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AIR
FLOW METERS
Identify, Troubleshoot and Cure Air Flow Meter problems
What is an air flow meter(AFM); how would you recognize it and what does it do?
The AFM is an important component in regulating fuel delivery to the engine. All modern petrol
and diesel engines have an AFM. Engines with carburettors do not need an AFM because the carb itself regulates the
amount of fuel and air going into the engine.
Look for the air intake opening, follow it to the air filter and usually you will find the AFM either in the filter
box or clipped into the pipework going to the engine
Photo shows a typical plastic cylinder type of AFM which has been dismantled from its pipework. Most AFM have
an electrical connexion into them.
Why is it necessary?
The AFM tells the Engine Control Unit (ECU) how much air is flowing into the engine;
this allows the ECU to control the amount of fuel delivered to the injectors.
What are the different types of MAF used in modern cars and how do they
work?
The earlier type of AFM (eg Bosch Jetronic) used a vane or flap inside the meter body; as air passed by it opened
the flap. A spring then returned the flap towards closed position when the engine speed slowed down. Later types
are called hot wire; air flow (over the wires) is measured by how much current is required to keep the wires at a
predetermined temperature. Look inside and you can usually see two filaments; these have to be thin
to measure accurately the speed at which air is flowing into the engine.
Which companies make air flow meters? Are there pattern copies on the market? Original equipment fitted
to European cars is made by companies like Bosch, Siemens and Hella and, yes there are a lot of pattern copies on
the market. Depending on budget you may prefer to fit a genuine replacement.
How will you know if your MAF has failed? What effects will failure have on the
performance of your car? The easiest indication is, of course the malfuntion light showing on
your dashboard! Generally the car will start but will run erratically and may even cut out, using more
fuel and performing poorly.
How easy is it to change the air flow meter yourself?
Fortunately it's an easy job, requiring no special skills or tools.
© by Andy Hewing B.Sc. - 11th January 2010
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