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Changing the MAF is a straightforward task. Some say it can be done in five minutes. It took me a lot more than five minutes. Maybe it's because I took pictures, paid close attention to how things were laid out, and had never done it before.

By "MAF" I mean a mass air flow sensor. A MAF is used to measure the amount of air entering an engine. The ECU uses this information to help determine how much fuel is required. This page is for a Bosch hot wire mass air flow sensor and the car used in this demonstration was a Mercedes-Benz CLK430. The basic procedure is the same across many V6 and V8 Mercedes-Benz vehicles.

A little background

A hot wire air flow sensor does not measure air mass directly. It determines the amount of air entering the system. It does this by using electric current to heat a wire that is suspended in the engine's air stream. The electrical resistance of the wire increases as the wire's temperature increases. This limits the electrical current flowing through the circuit. When air flows past the wire, the wire cools. This decreases its resistance, which allows more current to flow through the circuit. As more current flows, the wire's temperature increases until the resistance reaches its desired state again. The amount of current required to maintain the wire's temperature is directly proportional to the mass of air flowing past the wire.

The key word there is "mass". The goal is to achieve an accurate air-fuel ratio. Air-fuel ratios are based on mass. Air density (mass) varies based on pressure and/or temperature. If the MAF simply measured air volume, the air-fuel mixture could be incorrect. Since a MAF measures air density, it does a better job than vane air flow sensors. Like a vane air flow sensor, a hot wire air flow sensor is a normal wear item. It takes a long while to wear one out, but they do wear out. This page explains how a MAF was changed when it did wear out. When a MAF wears out, it gives less than accurate readings. Those reading may trigger codes that point to other suspects. Some techs will simply follow whatever procedure the code points to. A more astute tech will note that after 150,000 or so miles, the MAF may be going out.

On to the job

This is what the tech sees when they raise the hood; a large square peg in a square hole, covered by an air cleaner.

A new MAF will come in a sealed Bosch box.

The air cleaner assembly has to be removed first. The original MAF is the circular black thing next to the firewall. The new MAF will have dust covers on both sides.

This is a closer look at the old unit.

The MAF is held in place by a clamp on the top and a clip in the back. Note: A white cap is on to keep the dirt out.

The first thing to be removed is the electrical plug. Two tabs are at the top and bottom of the plug. After those were pressed in the plug was slipped off.

The two ears on the clamp were pressed together slightly and needle nose pliers were used to free the catch on the clamp. After rerouting a vacuum hose, the clip was slipped over the MAF.

The clip on the rear of the MAF was pushed back with a screwdriver. At that point the MAF was slid straight up and out.

Under the MAF was a rubber seal that sits in an intake air tube. This was dirty, so it was removed, inspected for cracks, and wiped clean. If it is removed, care should be taken when reinstalling it. The proper orientation is for the embossed part number to be under the clip. This allows the nubs to properly fit into the holes in the intake air tube. If it doesn't seat properly, unmetered air may enter the system.

Before installing the new MAF, I attached the upper clip. The catch on the clip should look exactly like it did when removed. Not off to the side. Not partially latched, but exactly as it did when removed. When the clip was initially installed, its latch was not properly in place. To make the latch fall into place, the MAF was twisted while holding the clip.

At that point, the MAF was slid into the rubber seal on the intake air tube. If the clip is properly positioned, each of its ears should slide over metal tabs on the intake manifold. The rear clip should also be in the same position it was in when the old MAF was removed. Note: The white dust cap is in place.

The electrical plug was then reinstalled. After installing new air filters, the dust cap was removed, and the air cleaner assembly was reinstalled. Regarding the assembly, it's a good idea to make sure the rubber seal that goes around the MAF is fully seated and to ensure the four tabs that go over the engine are connected (the fifth tab cannot be seen with the cover in place).

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© 2011 Marcus Blair Fitzhugh