What should MAF sensor numbers be?

sunliner

Well-Known Member
I'm still learning to make sense of the data on my scan tool. Today's topic is the MAF sensor...
If I understand at all, I should be seeing ~4.2 gr/se for my 4.2 6 cyl engine. At idle, I've got anywhere from 4 to 7 or 8-something, but driving, both around town speeds and on the highway, it can be around 30 gr/se...glanced at it at one point and saw 64 gr/se. It tends to bounce around a lot, even at constant vehicle speed.
Now I understand that going 70 mph, more air is going to be rammed into the air box, but are those numbers too high, or is this normal?
 

IllogicTC

Well-Known Member
Maybe have a read of this (MOTOR Magazine)

It doesn't describe how to determine exact rates, but it does show you how to correlate data from other sensors to determine if your MAF is good or not.

I would say as long as you don't have any P0171 or P0172 codes, it seems to be working well enough. I believe the 06+, while having a MAF, are able to use the IAT and MAP as a backup if it confirms the MAF is non-functioning, or perhaps as a sanity check for the MAF to make sure it's reporting properly (likely part of its determination to go into the "backup")
 

Royalwapiti

Well-Known Member
Mine bounces too. It's normal, as you use the accelerator the amount of air passing through the intake changes. My scan gauge reads in lbs/min. At idle it will be around 0.750 and as I accelartate and increase engine speed, it increases, at 70mph at 2100rpm it will be in the 5 to 7 lb/min range. (5 lbs is equal to 37.8 grams/second - 0.750 lbs/min is 5.6 grams/second) Our numbers are similar.

On the 2006 MAF is built together with the IAT, my IAT was reading 50-60 degrees higher than actual ambient air tempurture. So I cleaned it with electronic contact cleaner. Now my temps are constantly about 10 to 20 degrees higher than ambient.
 

bobdec

Well-Known Member
sunliner, air flow will vary idle as the PCM adjusts the TB opening to hold desired RPM, Assuming a constant coolant temp. Varying the engine load by shifting to P/N or in gear, or AC on/off will result in 4 different air flow conditions. The article in illogicTC's post is very good reading.
Bottom line the PCM uses the MAF to determine the injector pulse width (how long to squirt fuel). Perfect AFR s/b 14.7 grams of air to 1 gram of fuel. So if MAF sees 4.2 grams of air per sec the PCM will fire the injectors to allow 4.2/14.7 or 0.286 grans of fuel per second to flow. Now the PCM reads the O2 actual burn voltage and if it's not 14.7 AFR then it tweaks the LT trim to add/delete fuel during the next calculation. Your LT trim problem of +18% says the PCM has to add 18% fuel after the calculation because the O2 is redaing an AFR above 14.7 more line 18. As article states, vac leak (air bypassing the MAF), fuel delivery (injectors/FP not delivering correct fuel), defective O2 , incorrect MAF reading, exhaust leak in front of O2 (pulling in clean air, manifold, gasket, crack)
In answer to your question 4.2 to 6 is correct MAF reading for warmed up idle, remember it's calculated PCM looks at IAT and MAS (sensor frequency) to calculate the weight of the air in grams per/sec. If MAF was bad to run 18% lean you MAF would have to be in the 3.4-5.0 range (18% less than normal). Also as stated the MAF results are constantly being compared to the MAP/RPM (speed density fueling) results and an 18% difference would throw a MAF DTC.
 

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