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Idle Air Stabilizer valve test

For motronic 1.1 system 535I E28

symptoms of a bad idle air stabilizer valve

  • The engine has erratic Idle speed
  • RPM is high (about 2500 RPM)

Learning context

The idle air stabilizing system works to maintain the vehicle’s idle speed within a 200 RPM range regardless of varying engine load at idle. For example, if the idle speed drops below 750 RPM the electric valve opens allowing airflow through the throttle plate, and when your idle speed rises above 950 RPM the electric valve closes restricting the airflow through the throttle plate and reducing the idle speed. A poor-performing IAS will cause your vehicle’s RPM to be erratic and high causing the vehicle to run poorly.

Motronic VS Motronic 1.1

When testing the Idle air stabilizing valve you will need to identify what motronic system your vehicle is equipped with. To identify your E28’s type of idle air stabilizer valves; early models (before 3/1987) were equipped with a two-wire valve (Motronic) while later models (4/1987 and on) were equipped with a three-wire valve ( Motronic 1.1).

My E28 is equipped with the Motronic 1.1 system, but if you want to learn about the motronic system I will create a write-up for motronic systems but will not have photos. I will try to find a friend with a vehicle with a motronic system and perform the test for you guys.

Why am I testing the IAS valve?

As I am going down the list of what can be causing excessive fuel consumption in my E28, I want to confirm that the IAS valve is working properly. This part won’t cause the issue of excessive fuel consumption, but it is a vital part of the engine operating properly.


  1. Confirm proper operation of the IAS valve
  2. Resistance test of the IAS valve
    • A resistance reading- 40Ω
  3. Voltage test of IAS valve
    • The voltage between the center pin and outer pins- 10V
  4. Tools/Parts
    • Volt meter
    • Idle air stabilizing valve part # 13411286065

preliminary Check

Before you perform a test on the IAS valve make sure these conditions are met.

  • The engine is at operating temperature (140 degrees F)
  • Turn off electric accessories. Ex-Air conditioner, heater control, headlights
  • The throttle position sensor operating correctly
  • No exhaust leaks
  • No vacuum leaks
  • The oxygen sensor operating properly


  1. With the engine running, disconnect the IAS electric connection. The vehicle RPM should increase to about 2,000 RPM. After I disconnected my IAS my RPM increased by 100, and automatically my IAS FAILED and needs a replacement.
Motronic 1.1 three wire IAC
Motronic 1.1 three-wire IAC, disconnect while the car is on

2. If the RPM does not increase, turn the engine off and check the resistance between the two outer terminals of the valve. There should be about 40Ω of resistance total, 20Ω between the outer and center pin. Between the two outer pins, the IAS had a resistance reading of 44.4Ω.

Center with left pin Ressitance
Center pin with the left outer Resistance reading
Ressitance test of the IAC
Center with Outer right pin resistance reading

3. If the IAS valve fails the previous test replace it with a new one.

IAC failed test
My IAC failed the voltage and RPM test and needs a replacement

4. If the valve passes the test, check the voltage at the electrical connection. With the ignition on, there should be battery voltage present at the center terminal. You should read about 10V

IAC voltage reading

5. If no voltage reading is present have the idle speed control unit (if equipped with Motronic) or the ECU (Motronic 1.1) diagnosed by a repair shop.

6. The last thing you want to do if the valve passed all the tests, is taking some carb cleaner and clean the build-up inside the IAS valve.


My Idle air stabilizing valve failed the RPM test and the resistance readings were higher than listed. I will be replacing the valve and performing the tests again. If your idle air stabilizer valve is working properly you should check that all of the vacuum hoses and intake tubbing are not cracked or leaking. This can also cause a rough or erratic RPM.

UPDATE 03/30/23

I explored the internet and found a aftermarket IAS valve on Ebay for forty dollars, I know what most of yall are thinking. But I said what the heck, I will test this product so you dont have too.

Ebay IAS valve

I went ahead and repalced my IAS valve, and took the car out for a 30 miles testdrive. As most of you have already guessed it DID NOT WORK, the RPM’s were speratically jumping while underload. I am leaving it on untill the second IAS valve arrives from ECS tunning.

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