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How to Diagnose and Limp-In your Subaru with a fault in the

Dealing with a Mass Airflow Sensor problem can be no picnic. Even to most technicans in the field, this part can seem mystical, and other than knowing that it meters airflow, how it does it, and how it communicates with fuel control computer is mystical. Before the invention of fuel injection, this was a topic that no techician needed worry about with a normally aspirated engine because physics took care of the problem for us. The theory behind the necesessity of this part can be overlooked by someone only familair with carburation.

The MAF sensor tells the fuel computer how large the mass of air entering the intake is, usually calculated by grams per second. To maintain a stocheometric mixture, 14 units Atmosphere/Air to 1 unit Hydrocarbon/Fuel, the Fuel Computer adjusts the time that the fuel injectors are opened based on it’s programing. If this ratio, 14:1, cannot be closely maintained, the fuel economy and performance suffers. If the ratio changes up or down 4 points or more, the engine will stall, flood or misfire, or in extreme cases, not start at all.

Some fuel injected motors have no MAF sensor at all - they work by calcuation. These are generally the central or one fuel injector for all cylinders type, unlike our MPFI Subaru’s. An accurate estimate of airflow can be taken by the displacement of any motor, and applying a formula based on the engine RPM to arrive at a managable airflow measure that can be used to calculate fuel injector on time.

This known, will explain the therory behind what I tell you next.

In most pre-OBD-II (pre-1994 emission legislation) Bosch-type EFI systems did not have parameter for setting a fault code based on the MAF’s reading. There are codes for open or shorted MAF circuit (but they and any manufactuer will use tech jargon to create an illusion that it is more complicated than this, lol,) but no code was created that checks for accuracy of the signal from the MAF sensor itself. It’s a shame becasue they have all the tools in the fuel computer to do this. SO - a MAF sensor failure will NOT be detected by the self-diagnostic system! Tech who rely on the “computer” to tell them what is wrong stop being able to fix cars right here!

When the car first starts up, it does not use the information from the MAF sensor to start, instead it calcuates the air for 3-5 seconds based on RPM, and then switches to the MAF reading.

Sound familiar? Frustrated on why the damn thing starts for 3-5 seconds, then dies??? It is because as soon as the fuel computer aboandons the RPM based air flow calcuation, and uses bad input from the failed MAF sensor, it is the classic computer problem - Garbage in, garbage out, and the fuel is not metered correctly, and the engine dies.

So what can you do?

Well, first, don’t automatically concude the MAF Sensor is a POS and replace it. Check the wiring for continuity, ensure a clean ground, and most of all, and you don’t even need a manual for this - LOOK INSIDE THE CONNECTOR! ENSURE ALL THE TERMINALS ARE STRAIGHT, CLEAN and IN THEIR SOCKETS!!

I can’t stress this enough. 65% of the time an electrical part is replaced, it only appeared to be the part at fault, because the true failure was a poor connection, and it was restored in the process of replaceing the part - Another article on the CTS pigtsil replacement by another author is a prime example of this all too common oversight. It can save you hundreds of dollars and hours of frustration.

Now, if you have read this far, then I will bestow apon you a guarded secret from the back of the shop.

Just unplug the damn MAF sensor connector, start your car, and drive away :)

This works becasue this trips the MAF code in the computer because we have opened the MAF sensor circuit. The fuel computer is programmed to revert to the calcualted form of airflow measurement when this code is set. Now the air/fuel ration can be determined within 2-3 points, and the car will start again!

You don’t want to drive it like this long, and fuel economy will suffer, but at least your mobile again!

Written by

Tom Anderson