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I have a 2002 Audi A4 Quattro 3.0. I have a Vagcom USB cable and I am trying to diagnose the MAF sensor. I had a friend get some readings from an OBD2 scanner, which indicated that both precat O2 sensors (passenger side and driver side) had negative values. He said that it probably isn't the O2 sensors, since both of them are reading similar readings. He also said it is possibly the MAF sensor or a vacuum leak somewhere.

I have taken these readings from the MAF sensor. It appears that the "grams per second" is not going above 50, even when the engine is revving at 7000 rpm.

enter image description here

Does this indicate that the MAF is faulty? Is there anything else I can do to see if this really is the problem?

I have found this PDF:

http://www.ross-tech.net/vag-com/advertising/DiagnosingA_MAF_TechTip.pdf

Which indicates that this value should peak at about 160 g/s in a 200 horse power car. However, the altitude could be a factor in calculating this. I am located in Denver Colorado, roughly mile above sea level.

Any advice would be a massive help,

UPDATE: I wanted to add that the CEL's that came on are P0171 and P0174. I have tried spraying the MAF sensor with MAF Sensor cleaner that I got from Autozone, it didn't seem to make any difference, but I possibly didn't clean it properly? I took the sensor off an sprayed it all over, let it dry, and then popped it back in.

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2 Answers 2

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The 50 g/s max flow reading that you are getting is far lower than what merely altitude can explain. enter image description here

The chart above shows that at about 5,200 ft above sea level, the density of air is 83% of what it is at sea level (0.062 lb/cu.ft vs. .075 lb/cu.ft).

So to account for the altitude, you should multiply your expected mass flow by 0.83. According to the VAG-COM diagnosis guide, which I have no reason to doubt, a 200bhp naturally-aspirated engine should flow about 160g/s of air at full-throttle peak power when at sea level. Therefore, at 5,200 ft above sea level, it should flow 160 g/s * 0.83 = 133 g/s. You're seeing 50 g/s.

You're certianly not metering enough air through the MAF. This could be a MAF failure, or a big leak after the MAF.

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Excellent, thank you very much. I am going to replace the MAF sensor. It's not too expensive and I know how to install one, since I have previously cleaned it. I appreciate your time and input! –  SSH This Dec 10 '13 at 16:31
1  
I have been given it that the highest 'road' in the world is at 8000 feet. :-) –  Allan Osborne Dec 10 '13 at 20:25

The fault code P0171 descriptor is Bank 1 running weak. The fault code P0174 descriptor is Bank 2 running weak. This means that your engine has too much air or not enough fuel for proper ignition.

This can be a faulty MAF sensor as it is common to both banks. It can also mean that you have an air leak between the MAF sensor and the throttle body, a faulty intake air sensor, faulty manifold air control system solonoid, an exhaust or inlet manifold leak, a PCV valve or hose leaking air, a faulty fuel pump delivering low pressure, or a partially blocked fuel filter restricting flow of fuel, faulty injectors, secondry air system not shutting off completely and allowing an air leak.

VAG readouts are not the most intuitive and they rely on you having specific information to hand to fully diagnose a fault. I do not have a VAGcom, but do have several other scanners, (VCDS, Autel, and VAG handhelds). If you are able to view the MAF sensor signal voltage on an OBD2 scanner, then do so. The voltages are around 0.5V at idle to around 4-4.5V when snapping the throttle open and closed. OBD2 readings are always the actual readings from a component, OEM side readings may be substituted to keep the vehicle running so they can be misleading.

Altitude will effect the air density reading from your system. Adjustments are calculated from a BARO imbedded in your vehicles ECU.

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