edsg2

I took my 03 m271 to the dealer for a diagnostic on a check engine light.


Had the p0172 code before and tried a few things to rectify the issue. I cleaned the MAF with the maf spray, cleaned the various electrical connections on the engine with electrical spray, changed spark plugs and valve cover gasket, visually inspected the vac hose in engine bay (no tears)and replaced the O2 sensor (upstream).


They said the "cause" with a leaking intercooler hose(passenger side) so we replaced it few days ago even though we noticed no tears, reset the ecu as instructed and drove about 70 miles. Today, the same p0172 code came back with the check engine light. So I figured this is somehow fuel related and dumped a BG44K fuel injector cleaner in tank, reset ecu, and then drove about 35 miles.


The check engine light returned with the code only after a few miles of driving. The dealer now assumes the O2 sensor but it can't be since it is new (2 weeks old). Attached are the pics of the obd scanner fuel trim readings. It drive fine otherwise. I don't know what to do. DMV inspection due.


PLEASE HELP. thanks guys

Plutoe

One problem with those home scanners is that they can't compare actual values with design data, however great screen shots----I would start with what the dealer indicates and change the O2 sensor---I know you say you did that but you didn't say that it was new, used, new to the car or OEM.

edsg2

Thanks for your response. I replaced the upstream 02 sensor with a new one a week before dealer visit.

richie321e

have you replaced the hose at the back of the manifold? it goes from the rocker breather down over, there is also a pipe that goes on the front side of the cylinder to the inlet manifold. ive personally have this fault and ive done everything from wiring to updates even checking the timing chain, and eventually ive found the fault and it was the supercharger,not that this could be your fault but even with merc xentry the readings were within the tolerances regards airmass just that the upstream readings were around 950mv instead of 1100 to 1500mv. check pise first though and valve timing. i no this is a lott but this is not a nice fault. good luck

wvadam

did you ever find the cause of the problem? I have the same code with fuel trim too high and suspect some kind of vacuum leak. Bad mpg too.

sajo1973

Got the same problem here. P0172, same engine M271 c class 2002.

LTFT is 15.6% but code is P0172, not sure what to do. When you rev the engine hard, LTFT comes down briefly by 3% then goes right back up, gradual increase in rpm's no change in reading.
Checked fuel pressure and it is bouncing very fast between 52psi and 60psi.

New o2 sensor upstream, same result and also cleaned MAF sensor. Also checked the hose under the airbox aswell. I checked all the main sensors readings with star and they all seem fine, except intake air temperature reading
voltage was out of normal readings, Could IAT have a drastic effect on the stoichiometric mixture 14.7:1. Reading using torque pro I'm getting 13.2:1 air to fuel ratio, Rich.

I'm thinking it has something to do with the fuel delivery system.
Any help much appreciated
Thanks
Sam

RedGray

https://mbworld.org/forums/c-class-w...placement.html

sajo1973

Thanks for replying

Will check it out

mtnman82

sajo1973 - please get back to us and let us know if you found the problem/remedy. Thanks!

sajo1973

Ok

So i smoke tested for any leaks, couldn't find any. So i decided to change the downstream o2 sensor, air to fuel ratio got better. The Stft was hovering around 0% but long term fuel trim was pegged at 15.6% and not moving after driving the car for several miles.

So I reset fuel trims and learning processes using das and now the engine is running lean. Air to fuel ratio, jumped up to 15.3 - 1. Stft is +25% and ltft is at 0% and not changing.

Not sure if my ecu is working properly

Geri-go

how did you delete fuel trims?