[Tom]

How do I read / diagnose the mass air flow sensor on a scanner? Fuel trims? Vehicle is a 2005 Chevy Silverado 1500 with a 5.3 v8 engine.

[Wesley]

There are a few clues, depending on what you’re chasing. If it’s fuel trim codes, Freeze Frame data will give one clue. A contaminated MAF tends to over report at idle and under report under load.

On a naturally aspirated engine, MAF gm/sec will generally be a few tenths over the engine size in liters at hot idle. At 5000 RPM WOT it will be about 42 gm/sec per liter engine size assuming 85% volumetric efficiency.

As examples, a 5.3L will be about 5.6 gm/sec at idle, at least 222 gm/sec at 5000 RPM WOT.

Bear in mind anything that restricts engine breathing will reduce MAF readings under load (extremely dirty air filter, restricted exhaust, mechanical timing).

[Marc]

Fuel trims are calculated using O2 sensor feedback…..upstream is usually short term….downstream .long term… If your scantool does, graphing should be linear with rpm and no drop-outs.

[Steve]

Volumetric efficiency test, graph MAF/GPS, IAC, and rpm’s from a stop do a full throttle start in first gear until you hit around 5000 rpm’s while recording data, then input your data into a VE app that you can find pretty readily.

You should see at least 80% efficiency on most naturally aspirated cars with port fuel injection, and 90% on newer DI cars.

If not, clean MAF sensor and repeat test, if numbers improved you likely found the problem.

[Mike]

The sensor should read grams per second that match the displacement of the engine within 1 gs. So a 5.3 should read close to 5.3 gs. And when you go wide open throttle, the calculated load should be 100%.

[Chris]

An easy way is to look at calculated load range while under WOT, if it’s not close to 100% the MAF sensor is usually bad.

[Author]

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