Mass Air Flow (MAF) systems
How does the MAF work?
First, let's talk a little about engine load. The vehicles Powertrain Control Module or PCM calculates the load of the engine buy comparing how much air the engine can displace, and how much air is entering the engine. For example, if you have an engine that is capable of displacing 5.0 liters of air, but is only bringing in 2.5 liters, this engine would be at 50% load.
A MAF sensor is used to help determine the load of an engine. The sensor works by heating a wire inside to a constant setpoint. As air flows past the wire, the wire is cooled. The measurement of the voltage to keep the wire at the setpoint is out putted to the PCM between 0 and 5 volts.
In the performance aftermarket the MUST thing to have is a calibrated MAF sensor. These are sensors that are usually larger than stock, and have a calibration for popular aftermarket injectors. A common application would be a 5.0L Mustang with 24# injectors and a MAF that has been calibrated for the injector size. One thing to keep in the back of your mind is that the factory PCM is calibrated for 19# injectors and has a series of set parameters for the factory MAF sensor. A very common problem people face is installing 24# injectors without replacing the MAF. This will result in a rich condition, mostly noticeable at an idle or off idle situation. The reasoning is the PCM still believes it is opening a 19# injector, but in reality it is opening an injector 26% larger. Now how can a calibrated MAF sensor correct this? Let�s go back to our example. In this situation the calibrated MAF sensor would tell the PCM is has 26% LESS air, while in reality it does not. By the time the actual air and fuel reach the cylinder all is well.
Is the MAF whacked?
Troubleshooting a MAF related problem can be made easy by understanding what could go wrong if the sensor is not installed properly, has reached its limit or is bad.
First, look at were and how the MAF is installed. Sharp turns or elbows directly in front of the MAF will cause turbulent flow across the meter and cause erratic readings being sent to the PCM. Turning the MAF so the sensor tube in the meter housing is in line with the long radius of the upstream elbow will improve these erratic readings and in most cases cure them.
In some cases the MAF maybe 'pegged' or beyond it's limit, the only good way to verify this is with the use of a wide band O2 sensor. This is caused by the MAF being undersized for the application. When this happens the engine will go lean. Why? In 'PART 1' we said the MAF uses a 0-5 volt output. In the PCM there is a function of 'MAF volts vs. Airflow (kg/hr)'. As an example let's take an engine that is capable of sucking in 2000kg/hr of airflow, and in the PCM our MAF sensor function at 5 volts shows 1500kg/hr airflow. At 5 volts the PCM stops adding fuel for the incoming air charge, but the engine is still bringing in more air. In most normally aspirated motors and forced induction applications were the MAF is positioned in a 'suck-through' situation, if the fuel injectors are sized appropriately and the MAF is sized for the injectors, the meter should have enough range. Having a 'pegged' MAF sensor is more prone in a 'blow-through' application. This is were the MAF sensor is placed in the discharge tube of a supercharger or turbocharger. If this is the case you can expect to use only 65-85% of the total range of the MAF sensor depending on your boost level.
Make sure the MAF is not bad. A good thing to check is the output voltage to the PCM at an idle. Mostly, what I have seen on the dyno is if the MAF volts are between 0.7-0.9 the sensor should be good for the most part. I normally don't like to see the output voltage on a 'suck-through' style MAF sensor be above 0.9 volts, this usually means the engine calculated load will be high. However, an exception to this would be if the MAF is used in "blow-through" applications were the volts may be above 1.0, but not by much. Another good test is with the engine running, unplug the MAF sensor and see if the tone of the engine changes. In almost every case if you unplug an engine sensor there should be a positive or negative effect. If there is no change there is a good possibility that the sensor is bad.
The above information mostly is based upon the Ford platform. GM Mass Air Flow sensors are usually more giving, by the way of GM using both a MAF sensor and a MAP sensor to interpolate engine load.
Manifold Absolute Pressure (MAP) systems
The MAP sensor has the same function as a MAF sensor. However, instead of a heated wire it reads manifold vacuum and is usually read in Kilopascals (kPa) having an output range between 0.5 to 5 volts DC. A positive vacuum is read as low load and a negative vacuum is read as high load. It is very important to choose the correct MAP sensor for your application. A problem occurs when applying boost to a MAP sensor with a low range of operation. Boost is basically a measure of resistance in the intake. You can see if you pressurize the intake manifold with a positive pressure a lean condition may occur since the PCM will think you are still at low load when actually you are not. Below is a table showing the recommended applications for different types of MAP sensors.
|Normally Aspirated||-||1 Bar|
|Forced Induction||to 14.5 psi||2 Bar|
|Forced Induction||to 29 psi||3 Bar|
Fuel Injectors: Sizing for all!
Below is a formula to figure what size fuel injector is needed for a certain horsepower.
HP x BSFC
--------------- = Required Injector Size (lbs./hr)
# Inj. x DC
- HP = Estimated Horsepower.
- BSFC = Brake Specific Fuel Consumption.
- This is the amount of fuel it takes
- to make one horsepower for one
- hour. Use .45 to .50 for efficient
- normally aspirated performance
- engines and .55 to .60 on forced
- induction applications.
- # Inj. = Number of fuel injectors used.
- DC = Maximum duty cycle of the injectors.
- Try to keep your duty cycle below
- 80%, so a good value here is .80.
To convert lbs./hr. to cc/min - multiply by 10.5