MAF Contamination & Lambda

Description
A few days ago while using my tech's AST Retreiver scanner at his shop, I checked "live data" from the LH-SFI (N4/1) module and found some questionable readings. The ON-OFF ratio (Lambda) was reading from 65 - 75% with the center around 70%. The MAF (Mass Air Flow) air mass was measured low, about 18 Kgm/h.*  There were NO CODES in the Diag or LH-SFI Modules. This menu item discusses what steps I took to diagnose and resolve the problem.

What is the cause . . .
At first glance you might think that it's the O2 sensor since it's not at "50%" point, but you would be wrong in this instance. Many O2 sensors are replaced for $125 a pop but that does not correct the problem. Normally an O2 sweeps a voltage range from .1V to .8V, from lean to rich. If the ECU gets a lean signal from the O2 it will richen the fuel mixture to compensate. And if it gets a rich signal, it will lean out the mixture. The Fuel Computer (LH-SFI) is looking for the ideal voltage signal of .45V from the O2 sensor or at least the 'average' to be about 0.45V (Menu#4). So the O2 signals ranges up and down around the 'perfect' 0.45V signal.

The readout showed that indeed the O2 sensor was ranging from lean to rich, so that appeared normal. But the MAF reading was very low . . at least as I remembered the former readings so that prompted me to go on to do some experiments. Jumping ahead, this problem may have different causes, but in almost every Mercedes in the '90s, the most likely cause is the Mass Air Flow (MAF) sensor!

MAF Functions . . .
The MAF (B2/2) is mounted in the center of the engine under the air cleaner. My S500 Cpe employs the "hot wire" type sensor. A constant voltage is applied to the heated wire. This wire is positioned in the air stream and is heated by the electrical current that the voltage produces. As air flows across it, it cools down. The heated wire is a positive temperature coefficient (PTC) resistor, meaning that the resistance drops when the temperature drops. The drop in resistance allows more current to flow through it in order to maintain the programmed temperature. This changing voltage is sent to the fuel computer (LH-SFI). The voltage level is interpreted as airflow and the voltage changes are interpreted as change in the airflow. Straightforward but elegant stuff!

Mercedes has a cleaning cycle built into the MAF circuit. When the key is turned off a larger current is sent through this hot wire causing it to glow red hot, thus burning off most contaminants. It usually does a good job for most contaminants but if it's contaminated by "silicone" then it's resistant to this burn cycle. Or in other words, it's not usable until it's replaced. It can also become silicone contaminated via the silicone that's used on the circuitry in these sensors because it migrates and then sticks to wire surface.

It can also become contaminated by use of sprays that contain silicone! If you look at MENU#22, you'll see that the 'tire-shine' contains silicone and, most likely is the cause of my contaminated MAF. As you see from the pictures, the silicone was applied to most 'everything' including the air breather tubes and the air box itself. A very short path from there to the MAF!  The other 'co-contaminate is the filter oil used in the K&N filters. Read the second footnote at the end of this article.

What Happens. . .
The MAF sensor can also be contaminated from a variety of other sources such as: dirt, oil for vapor, spider webs, potting compound from the sensor itself and the filter oil used in reusable air filters.**

When a MAF sensor is contaminated, it skews the transfer function such that the sensor overestimates air flow at idle and that causes the fuel system to go rich and it underestimates air flow at high air flows and that causes fuel system to go lean. This means long-term fuel adjustments will learn lean corrections at idle and learn rich corrections at higher airflows. So my observations at idle showing 70% Lambda and 18 Kgm/h air mass now make sense.