E30/E28: Faulty AFM (Air Flow Meter) repairs, worn carbon-track quick fix

** FOR ADJUSTING AFM AIR/FUEL RATIO SEE THIS POST **

The AFM or Air Flow Meter uses a metal flap that is opened air entering the engine to move a brass arm up and down a carbon-track. The position of the arm along the carbon track alters the electrical resistance through the circuit and this lets the ECU know how much air is coming in, so it can deliver the right amount of fuel to the injectors. The E30 uses a BOSCH AFM unit, also used in many other BMWs with Motronic or Jetronic injection systems.

The analog electrics in the BOSCH AFMs rarely ever fail, but there are two areas that are failure prone after higher mileage - a worn carbon track and a faulty air-temperature sensor.

WORN CARBON TRACK FIX:

This is what tends to junk most AFM of this type. The brass tip of the rotor-arm is split into two sections, each with a small stud that contacts the carbon-track. Over time and with overuse, the studs wear two lines into the carbon-track until they scrape through into the PCB underneath. The PCB does not conduct, so as the rotor-arm passes over the worn areas continuity of the circuit is reduced or broken, which causes the AFM to temporarily read incorrect (usually lower) values and the ECU finds itself jumping between values and not following the linear range up and down the rev range. This means the injectors are choking fuel to the engine when more air is entering than the AFM knows about. In extreme cases, the breaks in the carbon-track will give a sudden zero-value, which the ECU interprets as a faulty AFM and resets the fuel delivery system back to a range of default values. The AFM then jumps back to a correct reading and the ECU switches back to deliver the correct fuel amount to the injectors. This all happens in a split-second and the ECU struggles to keep up, giving the effect of fuel cut-off at certain bands in the rev range, which makes for rough, juddery throttle-response.

The fix to this problem is very simple, but will require the sealed plastic box with the AFM electronics inside to be cracked open. This isn't too difficult and can be achieved without damage to either side of the plastic surface.

The studs on the brass arm need moving in or out to sit either side of the grooves worn into the track, where there is still fresh carbon. Bend the arm at the point where it already kinks slightly or try and straighten it out to move the tip of the brass arm onto a fresh area of carbon. The differences made to air/fuel ratio by bending the arm in this way are minuscule, so this fix should clear up any worn carbon malfunctions. If the problem with your idle / throttle is still there, then the AFM could be toast, or its time to look elsewhere. More often than not, its an air leak somewhere giving the same symptoms, but doing this fix won't make the AFM any worse.

FAULTY AIR-TEMP SENSOR:

This is fairly common due to the age of the AFM units and the sensor being permanently exposed to moist air. It seems that in most cases a totally inoperative sensor will not affect the running of the engine noticeably, as the ECU will use a set of default values when receiving no air-temperature signal, but when a faulty sensor is giving incorrect values then rough throttle response can occur. If you feel this is the problem, then a replacement AFM is the way forward, though if you can find the correct temp-sensor for your AFM, then replacing just that component can be achieved by breaking open the sealed box with the electronics in and de-soldering the two pins from the sensor. For this effort, considering it may not be the fix you want, I would recommend replacing the AFM unit, which can be picked up off eBay for less than £50.

The sensor can be found just in front of the metal flap on the inlet side of the AFM after the air-box.