Any MAF experts around?

[Vikas]

Here are the specification for the MAF (2000 Maxima). These are reported by the car computer.

Code:
Idle: 2-6 grams/sec

2500 rpm: 7-20 grams/sec


My question:-

How can the specification have that much latitude? This sensor needs to measure absolute air mass flow. This is different than say throttle position sensor where only relative numbers are needed and the computer can "calibrate" the TPS.

MAF has to give accurate air flow measurement otherwise the car will run very rich or very lean. 3X variation would kill the engine in short order!

Another specification given for that sensor is in voltage.

Code:
Idle: 1.2V - 1.8V

2500 rpm: 1.6V - 2.2V

This makes little more sense as the variation is from 50% to 33%. That still does not tell me how computer is able to take the raw voltage and compute the airflow and especially how it accounts for the individual sensor variations. What kind of feedback is used? I don't think O2 output can be used for it because that information is used to compute the fuel trim data and I do not think it can be used to figure out the grams/sec coefficient.

Thanks,
- Vikas

 

[Thermo1223]

Well, air temperature changes air density as does humidity.

Maybe compensating for a slight ram air effect at speed as well.

MAP systems are so much easier!

 

[NateDN10]

A MAF alone isn't enough to know how much air is coming in. The computer takes into account temperature too.

According to this, air density is 0.081 lb/ft^3 at 30F and sea level, and 0.071 lb/ft^3 at 100F sea level. That's only about a 13% difference. So it seems like there has to be something besides temperature!

 

[UNC017]

I think what you're seeing is the expected range of what you might see the MAF reading at idle and 2500 RPM, not what its precision or actual tolerance is.

In other words, with the engine stone cold and the air temperature at 40F, the mass flowing to maintain idle or 2500 RPM (after cold engine enrichment and air temperature compensation) may be very different than the mass flowing at 100F ambient and a fully warmed up engine.

 

[Shuttler]

MAF Metrology for Dummies

 

[brianl703]

Originally Posted By: Vikas
]
Code:
Idle: 2-6 grams/sec

2500 rpm: 7-20 grams/sec


My question:-

How can the specification have that much latitude?


Because the engine load AND speed determines how much air the engine takes in. Is the AC compressor running, how much load is on the alternator, etc.

 

[Vikas]

I found couple of great articles explaining this. They are from Mark Warren of Motor Magazine. I will see if I can find the link.

- Vikas