Technical delayed revving down to idle

[tviscor]

I have a Fiat Cinquecento 0.9L from 1996 that works perfectly apart from one small thing.
When I give gas in idle (ie. rev up the engine) it stays in high revs for a few seconds (around two seconds) before then dropping down to what I assume is normal idle revs. Is this normal? If not are there any suggestions as to what it could be?
First I thought it might be the engine temperature sensor, so I changed that but to no effect (luckily they aren’t to expensive).
I have tried searching the boards and internet not to mention my manual for any clues but so far have found nothing.
Hope for help
a new Fiat owner (Tviscor)

 

[fingers99]

Very difficult to tell. Could it simply be that the throttle cable needs lubricating (any old spray grease -- often pound shops have it).

 

[tviscor]

I have checked that, and it isn't it. i tried revving the engine directly on the leaver on the engine block (directly where the cable is attached). also when i rev high revs it falls fast down to moderate stays there for a few seconds and then falls down to normal idle. Also found out yesterday that my fuel economy is quite bad (around 12 km/liter = 28,2 mpg it should be about 50 % higher as far as I have read).
Tviscor

 

[fingers99]

The sensor most often related to high fuel consumption is the MAP sensor. Would be worthwhile checking the pipe and connection, and maybe seeing if you can get one off a scrapper.