mass airflow sensor question.

[thayer1536]

I have a 94 ranger 4.0 4x2. Ran really rough when started this morning to the point where you couldn't drive it. It felt like you were in too high a gear to start out and it didn't matter how fast or what gear you were in. I let it idle in neutral and it stalled after a couple of minutes. I repeated this several times with the same result. I replaced the fuel filter because I'd been meaning too anyways.

I put a OBD1 reader on it and got code 159-mass airflow sensor out of range. I replaced the sensor this summer to get rid of a check engine light and had no problems. I read that if you unplug the harness while running, the engine should shut off. It didn't when I disconnected this. I'm unsure if this advice is true. I wonder what people would do in my shoes. Try cleaning it, take it back to Autozone and get a new one or could there be another issue I'm overlooking. Feedback welcome.

 

[martin]

If you disconnect the maf they will usually run ok but the mixture is off and the throttle response is different but driveable.

 

[RonD]

As said, no, engine shouldn't stall when MAF is disconnected, CEL(check engine light should come on) and vehicle will be drivable with MAF unhooked but computer will be running preset fuel/air mix from tables in memory based on RPM and Throttle position.
That being said, the engine "can" stall when MAF is first unplugged, it really depends on the computer software and its timing, the computer may think engine is being shutting off so stops fuel injectors and spark, when MAF data stops(no air flow).
But most computers will recover in time so engine stays running.
If engine does die simply restart it with MAF still unhooked.

The MAF(mass air flow) sensor measures air volume coming into the engine, it uses a heated wire to do this.
The wire is cooled by the air passing over it, how much it is cooled is used to calculate the air volume.
There is also an IAT(intake air temperature) sensor that is used to correct air volume based on temperature.
The IAT sensor can be part of the MAF sensor(6 wire MAF) or be in the air filter box or on the upper intake manifold.
On the '94 4.0l it is on the upper intake near the vacuum manifold.

The computer does a few checks on the MAF, one of these is to compare RPMs to MAF data, computer has tables in memory that tell it the expected air flow at X,XXX RPM for a 4.0l engine.
If MAF data isn't within this expected Range code 159 will be set
This could be dirty or bad MAF sensor, or it could simply be an air leak between MAF and intake(or vacuum leak in intake manifold), a crack in the large air tube or a bad gasket at either end allows unmetered air to come in, so MAF sensor can't report all the air coming in and computer picks that up.
I.E. a 4liter engine uses 4liters of air every 2 revolutions of crank(4 stroke engine)
So at 1,000rpms(500 4liter breaths) that would be 2,000 liters of air, if MAF is reporting 1,800 liters passing thru it computer would set "out of range" code for MAF