% healthy, more so if it is not a genuine MB one.

Also please clarify :
MAP - engine off = 100kpa <<< is this with ignition ON to provide all sensors with power ? Some in USA call it Key-ON-Engine-OFF, meaning ignition system standing-by, dashboard lights up all its warning. Another step is engine cranking/starting.
MAP - idle. = 104kpa

Now check if your engine is a DES type and actually has a MAF sensor ? If it does not have MAF sensor.... well, your scanner should not read it as having a MAF.

Borrow a vacuum hand pump from your friend. Key-ON-Engine-OFF to activate the engine computer, read your MAP value in open air and while sucking the sensor with a vacuum pump and blow air into it a bit if possible to see what it will actually read.
With a bit of work, the MAP sensor pipe shape part can be hooked to the vacuum pump's hose.

Below is my MAP sensor B28/7 , most likely your engine use the same type






Have fun...........

Ali123456789

Thanks for your above post! Sorry should have clarified this in the first post as I thought i had written it, sorry for any confusion. The engine is a 350 diesel, (believe om642)? Which is why the readings start at 100kpa and rise, so under small amounts of boost when engine revs are increased, positive pressure as opposed to vacuum which I put earlier 🤦🏻‍♂️. Readings have only been taken when vehicle is stationary so although positive pressure will rise it won’t boost properly unless under load.
the two MAF sensors replaced are the two in the y piece plastic inlet pipe, I replaced the two sensors and pipe by ordering a replacement. And replaced map sensor today.

All readings etc taken with ignition on when engine is off to supply live data to the diagnostic tool.

I can’t understand why the code would instantaneously re appear after being deleted, while the engine is still off? surely the fault would have to be a sensor giving a massively false reading constantly for the ecu to know the fault is constantly there even when the engine is off? With the map reading 100kpa when engine off this appears to be ok and shouldn’t cause a fault in itself.

Thanks

S-Prihadi

LOL, diesel engine. No throttle body, no wonder no vacuum happening.
Sorry, I do not know much about this engine to assist you.

You best list your exact DTC/troublecode number, so that those who knows about diesel may help.
Xentry scanner ( Mercedes official one ) usually have explanation about limits/value for a DTC to be triggered.

Take a read here : https://mbworld.org/forums/e-class-w211/795571-p2633-need-help.html

HFM ( Hot Film Mass ), reset drift compensation value = MAF reset. Has that been done using Xentry ?

S-Prihadi

One more thing.
When you want to search issues, do not use MBWorld search engine. Use Google, it is faster.
Copy the exact troublecode sentence using Google , that is how I found the MB link I posted earlier.
Good luck