Symptoms of a bad Mass Airflow Sensor
Symptoms of a bad MAF sensor is an engine that is hard to start or stalls immediately after starting. It may also hesitate under load, idle roughly, or run either rich or lean. The engine may also not respond well to sudden changes in throttle position. Similar driveability symptoms may occur from other ailments that are often blamed on a malfunctioning MAF sensor. They are low fuel pressure, leaky or dirty injectors, ignition misfire, excessive exhaust backpressure, low engine compression, and low vacuum.
A bad mass air flow sensor may not always set a computer code. If the sensor is giving an erroneous reading, as long as it is within its allowable range, no computer code will be set. However, scanning your computer codes is a good place to start. The code will be set if the frequency is either too high or too low or if the burn off cycle after shutdown fails.
In many cases a mass airflow sensor may be blamed for your problem when it is doing its job just fine. The problem is often unmetered air that is entering the engine. Remember, all air that enters the engine that is past the sensor must be accounted for or an inaccurate calculation will result. In the past I have found many things that have contributed to this problem including leaking injector o-ring seals, leaky intake manifold gasket, and vacuum hose leaks. An easy way to diagnose this is to hook up a live data feed from a scan tool. You will notice that your engine may be running with a rich fuel trim as the oxygen sensors will be giving the PCM feedback that the current program is too lean. This is due to unmetered air entering the engine. With the same live data feed, monitor your oxygen sensor data. Spray some carburetor cleaner at your fuel injectors, intake sealing surfaces and vacuum lines. If anything is leaking, you will be able to tell immediately by the way the oxygen sensor responds to the carb cleaner being sucked into the engine, even if you can’t hear the engine rpms change.