Xavier Smith II

What is the proper fuel trim for the M156?

I plugged in my cheap BT scanner and got a fuel trim level of -95 for Bank 1 and 1 for Bank 2. I confirmed this with a mechanic level scan tool that I borrowed from a friend

No CEL is on.

Now my reason for checking this, I hear a light plastic on plastic or rubber on rubber tick/knock at idle. The sound occurs regardless of the engine temp, and disappears if the accelerator pedal is depressed at all. The sound sometimes goes away if I fully depress the brakes. The sound isn’t audible in the engine bay only the cabin, and there are some days where the sound never occurs.

BLKROKT

Proper trim is 0. Most running ROWs or Carbonio see -20’ish. You are way off. You have a leak.

Xavier Smith II

Leaking Fuel injector? Just replace all 8? While I’m replacing them, is there anything else I should do/look for? I already have new spark plugs and new air filters that need to go in.

Can I go to a bigger Injector size without a tune/is it worth it?

skratch77

What injectors did you put in there? you are running really rich.

BLKROKT

I was thinking massive post-MAF air leak, not injectors

Xavier Smith II

Stock Injectors

BalanBro

That's what I thought at first, but a post MAF air leak should cause a high positive fuel trim, and he's saying it's negative. I also just noticed that the issue is just on one bank. It actually might be an injector stuck open, or a bad O2 sensor.

EDIT: ....or a bad MAF. You should be able to check the air flow readings on both banks with your scan tool. They should read similar voltage at idle if both MAFs are in good working order.

Jasonoff

The ECU thinks there's way too much fuel on that bank so it's trying to remove fuel with the negative trim.

It either actually has too much fuel or not enough air. Or whatever is reading the output thinks that.

Xavier Smith II

I will update you all when I get home

BLKROKT

I think it’s this ^. A post MAF air leak will run rich so the trims go negative to try and balance. Big negative number suggests that not as much air made it into the A/F mix as what the MAF calculated going through it. At least I think that’s how it works.

deadlyvt

Ya something is out of wack between the maf reading and the o2 reading. Could be one of those not reading correctly or as stated too much air post maf entering the system

hachiroku

post pictures...honestly don't think -95 is possible without fault and limp mode.

do you have Torque for Android?

Jasonoff

Those fuel trims reported to OBDII is not what the ECU uses to throw codes though.

Xavier Smith II

Car runs perfectly fine

hachiroku

if the ECU cannot correct quickly or enough at enough data points...usually at throttle tip in. it'll throw into limp mode.

if true, at -95...if you reset all adaptations, you'll have a very tough time starting your engine without giving it gas while cranking.

with 596cc injectors the largest long term fuel trim i ever saw on a stock injector tune was -35%. only for a short while. not sure what the combined long term and short term was.

-95 is pretty nuts.

hachiroku

can you separate Long Term Fuel Trims and Short Term Fuel trims

maybe you're displaying combination of both LTFT and STFT. can't really tell from the screenshot.

Xavier Smith II

The top right bank 1 is STFT
bottom is LTFT

BLKROKT

Intake Manifold Pressure at idle should be closer to 8.5 vs your 4.8.

hachiroku

like this...

hachiroku

yeah you have a leak or some mechanical issue. if its your STFT and not your LTFT has corrections and your Manifold readout is wrong as BLKROKT stated

Xavier Smith II

Mechanically I’m pretty solid I think. No oil consumption/blue smoke.

Service A was completed at the beginning of this month, where I was advised the following:
Rear brake pads at 4mm: Done myself
Air filter based on Wear/Mileage: Not done yet
Spark Plugs based on Wear/Mileage: Not done yet
Transmission Fluid change due by June: Not done yet.

The weird part is that the car definitely isn’t down on power, there’s just a strange noise in the cabin.

Xavier Smith II

Shouldn’t this be throwing a code? I’m at almost half

PeterA90

You have a leak in the intake manifold. I had the exact same issue a couple months ago. Was going crazy. I had new injectors. New o2’s. New intake mainifold gasket+pcv valve.

One bank was running lean. The other rich. Took the manifold off and smoke tested it on bench. Found the problem... rear of the manifold was leaking

Xavier Smith II

Is there anything else that I should check while I’m in there, car has 117K miles. I’m thinking the injectors should go as well.

How do I prep the surface that mates with the manifold?

PeterA90

I would do new injectors, new plugs, new o2’s.

check this thread out. https://mbworld.org/forums/c63-amg-w...valve-diy.html