my car fiat bravo 1.2 16v 1999 is revving itself when idling - when cold it oscilates btwn 1500rpm & 2000rpm then soon after it shoots to 3000rpm and never falls back.
there is no engine light. changed the throttle body twice still no effect - traced for any leaks from servo to breathers and all is fine. when i check all spark plugs they are dry sooty black, also the lamda sensor is also sooty. the car used to misfire but change spark plugs before the problem and never misfired.

this problem happened after filling a full tank of petrol and pouring stp injector cleaner ( that was to solve the misfiring - but since changed plugs and leads)
I aslo put oil treatment as the car will make blue smoke after being parked for days implying the valve seals had hardened - the treatment was stop leak that acts as a valve seal softener. since then there has been no blue smoke but normal condensation colour. however this colour tend to be a very light thin grey to white which is consistent with the high rev problem i have. i seem to lose water slightly but the is no leaks and no mixing with oil. the fuel pump starts normal with key.

i have run out of ideas please help advise

try adding washers between the throttle positon sensor and the throttlebody.