Regardless of the specific design be it “hot wire,” “cold wire,” “vane-type,” “laminar” or any other a mass airflow sensor’s job is essentially the same: to tell the computer how much air flows into the engine.
Mass airflow systems are generally similar regardless of the specific type of MAF sensor used, since most modes of failure will do the same thing: tell the computer that there’s less air going in than there really is.
Engines typically run at their most powerful and efficient at an air-to-fuel ratio of about 14 parts air to 1 part fuel. Some engines, depending on the specific design and efficiency, may produce a bit more power with more fuel, and get better fuel economy with less fuel, but large variances in air-to-fuel ratio will hurt both.
Power is generally the first thing to suffer from a bad MAF, since leaning the mixture will kill the flame travel through the cylinders and reduce cylinder pressure.
Loss of Fuel Economy
You might think that a leaner air-to-fuel mixture would do nothing but good for fuel economy; after all, the mythical “vapor” carb worked on exactly this principle. But to burn completely, the fuel in your cylinders needs to burn steadily from one end to the other.
Excess air in the cylinders will create gaps between the fuel molecules, inhibiting flame travel and causing a certain amount of the fuel to exit unburned. For this reason, and because of compensation from the computer, a bad MAF sensor will typically cause a noticeable loss in fuel economy.
Stumbling Under Acceleration
When you open the throttle plate, your pistons suck air through the MAF sensor, throttle plate, intake manifold and intake valves, after which it combines with fuel to make power.
Normally, the MAF sensor would immediately recognize the change in airflow after opening the throttle; however, MAF sensors will often slow down in terms of response time before failing completely. So, the first thing you may notice, even before a loss in net power, will be a slight bog during acceleration.
Knock and Ping
A fuel-rich mixture may seem wasteful because a certain amount of the fuel introduced will end up going out of the exhaust without burning. But, that wasted liquid performs a very important function: namely, to ferry heat out of the combustion chamber.
Lean mixtures will tend to cause overheating in the combustion chamber, which in turn may lead to engine knock, overheating and eventual serious damage to the engine. If you hear anything that sounds remotely like engine knock or notice the temperature needle running high, stop and address the MAF immediately.