| What is an air flow meter (AFM); how would you | | | | The earlier type of AFM (eg Bosch Jetronic) used a |
| recognize it and what does it do? | | | | vane or flap inside the meter body; as air passed by |
| Did you know that as you press the accelerator to | | | | it opened the flap. A spring then returned the flap |
| go faster in your car so you allow more air and fuel | | | | towards closed position when the engine speed |
| into the engine. All this takes place under the bonnet | | | | slowed down. These vane types are cumbersome |
| or hood but how is it controlled? Well the AFM is an | | | | and expensive to make. Later types of AFM are |
| important component in regulating the amount of fuel | | | | often called Air Flow Sensors and work on a hot wire |
| injected into the engine of a modern car so all | | | | system; air flow (over the wires) is measured by |
| modern petrol and diesel engines have an AFM. | | | | how much current is required to keep one of the |
| As you go faster (press the pedal) the AFM senses | | | | wires at a predetermined temperature. Look inside |
| that more air is flowing and tells the engine's brain | | | | and you can usually see two filaments; these have to |
| (ECU) which then instructs the injectors to pump | | | | be thin to measure accurately the speed at which air |
| more fuel. It's a bit like running faster or up a hill; your | | | | is flowing into the engine. |
| brain senses your muscles need more oxygen; your | | | | Which companies make air flow meters? Are there |
| heart rate rises and you breath faster. | | | | pattern copies on the market? Original equipment |
| Engines with carburettors (remember them?) do not | | | | fitted to European cars is made by companies like |
| need an AFM because the carb itself regulates the | | | | Bosch, Siemens and Hella and, yes there are a lot of |
| amount of fuel and air going into the engine. So when | | | | pattern copies on the market. Depending on your |
| your engine management panel indicates a fault in the | | | | budget you may prefer to fit a genuine replacement. |
| AFM you'll need to find it and replace it. This can | | | | How will you know if your MAF has failed and what |
| often be done without specialist tools and using a bit | | | | effects will failure have on the performance of your |
| of 'savvy', common sense and a workshop manual.. | | | | car? The easiest indication is, of course, the |
| Look for the air intake opening to the engine. This is | | | | malfunction light showing on your dashboard! |
| usually plastic, shaped a bit like a small trumpet. Follow | | | | Generally the car will start but will run erratically and |
| it to the air filter and usually you will find the AFM | | | | may even cut out, using more fuel and performing |
| either in the filter box or clipped into the pipework | | | | poorly. |
| going to the engine. | | | | How easy is it to change the air flow meter |
| Most AFMs have a 12 volt (not dangerous) electrical | | | | yourself? |
| plug into them which is easily disconnected. | | | | Fortunately it's an easy job, requiring no special skills |
| What are the different types of MAF used in modern | | | | or tools. Often the meter is simply clipped into place |
| cars and how do they work? | | | | or held with torx or hexagonal bolts. |