Changing injector pulse rate at idle?

[robert dingli]

John,

> In my original posting I had said that the injector pulse period I 
> had observed at idle was varying between about 40 milliseconds 
> and about 45 milliseconds with a constant idle speed.  
> 
> While a slightly wandering idle speed might cause the pulse period
> to drift THROUGH the range from 40 to 45 milliseconds, what I 
> observed was a period that was EITHER 40 milliseconds OR 45 
> milliseconds, with no intermediate values. I would expect idle speed 
> wander to yield periods BETWEEN these extremes as well as the 
> extremes themselves.
> 
> The apparantly deterministic nature of the variation in period  was what
>  led me to ask about possible reasons. It seems like the ECM is doing 
> this intentionally, but for what reason??  
> 
> BTW, the engine in question is throttle-body injected, not port 
> injected as Robert mentioned in his reponse.  While some averaging of 
> the injector duty cycle, and hence the air/fuel ratio would occur in the 
> intake manifold, varying the pulse rate still seems like a strange thing 
> for the ECM to do.
> 
> Any suggestions??

The simple solution would be that 5mS is the limit of your system's 
resolution (as some earlier has pointed out).