The mass airflow sensor (MAF) is located inside the engine compartment and is used to measure the amount and temperature of air that is entering the engine at any one time. Older-style meters used on fuel injection systems in the 1980s measured air volumetric flow, which worked fine, but then you also needed a separate sensor to figure out how cold or dense the air was. The mass airflow sensor senses the total amount of air passing the sensor and allows the fuel injection system to adjust the fuel mixture to compensate for cold weather and/or high altitude conditions. The MAF also incorporates an internal intake air temperature sensor that measures the temperature of the intake air.
The first indicator that you might have a problem with the MAF is the presence of a check engine light (CEL) on your dashboard. The check engine lamp can be caused by a wide variety of problems with the engine--you need to read the codes from the computer to get a starting clue as to what the problem is (see Project 20 for details on reading the codes). It's perfectly safe to continue to drive the car while the CEL is on, as long as it is not flashing. However, the engine will not be operating at peak efficiency, and you will most likely experience a loss in power and a decrease in gas mileage as a result. It's best to get the problem taken care of relatively quickly, as running the engine in this condition can potentially cause damage to other components, such as the catalytic converters.
The computer will know if something is wrong with the MAF because it will compare the values being output by the sensor to "expected" values that it should be receiving. This common sense check by the computer helps diagnose problems with every component in the system. If the MAF becomes dirty and is falsely indicating to the engine that the car is receiving very little air while at full throttle, then the computer will most likely kick back an error code.
To gain more information about the problem, you can try disconnecting the sensor completely and take the car for a drive. If you take short drives (30 minutes or less) with the sensor disconnected, it shouldn't cause any major damage to your car. The engine management system (DME) will enter into a type of "limp mode" that will compensate for the missing MAF. If engine performance improves dramatically when disconnecting the MAF, then the problem quite likely lies with the MAF.
Vacuum leaks and other air leaks in the system can cause MAF sensor errors. If you have a crack or leak in your air intake downstream of the sensor, then the MAF will be sensing less air than the engine is actually receiving. If the clamp on the throttle body happens to come loose and fall off, then the MAF will indicate almost no air being sucked through the intake, yet the engine will be sucking air directly from the engine compartment into the throttle body. The bottom line is that you should carefully inspect all of your hoses, clamps, and intake tubes for air leaks prior to replacing the sensor (see Pelican Technical Article: Finding Vacuum Leaks on the Porsche 911 Carrera).
The MAF is located on the left side of the engine compartment, just behind the air filter (see Project 3 for access to the engine compartment). For some reason, Porsche made it unusually difficult to remove the MAF by securing it with a T20 tamper-proof Torx screw. You need the special tamper-proof Torx drivers, which are not typically found in everyone's toolbox, but usually can be purchased at a good local auto parts store. Although the holes on the MAF look symmetrical, they are not, and the unit can only be installed in one direction. See Figure 1 for a close-up of the MAF. Removal is easy once you have the tool. Simply remove both screws holding it in place and pull it out.
It's very important to keep the sensor clean. If the air cleaner isn't working too well, it could allow dust and debris to collect on the MAF. If you've had a problem with your air-oil separator, it could have contaminated the sensor as well. Oil sucked into the engine intake from a defective separator can easily find its way back to the intake tube. If you have had major engine problems (like our project car with the blown-up engine), then you may find a ruined MAF. On our donor car, the MAF was soaked in oil and coolant residue that had found its way all over the engine. If you have an aftermarket reusable air filter, beware of how much cleaning and filtering oil you use on it. Excess oil may get sucked into the intake and find its way onto the MAF. To keep your MAF healthy, I recommend changing or cleaning your air filter often (see Pelican Technical Article: Replacing Your Porsche Carrera Air Filter (996/997)).
If you are replacing your sensor, it is extremely important that you get the proper one for your car. There are two basic types, one for the cars that use a traditional throttle cable (1999 only) and one for cars with an E-gas electronic throttle (2000 and later). In addition, the later-style E-gas sensor has been updated at least twice as of this writing. Porsche updated the sensors in the Carrera in 2002 with the introduction of the 3.6 liter engine. Here is a chart that shows the differences between all of the sensors:
Porsche Part #
BOSCH Part #
1999 996 Carrera 2 (2 wheel drive)
Sensor for use with cable throttle cars
1999-2001 996 Carrera 4 (4 wheel drive)
2000-2001 All 996 Carreras
2001-2005 996 Turbo
2005 996 Turbo S
Original sensor for E-gas cars
2002-2005 996 Carreras
2007-2008 997 Carreras
2002-2004 Carrera 4
2007-2008 Carrera 4
2002-2005 Carrera 4S
2007-2008 Carrera 4S
2005 Carrera S
2007-2008 Carrera S
2007-2008 Targa 4
2007-2008 Targa 4S
Latest updated sensor
2007-2009 997 Turbo
Shared with Cayenne
In general, if the old sensor your removing has is a 123.00 or 124.00 sensor, then you should replace it with a sensor with the same part number. If the old sensor you're removing ends in 125.01, then replace it with 986.606.125.01 (the latest version available).
After reinstallation, reset your check engine light (CEL) using your code reader (see Pelican Technical Article: Reading Porsche 911 Carrera Fuel Injection Fault Codes). You can also disconnect the battery for a short while to reset the lamp, but I don't really recommend this approach (see Pelican Technical Article: Reading Porsche 911 Carrera Fuel Injection Fault Codes). On 1999-2002 Carreras, you can disconnect the battery for more than 20 seconds, but less than 50 seconds, to clear the trouble codes without having to enter your code back into your radio. On pre-2003 cars, the computer's CEL memory is cleared after being disconnected for 20 seconds, but the radio code is needed after 50 seconds disconnected.
After you have replaced or cleaned the sensor and cleared the code, you need to go drive the car and see if the code returns. If the same error code appears, then the problem probably lies elsewhere. Most of the time when you have an error code indicating a problem with the mass airflow sensor, it is usually solved by the installation of a new sensor. However, the computer can become confused sometimes and give misleading error messages. Wire harness issues, DME problems, and secondary air injection equipment problems may all give false MAF error codes. At this point, it's best to dive into the factory manuals and start going through the laborious test procedures contained in there.
Shown here is the mass airflow sensor (MAF). The main sensor fits in a hole in the air intake right downstream of the air filter. The green O-ring seals the sensor to the intake tube (yellow arrow). If you're having trouble with your MAF, you can try to resurrect it by cleaning it. Lightly spray the areas shown with the blue arrow with electrical contact cleaner--the one that I recommend is CRC Mass Air Flow Sensor Cleaner. Spray it and then shake the sensor so that any dirt or debris is washed away. Don't touch any of the sensor elements with anything (like your finger or a brush), as this will damage them almost immediately. Let it dry completely prior to reinstallation.
You need a T20 tamper-proof Torx bit (inset lower left) to remove the mass airflow sensor from its home in the intake pipe (insert upper left, blue arrows). It's typically easier to pull the sensor out of the intake tube first, and then disconnect the electrical harness. Be sure not to touch any of the sensor elements that are exposed. Clean the entire housing area prior to installing your new sensor. Take note of the opening at the bottom of the sensor (green arrow). The new sensor must be oriented so that the opening faces the oncoming air going into the engine.