Hello,
Currently I am driving Infiniti Q50 2.0T, which has got the same engine as many of MBs - the M274.
I've been trying to find an answer on local forums, but had not any replies.

I need a help with high positive fuel trims and a bit odd and high O2 sensor readings in the same time.

While idling on hot engine, I've got:
- 30 kPa of intake pressure via MAP (40 kPa when idle in Drive gear)
- high LTFT trims - which goes even past 12-15% and higher, while STFT does not compensate.
- high O2S2 readings (rear one).
- only few random misfires during the cycles - it is not happening constantly or repeatly.

What got my attention:
- too much steam/fumes coming from exhaust - even when hot

I've started investigation when I saw those fumes and additionally sniffed out petrol in oil - I've replaced bad PCV (I had P052E) and thought the problem will be gone. After that I've changed the oil, but there was still the same smell.
Oil level is not raising - it is sitting still on max.

There are not any problems with drivability - I can't see any jerking, hesitating or something like that.

OBD Readings
I took a 6 minutes near-home ride with a 3 minute idling, which u can see on the pictures. Engine was very hot - after long ride and some idling.
BTW I think I had never seen LTFT going under 6%.

What wonders me, if I understand correct:
- when O2 sensor shows rich mixture (0.7-0.8v), why do LTFT stays also rich? Shouldn't trims be negative to make the mixture correct? It just does not cooperate.
- I can hear the hiss coming from the bottom of the airbox, but not sure where it comes from
- I've tried blowing some smoke in brake booster line - but the only thing I got was farting from the pipe and I think nothing went into the intake to locate the leaks.
- when I unplug the sensor right before Throttle body, which theoretically should be a MAP, there are still values coming from the sensor in OBD2 (so this is probably any other sensor).

I hope I wrote it clear enough.
Thanks!



OBD2 Readings:

 

1. Why does this matter to you? What other symptoms or problems are you experiencing?
2. Do a compression check, you may have a piston thinking about cracking. The M274 unfortunately has a reputation for piston cracking.

 

Your idle LTFT is near 7% which is not alarming... Your engine is burning well and ECU wants additional fuel in the mixture.

how many total miles on upstream Lambda/O2 ?

Check the fuel pressures are stable under load to rull out pumps and filters.

Leaky PCV maybe forcing too much crankcase air pressure back in the intake calling for more gas....

🤞

 

@chassis
1. I was worried about the smell of gasoline in the oil - shouldn't I?
2. In my country there was not so many problems with it - it is very hard to find someone. I think that mainly US has got this problem. Maybe it is caused by the petrol differences?
And.. wouldn't anything else be happening if it was related to the piston problem? I mean a lot more missfires, lack of power.

I've recorded latest 35 minutes city drive with some WOTs and fake WOTs (to make it reduce gear or two) and I had misfires:
- 0 on Cyl. 1 and 2
- 1 on Cyl. 3
- 2 on Cyl. 4

.. so even if it is happening it is not on Cyl. 1 - overall they are happening very rarely.


@CaliBenzDriver
Where did the 7% come from? My idle LTFT is exactly +15,62% on this screen. It is on 15:41 time mark.

Car has got 110kkm (~70k miles).
Fuel rail pressure seems to be OK - it shows 13k kPa on idle/stop and 19-20k kPa when driving.

As I mentioned before - PCV was replaced, the obd code is gone, so I don't think there is still any problem.


I will spray some brake cleaner/starter to check for the intake leaks near airbox where I hear the hiss from.
Second - I bought some Tec 2000 to clean the fuel system and injectors. Maybe it will make any difference.

If I won't find anything and nothing will change, I am going to accept this as a normal thing.

 

Some petrol smell in the oil is normal. Excessive petrol smell is not normal. If you want to check this further, do a compression check.

 

We understand things differently...
I see idle at 7%: that's not bad

LTFT graph showing idle near 7%

You report 15% LTFT :
That's too positive to be normal engine setting.


Your fuel pressure going.from 2000psi to 3000 psi: that is bad. It should be unchanged 3kPsi flat, not 30% variation


Overall this is consistent:
Your lean engine needs more fuel.

The key question becomes: why is you fuel pressure low? In that order:

• HPFP
• Filter
• LPFP

If low pressure supply stays flat then its the high side.

 

@CaliBenzDriver

That's why I've included the entire set of readings in the screenshot, so you may have everything at hand.

The first plot is for engine rpms, where you can see idle is.
LTFT varying from 7 to 15 is when I'm driving.

This is where engine is idling:




Regarding to fuel pressure - are you sure about this?
M274 brochure says that it pumps 13-20 bar - so these are exactly the same values.

M276 & M278 brochure tells:



*It has not the same value when idling.*

 

My Lord, you're right: LTFT is indeed 15%, not 7% - If trim and rail pressure are all right, we are

++++++
Do you have a graph of rail pressure vs. RPM ?

 

@CaliBenzDriver

Yeah, but those graphs I've got for now are worth nothing.

I mean the app is not recording every sensor in every second. You have to get screen scrolled at right values or just check only few of them, to have them all time logged and there are moments that they were not registered for a while.

But the pattern I was able to see is very simple:
- when you idle or stay at red light it stays around 12000-13000 kPa (120-130 bar)
- when you drive or just MOVE slow/fast, no difference, it stays at 19000-20000 kPa (190-200 bar).

It would be nice if someone with M274/2.0T confirm this happening.

 

Can you plot HPFP control solenoid + output pressure vs RPM.
It may be interesting to see what ECU is commanding HPFP pressure to be.

The ECU LTFT is commanding more fuel... is the HPFP doing the same or questionable?

 

i am currentry working on a 17 metris with the same m274 engine. similar issues my o2 sensors voltages are pinned up high. no fuel trim codes. but it will misfire when accelerating and turbo starts to build pressure.

 

hey, I have an MB with the same M274.920 engine and I was also concerned about the LTFT readings, which are 10-15% plus while driving. STFTs fluctuate around 0%. I found information on the Internet that the fuel that was recently introduced in our country - Pb95 E10 (previously E5) may be responsible for this. Apparently 10% ethanol can be responsible for increasing the amount of fuel in the mixture and causing higher LTFT readings. In our country, only Pb98 fuel still has E5, so in the near future, after emptying the fuel tank, I will refill it with 98 fuel. We will see the result.

 

@tomtomas355 did you manage to solve the problem? I have the same engine but in MB GLC300 and the same problem.