Tuesday, 5 October 2021

E30/E28: Adjusting AFM (Air-Flow Meter) - change air/fuel rich/lean mixture


This is for BOSCH AFM units, fitted to most E30 and other BMWs fitted with Bosch Motronic or Jetronic injection systems.

The AFM, or Air Flow Meter, is a metal flap that is drawn open by air entering the engine and the position of the flap tells the ECU how much air is coming in so it can inject the right amount of fuel. The stock parameters for the AFM are set at the factory and unique to each car and BMW recommend that these settings should NEVER be adjusted... so do it at your own risk!!

Why would the AFM need adjusting??

  • You have done a ‘gas-test’ and the results show your car is running rich (too much fuel for amount of air) or lean (too much air for the fuel being supplied) and wish to adjust for emissions / fine-tuning purposes.
  • You have an air leak after the AFM, so more air is getting in than the ECU is supplying fuel for and the engine won’t run, especially when cold.
  • You have an over-fuelling problem where too much fuel is being supplied for the amount of air and flooding the engine.

Adjusting the AFM in the second two instances is a quick and dirty way to get your car running well enough to drive it, but is not a long-term fix for an underlying problem, so bear this in mind before messing with the AFM.


* To make adjustments to the AFM, the lid to the plastic-box holding the electronics will need removing. It is glued in place, but can be prised off carefully without damaging the plastic surfaces. *

There are two ways in which the AFM can be adjusted to change the fuel/air ratio, by changing the spring rate of the flap and by moving the rotor-arm on the carbon-track.

A. Adjusting the spring rate. - This is the recommended method as the spring is not factory-set for each vehicle, it just has a default position. The rate at which the spring reacts to incoming air can be altered to change the fuel delivery pattern. A tighter spring will require more air being drawn into the engine to push the flap open, meaning less fuel will be delivered for the same volume of air as stock and the engine will run leaner. Loosen the spring and less air will be needed to open the flap, meaning more fuel will be supplied for the given volume of air, making for a richer running engine. This method is time consuming, but gives a high degree of accuracy and fine-tuning, as the spring is adjusted in linear notches that also make it easy to keep track of alterations and return it to a stock setting.

1. Mark the notch where the locking arm of the spring is sitting in the stock position.

2. Lift the locking arm of the spring up and away from the wheel using a small flat screwdriver.

3.  Rotate the wheel using another screwdriver, clockwise for leaner, anti-clockwise for richer.

4. Release the locking arm of the spring back into the notches on the wheel.

5. Repeat this using the mark you've made in the stock position for reference.  

Thanks to Vern2765 for this rather crude video, but it shows what I'm talking about...

B. Adjusting the rotor-arm. - This is factory-set and has a blob of sealant to hold the screw in place so it can't be messed with, but the sealant can be picked out of the screw head and adjustments can be made. This method gives a lesser degree of accuracy, but gives noticeable changes with very minor adjustments being made. It's also near impossible to return to a stock setting, but the mark from the sealant does indicate the original position close enough. The arm is made of brass and rotates along a carbon track on the PCB as the flap is opened by incoming air. The distance the arm sits along the track gives higher and lower resistance levels in the circuit and the ECU uses this to determine fuel delivery. By moving the start position at which the rotor-arm sits on the track, fuel delivery can be increased or decreased for the same volume of incoming air. Having the arm sit further up the track while the flap is fully closed will mean more fuel will be delivered as the flap begins to open and maximum fuel delivery will be reached earlier. 

1. Pick out sealant from the screw head so it can be turned easily without rounding.

2. Loosen the screw slightly with a crosshead screwdriver.

3. Slide the hub of the arm along the slot that the screw sits in, clockwise increases the distance of carbon track from the start point of the arm, meaning less fuel per given air volume and a leaner mix. Adjusting the arm anti-clockwise will shorten the distance of carbon track from the arm start point and provide more fuel at the given air volume, making for a richer mix.

4. Re-tighten the screw. 

CO Pot or CO2 Pot: A third parameter can also be adjusted on older cars without a catalytic-converter, that don’t have an O2 sensor like modern cars, which is a rotary-potentiometer known as the ‘CO2 pot’. This is also factory-set and should only be adjusted for emissions purposes with the help of a gas-flow meter as it is only for fine tuning. Turning this dial will not affect the fuel/air ratio to a noticeable degree, but can be done easily without dismantling or removing the AFM using a flat screwdriver, again clockwise for richer, anti-clockwise for leaner.

** Adjusting the CO Pot will not alter the fuel/air idle-mixture, it only lightly trims carbon dioxide levels for emissions purposes. **


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