so just out of curiosity id figure is ask...
on my current car (b16 da) vtec wont work until i get the light (after a few seconds of driving) and on my old crx ls swap it bogged terribly until the check light came on.
odd, though ive heard of other people having the same thing.

 

the details are overwhelming

 

I would say, if an O2 or Map sensor (or some other sensor) readings are false, your ECU is trying to adjust fuel ratio according to that info, but when a CEL comes on, the sensor is considered bad and no adjustments would be made, so that you feel it works better, but its still not good. Read the code and replace the faulty sensor

 

^^basically, yea. when the light comes on, the pcm goes into what's called open loop mode, running off set parameters stored in the pcm. as opposed to closed loop, where the pcm receives feedback from (a) sensor(s).
so when the pcm is in closed loop, and the sensor is faulty, it will receive faulty readings from it until it recognizes a problem and goes into open loop

 

So what code is the cel throwing, that might be a good place to start.

 

well, the crx is long gone now but the b16 throws knock sensor code THEN vtec works. really odd. first vtec didnt work, then after i fixed it (it was the knock) this happens. not a worry, more of a curiosity.