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stepper motors

S

silvergoose

Tuned and Synch'ed
Joined
May 25, 2010
Messages
50
Location
Ft Smith, Ar.
Being new to the world of the modern Moto Guzzi, my others were BFI ( before fuel injection) Can some one give a run down on the stepper motor , why is it there, why is so many idle problems?
I have worked on many fuel injected bikes ( BMW, Can Am), But I must admit I do not what the stepper motor does. I have seen kits to improve the stepper moter operation and noticed an on/off switch. Under what conditions would the motor be turned on/off ? It seems strange to take an automatic system and make it manual. My other bikes include a BMW K1200 lt, fuel injected, no stepper motor in name but it does have an idle valve, same?
 
The purpose of the stepper motor is to control the idle speed of the engine in all conditions. If you start a gasoline engine when it's cold out and the engine is cold, if you didn't have the stepper motor you would have to manually open the throttle a bit to keep the engine going until the bike heated up. The ECU controls the stepper motor telling it how much air to let into the engine to keep the idle at 1100 RPMs or so. I've never had a problem with mine on my Norge even after 30K miles, but I did have an erratic idle which was cured by the removal of the evap canister and all related hoses. The only drawback to this is, if I go for a run and park the bike in the garage the tank fumes can now be smelt till the bike cools down.
 
It also, I believe, varies the idle mixture slightly from cylinder to cylinder by moving air flow around making one leaner then richer and then the other in sequence. this allows the catalytic converter to function properly and more efficiently I'm told.

I too have NEVER had an issue related to the stepper and have no problem with either it or Cat-Cons generally. I'm afraid I think that the need a lot of people seem to feel for isolating it or removing it is puritanical 'Old-Think' with little or no merit. Feel free to disagree. My bike runs and idles just fine.......

Pete
 
So Pete, you've never had the high idle problem? When the bike gets hot I will pull to a stop and the idle will stay around 2k. The only way to get it to return to idle is to let the clutch out slightly for a second to put a load on the engine then the idle will come down and stay down until the next time I stop. It doesn't happen all the time but it happens fairly regularly when I am putting around the city. I've always assumed this was due to the stepper motor. Someone mentioned a few days ago that maybe the programming for the stepper doesn't account for the charcoal canister that I assume you don't have.
 
The stepper motor has two different reasons for being on your motorcycle. One is to assist in maintaining idle at different temps and loads (on cars it even has to deal with huge load changes at idle due to things like the AC compressor kicking on and off, we should only see small changes of load). The other function it performs is to reduce emissions by cracking open when you are decelerating from higher rpms (throttle closed). This maintains whatever target air/fuel ratio some engineer has determined is best for emissions. This aspect of it can slightly reduce the amount of engine braking (compression braking) since the effect of this is much the same as cracking the throttle open slightly (or even setting the idle a little high).
The stepper motor as it came on my Guzzi cannot tell one cylinder from the other, it has one output that T's to go to both throttle bodies. It could open and shut at low throttle openings to cycle the mixture back and forth, rich and lean, as this does help the cat work properly. But since the O2 sensor and ECU normally work in that fashion anyway I would be surprised if they use the stepper motor for this (I am not sure it can even respond fast enough to do this). And obviously its effect would be diminished as you open the throttle.
I did a manual switch to tun mine on and off (before the kit existed). This allows it to assist in cold starting the bike but then I can turn it off and maintain a more stable idle and the shifting feels slightly better possibly due to the slight increase in compression braking. Yes, that last one could be in my head as feel is hard to quantify (but if it is in my head I am not alone as others have reported the same thing) but the more stable idle is definite.
 
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