Setting the Stepper Motor

Credit Roger Carpenter
Classic Mini NLA: Rare & Alternative parts

The Stepper Motor is also sometimes called the Idle Air Control Valve (IACV or just IAC)
MemsFCR will show the steps returned by IAC.

What does the Stepper Motor do?

I guess it’s best to start this post by explaining what the stepper motor is and why it’s kind of useful in the MEMS SPi system. At its basic level, the ECU uses the stepper motor to control the idle and fast idle speeds of the engine. It tweaks the throttle control on top of our human right-foot action, both in acceleration (it follows the throttle opening) and lifting off the accelerator pedal (it catches the throttle as it returns). This idle return feature will smooth the engine’s return to idle and will help prevent some engine stall conditions. It is a pretty cool and a sophisticated feature of MEMS.

Via the smattering of sensors around the engine, the ECU constantly monitors inputs such as engine speed, engine load, engine temperature, inlet air temperature, rate of throttle opening, exhaust oxygen content and battery charge. Once it has number-crunched all of that and used is mapping data, the ECU has a few significant outputs under its control; the fuel injector, ignition advance and the stepper motor.

The ECU drives the stepper motor via a cam, to a push rod (both are in the stepper housing lid) which acts directly on to what is called the Throttle Spindle Stop. This small round flat disc is attached, via the Idle Stop Screw, to the throttle body linkage (see pic) and therefore the throttle disc and air supply/flow into the engine. If you watch the push rod carefully when the engine is warming up, you will be able to see it moving in and out a bit.

If the stepper motor isn’t working or isn’t set up correctly then the ECU has lost an important control feature of its overall engine management and that is why that engine just won’t run properly.

Why the Stepper Motor needs adjusting

There are two sides to setting up the Stepper Motor to get it to work for the ECU as it should. One is electronic, the steps, requiring some sort of compatible diagnostic tool like MemsFCR and the other is physical.

The SPi (or TBi) system is often somewhat misleadingly described as “just an electronic carburettor”, there is good and bad in thinking about the system in that way. The physical and moving parts on the SPi throttle body do look pretty similar to the mechanical workings of a carb and with regards to the Stepper Motor setup there is a mechanical aspect that is more than just important, it’s critical. This physical aspect is the adjustment of the Idle Stop Screw or as it’s commonly but accurately known, “The screw that should never be turned”. Unfortunately, because of the ‘electronic carburettor’ thinking many, many helpful folks try and adjust the Idle Stop Screw in the false belief that it just adjusts the RPM (like it would on a carb) not having a clue that although it sounds like the RPM is adjusting, it is not because of the screw being turned, it is the ECU trying to work out what the heck is happening to the changing airflow and try and correct/adjust all manner of things.

Just a small adjustment of that screw can send the stepper motor to too high or too low steps (because of the other sensor inputs that the ECU receives) and likely adjust the ignition advance too. Once moved in either direction without a diagnostic tool, things can be difficult to get back to the correct baseline other than by blind luck. There is a direct relationship between the steps, the physical setup and ultimately the correct running of the engine management.

Adjusting the Stepper Motor

First, some do’s and some don’ts…

• Do not do this unless you have a diagnostic tool that can display the number of steps or the IAC value.
  
• Your engine needs to be at full running temperature! 88 degrees plus. This is really important. Take off your air filter box but ideally keep the sensor and vac pipe attached. I’ve found I can rest it a bit to the left of the throttle body without disconnecting anything. You need to do this to get decent access to the screw.
  
• You will need a 7mm spanner to unlock and lock the nut on the Idle Stop Screw so it can be adjusted.
  
• Once you start adjusting the screw and the steps, do not at any point blip the throttle or rev the engine! Doing so will cause problems which can be difficult to recover from.
  
• Plug MemsFCR into the white plug.

There are a few opinions on this but generally, you are aiming to get to between 30-35 steps in order for the engine to run at the recommended rpm at running temperature.

If your SPi doesn’t have many additions (Front spotlights, air conditioning or massive ICE system are good examples) then aim for 30, if you have additions then aim for nearer 35. Switching on the additions will increase the load on the engine and drain on the battery, both monitored by the ECU, so the additional steps just gives the ECU a bit more to play with at the right end of the stepper motor motion.

1. Using the spanner, loosen the nut at the back of the Idle Stop Screw so that the screw can be turned in or out with your thumb and finger on the very tip of the Idle Stop Screw.
   
   Don’t move it much at any one time! Watch the IAC/Step number carefully on your diagnostic tool as you move the screw, you will see the number change either up or down, that will show you which way you need to rotate the screw so that you can get it down or up to the step/IAC position you are aiming for.
   
   Be patient, you will only be able to do this slowly. Make a slight adjustment to the screw and wait for the number to stabilise, then do it again. In my experience it can take around 30-50 seconds for it to settle, so be patient, you are waiting for the ECU to compute what is going on and it will make its own adjustments while you are doing your changes to the screw. This is a good thing. You will hear the revs change on the car too.
   
2. Once you have it at your step position target, leave it idling a while so the ECU does its thing, you may find the step number slowly drops or raises. If that happens make any final adjustment to get it back to the figure you want and then tighten the nut. Be aware that when tightening the nut, it may change the steps again so you may have to loosen and do it again if it moves from the step/IAC number you want. Patience!
   
3. Once done. Switch the car ignition off, take the keys out to be sure.

That is it adjusted! At this point, I would recommend you take your Mini for a long drive so that the ECU can start to learn things again and you can see if it’s made any difference to the drivability.

The next thing to do is to check the Lost Motion Gap. The gap is now very likely to be out of alignment. If the Lost Motion Gap does need adjusting, then you will also need to go back and check and probably readjust the steps/IAC again afterwards. Tweaking the Lost Motion Gap will have an effect on the relative position of the Idle Stop Screw.

Once all this is done there should be no real reason to adjust the Idle Stop Screw again. Electronic tweaking of the steps should be fine but that probably doesn’t need to be done very often either if your Mini is regularly serviced and looked after.