Last month we fielded a question about a fault code a reader had been trying to repair for more than a year. To recap, the Corvette in question, a ’96 model, had a recurring P0171 code, or “System Too Lean, Bank One.” The car’s owner stated that his scanner indicated a defective fuel pump, but replacing the pump and filter (twice!) did not solve the problem. He then took the car to three different shops and had an array of sensors installed, to no avail.
Finally a friend told him about a Corvette expert in his area. He took the car to the technician, who, after a short session with his own scanner, proclaimed that the problem was a misreading mass-airflow sensor (MAF). Replacing the sensor eliminated the code, which hasn’t returned since. The reader wanted to know both how the technician diagnosed the problem so quickly and how he could test his other Corvettes for the same issue.
There are a number of seemingly unrelated fault codes and driveability problems that can be attributed to a misreading MAF (that is, one that is under- or over-reporting the amount of air entering the engine). In this article we will cover how to diagnose and repair many of these issues.
Let’s review the basics of how intake air is measured. The fuel-management systems in most modern Chevrolet engines employ either an MAF or a manifold absolute pressure sensor (MAP). These sensors measure the weight of the incoming air and then convert this measurement into a voltage or frequency signal that the engine controller can interpret. The controller then regulates the amount of fuel used to ensure complete combustion. Accordingly, a misreading MAF or MAP will almost always result in a less-than-ideal combustion process.
Symptoms of a Faulty MAF
The first step in diagnosing a faulty MAF sensor is to perform my favorite test—the tap test. Using a screwdriver, lightly tap on the sensor while the engine is running, noting any changes in engine rpm or idle quality with each tap. If you notice a change, the sensor is probably failing internally. (Note that it is possible for a MAF to fail without setting a fault code.)
Next, check to make sure fuel pressure and volume are correct. Also check the fuel filter for restrictions, and ensure that there are no vacuum leaks. Remember, the oxygen sensors must be functioning correctly to provide an accurate air/fuel ratio to the engine controller.
The most common symptom of a faulty MAF is a hesitation after the accelerator pedal is depressed. Some other common symptoms include the following:
Stalling shortly after starting
Hesitation under load
Jerking during acceleration
Erratic idle or operation
Excessively rich or lean idling, often causing black smoke from the tailpipe
Common Causes of MAF Failures
The majority of MAF failures are due to dirt, paper, or oil contamination.
Dirt can be introduced into the MAF if the air filter is punctured, or if it is so restricted that dirt particles pass through or around it. Do not use compressed air to blow the dirt out of a paper filter. This can actually punch microscopic holes in the filter, allowing the MAF to become contaminated.
Low-quality air filters have been known to shed paper fibers, which can accumulate on the heated wire of the MAF, causing it to fail. Remember, you get what you pay for, and saving a few dollars now can cost you much more in the future.
Oil can be introduced into the air-intake system by an incorrectly serviced aftermarket air filter, a faulty Positive Crankcase Ventilation (PCV) valve, or an engine that is burning oil. Oil buildup on the heated wire of the MAF can cause the sensor to fail.
Other problems, though less common, are worth checking. For example, the inlet and outlet tubes on many MAFs are the same size, making it easy to install the sensor backward. Most MAFs have an arrow on the sensor housing indicating the direction of airflow.
Many MAFs also look the same, even though they may be application specific. If you think you may have the wrong sensor for your car, have your local auto-parts store look up the part number in its application catalog.
Unmeasured air reaching the combustion chamber can also result in an inaccurate MAF reading. For this reason, it’s important to examine the intake ducting for stress cracks or loose clamps. A leak from a broken vacuum hose or dried-out intake gasket may also be the culprit.
Some aftermarket performance intake or “cold air” systems can increase intake-air turbulence. This may also cause the MAF to miscalculate the amount of air flowing into the engine.
How to Clean a Dirty Mass-Airflow Sensor
Even if you’re not having any driveability problems, it’s a good idea to clean your MAF every six months, or as part of your oil-change routine. To do so, first remove the MAF from the vehicle. Make sure you do not touch the wires inside of the sensor. This will keep any oils from your hands from containing the sensor, and also protect the delicate internal wires from breakage.
To clean the sensor, you’ll need to purchase a specially formulated MAF cleaner, available at most auto-parts stores. (Do not use carburetor cleaner or other harsh solvents, which could destroy the sensor.) Typically the directions will instruct you to spray the cleaner onto your sensor and let it sit for approximately 20 minutes to dry. Reinstall the MAF in the vehicle, and you’re done.
Keep in mind that cleaning the sensor will not always restore perfect calibration. If the MAF is too heavily fouled, it may need to be replaced.
Checking Fuel Trims
With the diagnosis and repair processes covered, it’s time to address the question of how that mechanic quickly determined that a faulty MAF was to blame. First of all he listened to all the things the reader had already tried, an important, but often overlooked, step.
Since he was using a scanner, I suspect he was looking at the short- and long-term fuel trims. If you’ve checked all the things we’ve talked about in this article so far, and everything tested “OK,” you can use fuel-trim numbers to determine if the MAF is measuring accurately.
The short- and long-term fuel-trim numbers should remain within single digits. If they do, we can be reasonably sure that the MAF sensor is functioning correctly. If the numbers read more than 10 percent positive or less than 10 percent negative, the MAF sensor could be at fault.
An MAF that over-reports airflow at idle will result in a rich condition and negative fuel-trim corrections. An under-reporting MAF, meanwhile, will result in a lean condition and positive fuel-trim corrections under load.
It can be helpful to familiarize yourself with looking at fuel-trim values on a vehicle that is running properly, as well as on one that has been experiencing driveability problems. We’ll be using these values to help us diagnose other problems in future columns.
Even if you’re not having any driveability problems, it’s a good idea to clean your MAF every six months
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