Last newsletter I looked at Oxygen Sensors, how they work, and some of the problems they can present as they age. This time we’ll look Air Flow Meters.
Air Flow Meters – Porsche use two types of air flow meters in their electronically injected cars.
Flap Type – 3.2 911, 964, 944, 944 Turbo, some early American and Japanese 928’s also used this type. These work by forcing intake air through a tunnel, and moving a flap at the end of the tunnel that is connected to a potentiometer. As the flap moves, the resistance in the circuit changes. This tells the control unit how much air is going into the engine. Using other sensor inputs like, coolant temp and throttle plate position. The DME control unit can then work out how much fuel to inject, and when. These metering units can wear out, as the circuit track that they use most becomes worn, over time the voltage supplied to the DME drops. This false or misleading signal causes the control unit to deliver not enough fuel or spark advance for any given situation. This can cause the engine to become sluggish, and can also present idle problems over time.
Hot Wire – 928, 968, 993, 996, Boxster and Cayenne all use this type of air flow meter. These units use a different measuring method. They use a single heated element of wire that sits in the air intake pipe, the resistance along this wire changes as it is cooled by the varying amount of intake air flowing over it. This change in the resistance value tells the control unit how much air is going into the engine. These sensors are prone to a different problem, they get dirty. I’ve found that some of the problems in this type of air flow meters are caused by performance filters. These after market, oiled filters, over time leave a film of oil on the measuring element of the air flow meter. This insulates that wire and makes it slower to react, the DME then reacts according to its inputs and engine performance is, over time, reduced. Some of these meters do have a self cleaning function. After the engine is switched off the signal wire is heated to almost white hot, to burn off any foreign substance. Even so, eventually the dirt will build up and performance can be lost. Presenting symptoms similar to that of the flap type unit.
Hints & Tips
Inline check valves for 944 washer jets – Tired of scratching your windscreen as you wait for the washer pump to spray water on the screen? Fit an inline check valve to keep the water up near the jets where it is needed, not back in the bottle.
Replace your DME relay – Do you know how old your DME relay is? Chances are it could be as old as the car. This little relay powers your fuel pump and the DME control unit. As a result it carries a lot of current. Eventually it won’t switch any more, and when it stops, so do you. For piece of mind get its date checked when you’re in the workshop. They’re not very expensive and you can stop worrying about being stranded on the side of the road on a rainy night.and the next thing you know is that it’s getting harder and harder to see at night.