Changing injector pulse rate at idle?

[John T Stein]

In response to my question about the varying frequency of the 
injector firing pulses I had observed on a TBI engine at idle, Robert 
wrote:


> > John
> > 
> John, while the engine speed may appear to be essentially constant, it isn't.
> A 30 rpm variation is not uncommon between cylinder firings and one would
> expect much more cycle by cycle.  The injection event is crank position
> dependent and the variation you have described (40 - 45 mS) corresponds to
> a 750 - 667 rpm variation, assuming one firing per cycle. 
> 
> I don't think anyone uses a variable period (speed independent) method for 
> controlling injection quantity for port injection.
>  
> Robert
> 


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??