I have noticed a curious thing in my '03 Civic. When cruising on a level road with the dash gauge indicating a steady 64 mpg, the engine will periodically surge, dropping the FE reading to about 40-48 mpg. A few seconds later whatever causes it will cut back out and the reading will rise back to 64. It's like an invisible foot that briefly steps on the accellerator. This happens with cruise control off, A/C off and with my own foot absolutely locked in position. Has anyone else noticed this behavior? Any ideas what causes it? Sometimes it seems to happen about every minute or so.

Help!

 

Sounds exactly like the lean burn/purge cycle that a stick shift Insight will do.

After the engine is warmed up, the engine will go into lean burn, mpg's will be high. Every few minutes, the speed will jump up slightly and mpg's will drop (10-25 mpg).
The engine control system is richening the mixture. Power is increased, and mpg's suffer.
This is the purge cycle, intended to clear nitrogen oxide (NOX) from the catalytic converter.

If you are trying to maximize mpg's and have the patience for it, you can lift off the gas pedal during the purge to maintain the instantaneous mpg's on the FCD.

Regards,

 

I believe that what your seeing is a purge of some of the NOx gases from being in lean burn. I saw a post on this phenomenon but I failed to find it again

I believe that the post also stated that it is documented in the owner's manual.

EDIT: Found this HCH forum entry with some discussion on lean burn purge:
https://www.greenhybrid.com/discuss/...ean+burn+purge

You may desire to look in the Insight Forum, also.

(Apologies to Don; I think we posted almost simutaneously.)