[zoso4534]

Car was in for service today. 2019 330xi which has 42K miles. Had engine oil, tire rotation, inspection, alignment, brake fluid done. As I'm pulling into my garage right after the service appt, CEL lights up. I pull the codes and it says mixture too lean on ECU #1.

One thing I noticed over the past few weeks is my fuel efficiency, according to the car, has gone down significantly. I usually get 25-30mpg driving around town and it's dropped to around 17mpg. It's not rough at idle and accelerating fine.

Anybody seen this before? The joys of German car ownership....

[Assassin]

Most likely one of the lambda sensors in the exhaust system is faulty. There are two (before and after catalytic converter) It measures the oxygen in the exhaust gases and adjusts the fuel/air mixture to ensure the engine is running optimally. If it goes wrong it can cause rough running, excessive emmisions and poor fuel consumption. Get the codes read to confirm. If it's that, its and easy fix, screw out, screw in replacement part.

[zoso4534]

Originally Posted by Assassin

Most likely one of the lambda sensors in the exhaust system is faulty. There are two (before and after catalytic converter) It measures the oxygen in the exhaust gases and adjusts the fuel/air mixture to ensure the engine is running optimally. If it goes wrong it can cause rough running, excessive emmisions and poor fuel consumption. Get the codes read to confirm. If it's that, its and easy fix, screw out, screw in replacement part.

Thanks for that. From what I've gathered, could be vacuum leak, MAF, or O2 sensors as you said. Too many possibilities for me to want to start taking crap apart! I dropped it off at the shop and we'll see what they say. I hope it's a simple vacuum leak. Much too early for those other parts to go.

[zoso4534]

Originally Posted by zoso4534

Thanks for that. From what I've gathered, could be vacuum leak, MAF, or O2 sensors as you said. Too many possibilities for me to want to start taking crap apart! I dropped it off at the shop and we'll see what they say. I hope it's a simple vacuum leak. Much too early for those other parts to go.

It was something simple, but what a nightmare it has been with this shop. They left something loose on the intake and that caused the lean code. They fix it, and I bring the car home. I pop up the hood, and I see one of the air box clips was not in place, that piece of plastic over the strut tower was not locked in place, and there was a piece of hose insulation just sitting loose under the hood. Boy was I frustrated. What if that piece of insulation went into the fan!? I called them, and they told me it was insulation from the PCV hose they forgot to put back in. They told me it was fine to leave it off, but I told them I wanted it put back in, so I get to bring the car back for the 3rd time tomorrow.

[zoso4534]

Originally Posted by zoso4534

It was something simple, but what a nightmare it has been with this shop. They left something loose on the intake and that caused the lean code. They fix it, and I bring the car home. I pop up the hood, and I see one of the air box clips was not in place, that piece of plastic over the strut tower was not locked in place, and there was a piece of hose insulation just sitting loose under the hood. Boy was I frustrated. What if that piece of insulation went into the fan!? I called them, and they told me it was insulation from the PCV hose they forgot to put back in. They told me it was fine to leave it off, but I told them I wanted it put back in, so I get to bring the car back for the 3rd time tomorrow.

Update for anyone who runs into this issue in the future. The shop can't figure out what's causing the lean CEL. They're going to try replacing the fuel vapor solenoid (as they called it) today pending getting the part from a local BMW dealer.

[zoso4534]

Shop replaced the fuel vapor solenoid, gave the car back. I drove it home and the CEL is back on again. Any ideas, guys? Can someone walk me through diagnosing the MAF and O2 sensors?

[Bc2005]

First check should be visual inspection of vacuum lines. Sometimes a small hose falls off the fitting, or on older cars the hose deteriorates and develops a leak.
I’ve never replaced the MAF sensor on my BMW, but on most cars you can unplug and remove the MAF sensor, use MAF cleaner on it, and reinstall.
If O2 sensors are bad I think the only choice is to replace them. When sensing O2 they make very small voltage changes that are fed back to the computer, and I think you would need special test equipment to know if they were working properly.

[zoso4534]

[QUOTE=Bc2005;30829158]First check should be visual inspection of vacuum lines. Sometimes a small hose falls off the fitting, or on older cars the hose deteriorates and develops a leak.
I’ve never replaced the MAF sensor on my BMW, but on most cars you can unplug and remove the MAF sensor, use MAF cleaner on it, and reinstall.
If O2 sensors are bad I think the only choice is to replace them. When sensing O2 they make very small voltage changes that are fed back to the computer, and I think you would need special test equipment to know if they were working properly.

Thank you! I was also thinking clean the MAF. Going to try that today.

[zoso4534]

I removed and cleaned the MAF this morning. Drove it around for about 20 minutes and no CEL so far. Not declaring victory yet. I want to drive it a bit more.

I've been using my scan tool to monitor the fuel trim data. I don't 100% understand it yet. At first, it looked out of spec for the MAF. After cleaning, it looks better. Car is showing a pending lean code but it hasn't thrown a CEL yet. Not sure what to think. I'm going to drive it more today and see what happens.

[AlexAnthony]

Originally Posted by zoso4534

I removed and cleaned the MAF this morning. Drove it around for about 20 minutes and no CEL so far. Not declaring victory yet. I want to drive it a bit more.

Car is showing a pending lean code but it hasn't thrown a CEL yet. Not sure what to think. I'm going to drive it more today and see what happens.

Any updates on this issue? I just picked up a ‘21 330i last week and the day after taking it home from the dealer I’ve also got a mysterious Lean error code. Just dropped it off at the dealer so we’ll see what they say, but they had literally just finished doing their full inspection an hour before I took it home. Wondering what to look out for in case I get a recurring CEL like you’ve described..

[zoso4534]

Originally Posted by AlexAnthony

Any updates on this issue? I just picked up a ‘21 330i last week and the day after taking it home from the dealer I’ve also got a mysterious Lean error code. Just dropped it off at the dealer so we’ll see what they say, but they had literally just finished doing their full inspection an hour before I took it home. Wondering what to look out for in case I get a recurring CEL like you’ve described..

Yep, cleaning the MAF did it. CEL never came back after that.