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535I E28 Aiflow meter test

For Motronic Systems

Symptoms of a bad airflow meter

  • Hard Engine start
  • Fails to start when cold
  • Engine misfires
  • Engine hesitates under load
  • Poor fuel economy

Learning Context

The airflow meter is installed between the engine air filter and the throttle body. The purpose of the airflow meter is to measure the amount of air mass entering the engine and send that information to the ECU. The ECU adjusts the air-fuel mix ratio based on this information. A poor-performing or non-performing air flow meter can cause a lot of problems that can diminish the drivability of the car.

Motronic Vs Motronic 1.1

When testing your AFM you have to identify which motronic system your E28 is equipped with. Early models (Before 3/1987) E28s are equipped with the motronic system and late models (4/1987 and on) are equipped with the Motronic 1.1 system. You can quickly identify which system your vehicle is equipped with by the number of pins your ECU has:

  • Motronic ECU has 35 pin
  • Motronic 1.1 ECU has 55 pin

The testing for the AFM between these two systems is identical except for the pins that you need to probe at the ECU electric connector. You can find the pins for each system in the summary section of this write-up.

Why am I testing the airflow meter?

As I have mentioned in a previous post I have an excessive fuel consumption problem with my E28, a poor-performing or non-performing air flow meter can be the cause of excessive fuel consumption. I want to eliminate the airflow meter as a culprit behind this excessive fuel consumption.

Summary

  1. Resistance test on Air flow meter
  2. ECU pins to probe for resistance test
    • Motronic- Pin #7 and #9
    • Motronic 1.1- Pin #7 and #12
  3. Check the resistance of the temperature sensor in the airflow meter
    • The resistance reading at 68°F= 2,200-2,700 Ω
  4. Tools required

Procedure

Before you start, disconnect the negative terminal from the battery, when performing any diagnosis involving the ECU. Disconnecting the ECU while the battery is connected can damage the unit and require a replacement.

E28 negative battery conenction
Disconnecting the 12v black connector before testing the ECU
  1. Locate your air flow meter between the engine’s air filter housing and the throttle body.
E28 Air-flow meter location
The air-flow meter is located between the engine filter housing and the throttle body.

2. Remove the intake boot to expose the flap inside the airflow meter, the clamps are 6mm, or a flat screwdriver to remove.

E28 air-flow meter flap
E28 air-flow meter flap

4. Open your glove box and remove the cover exposing the ECU, disconnect the ECU, and locate the appropriate pins for your car’s Motronic system.

ECU location in a 1987 535I E28

5. You can tell what Motronic system you have by the number of pins on your ECU. A Motronic ECU has 35 pins and the Motronic 1.1 has 55 pins.

Diargam of E28 metronic Pin out
Diagram of E28 Metronic ECU with labeled pins
Engine harnes side of he ECU connection
Engine harness side of the ECU Connection.

6. My E28 is equipped with the Motronic system so I will be probing pins #7 and #9. If your vehicle is equipped with the Motronic 1.1 system probe pins #7 and #12. Because I am working by myself I took two gauge 18 wires and a 6 ft retractable probe leads to be able to complete the sweeping resistance test.

Pin 7,9 probbed
6FT clip to help me diag
Gator clips on probed pins
Attached volt meter with Ohm setting

7. While probing the ECU open and close the inner flap of the airflow meter and you should see a steady increase of resistance as it opens without any flat spots or erratic resistance change.

8. If the airflow meter does not pass the resistance test, check the harness for any worn-out or damaged cables. If no harness damage was found the airflow meter has an internal short and it needs to be replaced.

9. If the airflow meter passes the resistance test, take some MAF cleaner and clean the inside flap as well as lubricate it to prevent binding or sticking.

Cleaner that can be used on your E28 AFM
Air flow meter safe cleaner

Conclusion

My air flow meter passed the resistance sweep test, and the temp sensor was within resistance spec. I removed completely and sprayed it down with some Air Flow meter cleaner and put it back together. If your air flow meter passed the resistance test and it does not bind or stick open, you can cross the AFM as the culprit of excessive fuel consumption. I would recommend testing the Idle air stabilizer valve, and the fuel injectors.

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