Hi all,

These forums have been big in helping me diagnose and fix my problem so I wanted to document my fix to pay it forward.

I have a 2002 C320 Sport Wagon with about 150K miles.

Recently the water pump failed and I replaced it. About the same time the check engine light (CEL) came on.

I borrowed an OBD scanner. It showed codes P0172 and P0175. This is a rich detection on both cylinder banks. As well, the car had a rough idle. The most common cause for a rich detection on both banks is the mass airflow sensor as it may detect more air flowing and needlessly increasing fuel flow. Also possible is the temp sensor where the sensor under reports the temp and can also increase fuel flow. The O2 sensors find too much fuel and try to lean the mixture, but the computer soon realizes that something's wrong.

If you've got an OBD scanner that displays system data, I've since learned that MAF readings of 4-5 grams/second is normal for idle and 14-25 at 2000 rpm. Mine was at about 7.5 gr/se at idle and 35-40 at 2000.

My fixes:

1) Bought CRC Mass Airflow Sensor Cleaner - I understand there may be substitutes, but it wasn't that expensive so I went with that.
- Removed giant plastic engine cover by lifting up for 4 spring clamps to release.
- Removed MAF (the 4" circular object at the back of the engine with the mesh screen), attached by 1 wiring harness, 1 spring clip on the back, and the plastic locking ring that encircles the MAF and clips to the manifold. It took 10 minutes the first time I did this, but can now swap a MAF in about 2 minutes.
- Sprayed the sensor (the stick inside the tube) vigorously, let dry and replaced.

This ultimately did not fix the problem.
The sensor is attached to the assembly by 2 Torx 25? security screws. I bought the security bit set from Harbor Freight for about $9.
I removed the MAF and the sensor from the body and sprayed more vigorously, but this didn't correct the problem either.

2) I replaced the temp sensor. It was about $12 from Amazon and took about 5 minutes to replace. Although optimistic for a cheap fix, this didn't do it in this case. Fun fact: if the engine isn't cool when you replace this part, hot antifreeze will pour out.

3) I bought a new MAF from Amazon for about $100. I replaced it and this seems to be the fix. Although I'm still waiting for one more monitor to set (EVAP) after resetting the codes through the OBD scanner, I'm confident I won't see a return of the CEL. MAF reading is now consistently in range and idle is very smooth.

I eliminated vacuum leaks since they tend to cause a lean condition code. Also eliminated as a problem source were faulty O2 sensors since they were both detecting the anomaly and the likelihood of both failing at the same time is statistically slim.

Amazon was a great source for parts. I bought everything except the CRC cleaner from there including the water pump replacement parts. I'm sure there are other good sources for parts, but that is who I used.

 

Awesome write up! 100 bucks for a new MAF? Please share that link!

 

Here's the Amazon link for the MAF:
http://www.amazon.com/Beck-Arnley-15.../dp/B001UEDHTG

Although the pic is only of the sensor, I received the entire assembly.

 

Very helpful write-up! So the o2 sensors were the soution to the problem? I have the p0172 code that reads bank 1 system is too rich. I believe my MAF is working correctly but i will double check how much it reads.