Karmakanix VW Audi Berkeley CA

MAF Sensor: Measure the Blow

Gear case (3)MAF and MAP sensors have a lot in common, they both measure intake air volume. Early mass air flow sensors from the 80’s, 90’s and this century have had a fairly common failure rate. Digifant and Digijet MAF’s are called air flow meters. A pivoting vane was connected to a wiper arm that rotated across resistance strips. The strips would get burn spots and bald spots over time, just like any active geezer. This would slowly, or sometimes suddenly, lean out the car on acceleration, causing power loss. Sometimes momentary, maybe terminal, and always worst when cold. These systems were also prone to ground system losses, which would affect the small voltages emitted by the MAF’s, or AFM’s.

The next type of MAF sensor is a hot wire type. The basic function is by heating a platinum wire with either a constant voltage over the wire or a constant current through the wire. Our cars used current. The electrical resistance increases as the temperature increases, which varies the current over the circuit. When air flows past the wire, the wire cools, decreasing its resistance, which in turn allows more current to flow through the circuit or causing a small voltage drop over the wire. The temperature of the hot wire increases until the resistance reaches equilibrium again. The current flow is proportional to the mass of air flowing past the wire. In some MAF’s, every time one shuts the engine down, the hot wire will burn off any built up contaminants. Those that do not fail so early. The next series MAF sensor had a hot film, starting with the Mark IV’s. They had a nickel film grid, and more or less had the same problems.

MAF sensor DSC07579 120x92One mode of failure of a MAF sensor was when engine heat would melt crappy insulation, and it would melt down onto the hot wire or film. MAF sensors would also get oil deposits from old engines with oily crankcase ventilation. Everybody thinks that because the PCV vents downstream the MAF, it should never contaminate it. The facts are that any engine works on pulses, both for intake and exhaust. These pulses can capillary the oil backwards onto the MAF sensor. The electronic circuitry that translated the hot wire/film signal into a .6 to 4.5 volt signal would also fail. Sometimes a sharp thump would make the MAF straighten out and fly right, momentarily. Sometimes a MAF sensor can be cleaned with carburetor cleaner, and recuperate its function. You should know that sometimes a MAF is way off kilter, yet does not throw a DTC. You should also know that the popular oil saturated air filters contribute to the demise of MAF sensors. The slight amount of oil they can shed can contaminate a MAF sensor over time. The oil carries with it a tiny amount of tiny dirt particles. Since the dirt has oil with it, it sticks to the hot wire. During the burn off cycle after shutdown, the tiny amount of dirt turns to a tiny amount of glass. MAF failure usually occurs within 4 to 6 years.

Gear case (3)