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Idle Control System.

Not really a how-to but a explaination of how the injection system controls its idle speed and what never to do...

You cant readjust the idle on a GTS, even with a diagnostic box.. the ECU controls the idle speed with a stepper motor actuated bypass valve in the throttle body and there starts todays lesson in MIU injection...

On these machines the throttle body and ECU is all in one unit, so it looks more complicated than it really is...

On a carb engine, when you close the throttle, it closes a butterfly valve in the inlet stream and the resulting airflow drop reduces the fuel atomisation and allows the engine rpm to drop... the carbs jet sizes determine how much fuel is drawn into the air supply and provides a fixed ratio of air to fuel... to control the idle speed on most carbs they simply have a screw that stops the throttle butterfly from closing all the way. The more open it is the higher the idle speed, the more closed it is, the lower the idle speed...

However the injection set up on the GTS is different... when you shut the throttle it again controls a butterfly valve, but on these it closes completely stopping all air into the engine, which should mean the engine suffocates and stalls... however to enable the ECU to be able to control its own idle speed, they build in whats known as a stepper motor controlled airflow system...

If you look into the mouth of the GTS throttle body, youll see the totally closed butterfly valve but before that a small opening.. this is the inlet to the stepper motor chamber...

Air enters through that hole and travels along the small external passageway cast into the aluminium body of throttle body as directed by the red arrow.

Once the air has come through there it enters a small chamber that has its exit blocked by the stepper motor (circled in red)

The air that gets passed the stepper motor then exits down a small opening and enters the inlet manifold and allows the engine to run..

The stepper motor itself is mounted in the throttle body and can wind its plunger in and outwards within a set distance, but highly accurately.. its something silly like a 14mm travel made up from 120 individual steps..

This is where the air exits and into the engine, all of which has bypassed the closed throttle butterfly.

So what goes wrong in this system? Well not much to be honest its proved to be very reliable indeed, occasionally due to the oiled foam air filter a small amount of oily vapour can make its way into the system and cause stalling issues, if this occurs a simple remove and wash out with solvent carb/throttle body cleaner does the trick and they work fine again. I have had a few stepper motors die, but its very rare indeed.

A really good function of the diagnostic box is the stepper motor counting, the ecu does a calculation of its preprogrammed data to work out what the expected stepper motor count should be for its current given state, this is known as "programmed steps" but it also shows how many steps it has actually done to reach its required idle speed, this is known as "carried out steps". Obviously these two should be nearly thr same, if its only a couple of figures out thats fine, but if its supposed to be 60 steps but its actually 90 steps, it wont show as a fault code but you can see that something is effecting the running and the machine is having to adjust a lot to keep idle correct, it could be anything like leaking injector, dirty air filter, tight inlet valve etc... its just a handy way of knowing if the machine is running like its supposed to.

What Not To Do.

These 2 things are common issues that untrained or simply people that dont know the system can easily do that cause havoc to the systems operation.

DO NOT EVER ADJUST THIS SCREW! This is not a idle screw, its the factory adjuster to set the throttle butterfly at 5.7 degrees open this is a set figure to let the ecu know that if it sees 5.7 degrees on the TPS then the throttle is shut and it operates the stepper motor to control idle, if you therefore adjust this, itll not see the 5.7 reading and therefore not know its at idle so not engage the idle control system, so youll get hunting, stalling, hard starting etc.

The next one is more for dealers, on the diagnostic tool there is an option to reset the TPS, why i dont know as it must NEVER be done after the throttle body has left the factory as it gets done when the butterfly valve is positioned correctly, the workshop manual tells you never to do it, the factory training course tells you never to do it, yet the amount of times ive known people to do it as they dont understand and see the word RESET and cant resist pressing it is beyond me... once pressed the bike now thinks 5.7 degrees is 0 degress and youll get haning idle speeds, stalling when slowing down etc... and the only way to cure it is a new ECU @ £350

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