Fuel economy loss
 

I have a 98 civic si and I was getting 525km per 40 litres in the
city.
Over last year the fuel usage has gone up and I can't figure out why.
Currently I am getting only about 450km per 40 litres in the city.
My idle is fine and the engine does not seem to run roughly.
I have noticed that around 2500 rpm I used to get a little surge of
power and then again at 3100 rpm.
As of the last while the surge of power at 2500 rpm is intermitent. I
would suggest that the power seems to go away with the engine getting
to running temperature however I have no proof just how it seems to
me.

Anyone have any ideas of what might be the difficulty.
I can tell you I have never replaced the PCV valve in the last 100,
000 km. Oh and the car currently has 190, 000 km on it.

Thanks for any help,

Brian