The Italian tune-up - at least in a diesel [although it applies to a gas car too - Ed] consists of taking a SOUND car out and maximally accelerating from stop or slow speed to some high speed OR driving at near full rpm with a heavy load (up long hill) for minutes at a time. The idea is to get maximal air and fuel flowing through the engine to develop near maximum heat. This will burn and eject residual carbon that has collected in the engine. Following such a regimen the idle is usually smoother and the amount of black smoke at full throttle should be reduced. This occasionally does not happen the 1st time and the idle might get rougher or a pronounced knock could occur. This is usually because some carbon has been "rearranged" but the prechamber environment isn't appropriate for smooth combustion. Further TREATMENT will clear this up as more carbon is burned up, but it is a little scary the 1st time it happens to YOU and your car! A few full throttle accelerations a week will usually keep most CLEAN engines cleaned out pretty well. More prolonged "treatment" is usually required for a car that has had a LOT of city driving. Just a long highway trip (50+ miles) with several two-three minute full throttle periods (long 6-8% grades are GREAT) will clean out an awful lot of carbon and crud that's build up, but with a really "city bound car" it may may take more than just a few sessions to really get it all out. My weekly 250 mile trips from Pittsburgh to Washington DC and back over the last 7 years have given me a chance to really examine the "Italian tune-up" and the concept of "flailing the heck out of the MB diesel engine" or at least running it at near full throttle/top speed for a long period of time.
I referred to a "SOUND" car. To me that means a car that NO serious mechanical problems. The chain stretch should be below 5 degrees or so, the valves should be properly adjusted, the air and fuel filters should be verifiably clean, and there should be clean and it would probably be best if the injection pump timing is known to be pretty close to correct (within a couple of degrees). The cars/engines I presently own, have THRIVED on this regimen. Idle smooths out, smoking diminishes, and fuel and oil consumption goes down. I can't promise that on an engine that's worn out, but nobody else can either ;-)

In the spirit of political incorrectness, we should not forget the other two miracle fixes given to us by other nationalities. (1) "Polish Brake Job" this , of course, repeatedly "getting on" your brakes hard in an attempt to "fix" pad or rotor problems. (2) "Irish Alignment" why waste a lot of money on an alignment? If your car is pulling to the right, just pump your right tire up to 50 pounds, and bleed the left one down to 15 or so! Watch how quick the differential in rolling resistance corrects your path!