Rough Idle and CEL - Mass Air Flow Sensor?

Hi All,

 

I have a 2005 Subaru Legacy GT. Today the engine idle became really rough (especially when stopped at a red light, while still in drive). The CEL turned on. I did some rough diagnostics by unplugging various sensors and monitoring the idle, and it seems the MAF sensor is the issue, as the idle improved once I unplugged it. I pulled out the actual MAF sensor and it looks pretty 'gunked up'. It's original, as far as I know.

 

Some questions for the forum:

 

1) Would you suggest I buy MAF sensor cleaner and see if there's improvement?

 

2) Instead, should I replace the part? If so, what replacement sensor do you recommend? I've attached a screen grab of the replacement options on Rock Auto.

 

3) Do you suggest I still have the CEL error code read by an auto shop? Or just proceed with the sensor cleaning/replacement and re-evaluate.

 

Thanks in advance for your advice - it's really appreciated.

What is the CEL code?

Unplugging the MAF just forces the ECU to use a predetermined AFR regardless of O2 readings (limp mode)

 

 

Sounds like a vacuum leak or vacuum line undone.

Thanks for the response. I figured unplugging the MAF and noticing the improved idle meant it was likely a sensor issue (rather than a mechanical one), but I am not overly experience with this type of stuff!

 

Would a vacuum line leak produce a unique sound? I did notice what seemed to be a high pitched whine with the bad idle....

 

As for the CEL code, I am not sure what the error is. Unfortunately I don't have a code reader. I noticed there's a 'DIY code reader' thread for the 2005 Legacy, but haven't given it a shot yet. Not sure how difficult it would be?

NAPA auto parts will read your code for free. Go there, write down the code, get to Googling.

 

My OEM MAF went at 202K. I was stalling all the time, some other symptoms similar to yours. Check engine light came on and found all four cylinders running lean/rough. Funny story, I feel like mentioning it here because it's just so damn surreal.

 

I had just bought the car as a bit of a project to become my daily driver and it broke down on me in SW Portland. I stopped in the middle of the road, propped the hood up, checked every single thing I could and didn't see anything wrong. Coolant, oil, vac leaks, nothing obvious was wrong. Some random guy walks by on the sidewalk adjacent to me and yells "Hey man, is that an 05? Legacy GT? I've got the same car dude." We have a quick yelling talk over the sound of traffic before he yells "I had the same problem with mine. Check your mass airflow sensor!"

 

After quite literally blowing on the electronic in the MAF sensor like an old Nintendo cartridge, the car started after a few extra cranks. I went home, cleaned the MAF sensor, and it ran OKish with the light gone. I posted the story on Reddit, hoping I'd find the guy again and thanking him, but never could find him

 

Replaced the MAF with this part and the car hasn't had a CEL since. https://www.amazon.com/gp/product/B007WMG8A0/ref=ppx_od_dt_b_asin_title_s00?ie=UTF8&psc=1

Part is 197-6040 if you can find it cheaper elsewhere.

Edited May 27, 2020 by Pleides

I'd definitely recommend cleaning it first. I use the special maf sensor cleaner, but the guy in the video below knows a ton, and says he has used brake cleaner without issues. Also make sure you're cleaning the right part of the sensor as he points out

 

FYI, the MAF sensor should be cleaned yearly when you check or clean the air filter. Still running the MAF the car came with.