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How To Diagnose A Bad MAF Sensor In 10 Minutes
Posted to Technical Tips Forum on 9/13/2011 62 Replies

A few months ago this vehicle came in with the MIL on. I scanned it and this is what I found. [1999 Chevrolet Express 3500, Scan Data] [1999 Chevrolet Express 3500, Waveform] This van has a P0131 and a pending P0151; the code descriptors and definitions are important here. These are not circuit codes, they can be cause by a lean condition.

What next? Well, it's your first job of the day and you don't feel like getting your hands dirty until you're done eating your morning donut.

When it comes to freeze frame I really only trust LTFTs and ECT. Both banks are lean in the Freeze Frame and both banks are setting codes. This tells me I am diagnosing something that would make both banks go lean. For a lean condition there are 3 common problems that I can rule out quickly: vacuum leak, bad fuel pump and bad MAF.

A vacuum leak would cause a lean condition at idle and would be better around 2500 RPM.

A bad MAF sensor will tend to have positive fuel trims at load. This is why I recommend testing fuel trims at a steady cruise when testing a MAF. Fuel trims may also be negative at idle with a bad or dirty MAF.

Here is the data from our subject vehicle. Idle [1999 Chevrolet Express 3500, Scan Data] 2500 RPM [1999 Chevrolet Express 3500, Scan Data] Steady Cruise [1999 Chevrolet Express 3500, Scan Data]

VE (Volumetric Efficiency) is the measurement of how well an engine can pump air, remember an ICE engine is like a large air compressor. What I do is monitor MAF in g/s, RPM, and both upstream O2 sensors. I then take the vehicle on the road and take a recording of a WOT 1-2 shift. [1999 Chevrolet Express 3500, Scan Data] I then plug the information into my VE calculator. [1999 Chevrolet Express 3500, Photo]

Now that we have capture all of our data it's time to go back to the shop and analyze it. This engine's VE is 53% which is very low. On this particular engine I would hope to see more than 75%. Our fuel trims go positive at a steady cruise and negative at idle. Notice the O2 sensors also go lean. If this were a bad fuel pump we would have good VE with lean O2 sensors.

With all of this data combine I am ready to condemn the MAF. A new MAF is installed and I take the van back out for a VE test. Here are the results. [1999 Chevrolet Express 3500, Scan Data] [1999 Chevrolet Express 3500, Photo] This van is fixed. It was diagnosed without even popping the hood, and the only tool that was used was a lap top and scanner.

Matthew from Illinois

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car Vehicle Data

1999 Chevrolet Express 3500 5.7L

Engine5.7 L
Trans4-speed Automatic (Electronic)