OBDII Data: Air Flow Rate

 Let's take a look at the data item  Air Flow rate from the Mass Airflow Sensor. The most straightforward way to look at this pid on a scan is grams per second however in my screen shots I have the tool set to imperial vs metric so we will need to convert. The reason I say it is more straightforward in metric readings is that your expected flow rate in grams per second at idle should be roughly the same as the engine size. Be aware of the caveats to that rule of thumb. The engine should be idling normally and steady, no accessories on and a fully warmed engine. Accessories on, cold engine, erratic idle, all those will affect your base air flow. I had a 2010 F150 in with a 4.6 and when the engine was idling cold the computer had raised the rpms for warmup and I was getting air flow of 0.86 lb per minute, which translates to 6.5 grams per second which, though steady, was significantly higher than the 4.6 expected. 

Once the engine was warm and the idle was slowed due to a warm engine the air flow was 4.0 grams per second and normal. See how closely that is to the 4.6 engine size? Now there was also a Toyota in the shop with a 5.7 engine so let's test that rule of thumb once again. The warm engine, good steady idle and no accessories brought me 0.66 lb per minute, or 5 grams per second. 

On a roll now so I took a look at the air flow on the 2011 GMC 5.3 and found a problem. The truck was here for an oil pressure switch replacement, which was done, but I have just found another problem simply by looking at the air flow pid. The idle speed was correct and stable but my air flow was erratic and varying from 0.86 to 1.06 lb per minute, or 6.5 to 8.0 grams per second. This Mass Airflow Sensor had not set a code but this signal indicated a problem. Being the GMC it would not be unlikely I was looking at a MAF that would soon fail but it was even more likely I was looking at a dirty MAF sensing wire. The sensor is easy to remove and very definitely needed cleaning. 

Once the sensor was cleaned and re-installed I had a steady airflow reading of  0.72 lb per minute, 5.45 grams per second. The sensor had been misreporting air flow by about 20%. So before the cleaning, and because the computer thought the air flow was more than it actually was it would have been adding extra fuel. We have just improved gas mileage and acceleration by providing the computer with an accurate air flow reading. 

Thanks for reading!

Kenny@GGAuto.Repair