Car running lean but no vacuum leak?
 

So basically a month ago i got a CEL. Pulled the code and P0101 (MAF), P0171 (lean bank 1), P0174 (lean bank 2) were showing up. Plus my car was having rough idling. All the symptoms of a vacuum leak.
I brought the car to the dealer and tell them i have a leak so please check and replace if necessary the intake manifold gasket. Few hours later i was right as the SA came back to me saying they had to replace PCV hose and the gasket/bolts all under warranty. I was happy. Took my car home.
Next morning on my way to the office CEL came on while entering the highway under acceleration! Same lean OBDII codes! Damn! Brought the car back to MB and asked them to have a second look. They kept the car a whole week without figuring out what the issue was!
That's where i need your help pinpoint the problem. They said they check the MAFs, they changed the fuel pressure pump, there is no more vacuum leak/no cracks, the purge valve is ok. What else should I investigate?
The MB tech said he tried to reset the self adaptation values but couldn't. Like the ECU won't allow him to do so. I don't know if it's true but could the tune be the issue? I doubt it since it has been on the car from day 1. So why all of a sudden? I will call Renntech and ask them if there is something they can do like an update or else :nix:
I'm thinking of putting my OEM filters and airboxes back to see if the CEL clears. As well I'm doubting about this being the root cause since when i installed the AFE filters and ROW airboxes i never had any CEL.
I'm running out of ideas....O2 sensors would unlikely be the problem as both would have to be at fault at the same time....

Any suggestions will be much appreciated.

I think you have a decent approach set up in that post...

Call Renntech and see what they have to say and maybe can update or reflash.
Then, I'd put the stock boxes back on just to see if that did the trick.
Then, address the issue with it possibly being the O2 sensors.

In other words, go from the less expensive option up to the most. But it seems like you know what you're doing. I'm not really sure of any other gremlins that have thrown those codes but hopefully someone who has been around longer will know of little things that could throw those codes.

Good luck

So i did a few tests today in order to rule out all air system parts one by one.
The outcome of this is i had to revert to my OEM airboxes + charcoal delete + AFE dry filters. I did a few drive cycles (15+ mls each) and so far CEL hasn't come back!
I really have no idea why should the ROW airboxes trigger the CEL. It never did it in the first place. Why some thousands miles later? :nix:
Now the next step is to see if I'll be CEL free for a while and at the same time I'll contact Renntech to see if any tune update is needed.

Possible the dealer removed the air boxes for access and then did not reseal them properly. Were the intake connector clamps displaced allowing air on the back side of the MAF ?

No everything was fine. Dealer ran several smoke tests to find any leaks but couldn't get none.
One thing weird (which is actually the root cause of the CEL) is when i checked my fuel trims SFT/LFT values (ROW airboxes + AFE filters) with my OBDII reader my short term values were lower than my long term values that would increase continuously until the max adjustement (>+25%) which triggered the lean condition CEL. In contrary with OEM airboxes + AFE filters my SFT and particularly LFT were as expected very close to 0.
How come this is happening? My understanding of fuel trims is LFT are basically based out of SFT trends and is a learn value where SFT are more instantaneous fuel trim corrections based on O2 sensors measurements. So if your SFT are low or equal to zero during drive cycles how come the LFT is high?
Again i have no more vacuum leak, the MAFs are OK. The only difference in this test is different airboxes.... Any ideas ? :nix:

Just saw these posts....you still have trouble with the cels? We have the exact same problem. I just removed carbonio intake as well and return to stock with 20 miles driven. Waiting for another 50-75 miles to see if cel comes back. If not, maybe something with car itself. I ran the carbonio for over 15k miles without issue. Surprise you had issues with row boxes.

Let me know

Actually i fixed my problem. It wasn't a ECU tune problem nor a new vacuum leak. It was just my airboxes. I had unmeasured air passing the MAFs.
So that is why i recommended you to put your stock airboxes back on. If you stay CEL free for a while this would indicate the carbonio are the culprit.

So your running the stock boxes now or did you find an issue with the row boxes, repaired them and put them back in. I may buy a new set of Mafs then seal them to the carbonio to see if that fixes it. Still running my box test now so will know in a bit. I am charcoal delete so will see if that effects it. One airbox is row and one is us spec on my car.

Yeah I just fixed the ROW boxes.

What area did you fix if you don't mind be asking? Mafs are pretty tight on the oem units vs the carbonio so curious as to what you repaired on them. Appreciate the input.

I had modified ROW airboxes. I thought it would be cool to have the boxes completely stripped like no duct to direct the airflow through the MAFs. So i removed the black plastic tube. Well I was wrong :D.
Air was flowing under the MAFs. Took me time to figure out I had to put the ducting back on.Et voila! It was fixed! no more CEL.

Got it....but weren't you running it for a while? I am thinking air leaking through maf ports on carbonio and seating of on unit not very good. I will try that after and see what happens. Weird cause I drove it for 15k miles with no issue. Now that I think of it, cel lights started happening when they did a major service back in feb which included cleaning filter. Thinking maf is loose but will see if trigger a cel with my stock box. Thanks man

Well I had the boxes for a while but the mod i did was at the same time as I had the vacuum leak. And actually i was running the normal ROW airboxes for a while not the modified ones.
I hope you'll fix your issue as well.

Ok great to know....points me in the right direction. The good thing is that through all this, they discovered an intake manifold leak and a bad 02, all covered work under warranty....a definite blessing if carbonio acted up on time. Warranty out in a few months.