The opportunity to get the engine up on the cam and run it until good and hot is rare.
As a result, engine oil doesn't get hot enough to boil off moisture and burn off the deposits caused by low speeds and incomplete warm-ups. Even with a fresh cleaning and tune-up, these cars may still show poor idle step-off, hesitation and non-linear throttle response.
A cure can sometimes be to carefully run an otherwise good engine hard so it gets hot enough to burn the excess carbon deposits off the piston crowns, clean the valve faces and blast deposits off the spark plugs, fuel injector nozzles and oxygen sensor and at the same time boil any lingering water out of the oil.
Warning #1:
The following procedure can blow your engine to: smithereens, bits, pieces, orts, perdition and kingdom-come. If you own a high-mileage, low-compression, oil-swilling, rod-knocking, piece o' crap car (you know who you are, don't you), don't bother with the "Italian Tune Up" because you will end up walking home from what's left of your sorry pile of smoking and hissing junk. OK? (I'm not kidding.)
:-)
Now. Assuming your car is otherwise in healthy condition already, and you have worked through the other steps on this site, let's begin.
The easiest way to "blow out the carbon" is to run the car under load at high engine speed. In other words, run the car right to the redline and hold it there in every gear. For decades, experienced drivers and mechanics have called this procedure the "Italian Tune Up".
Now this is a great trick on a race track, but hardly practical on the street.
The street alternative is to first check all fluids and top them up. Make sure you are running clean, high quality oil. I use regular dino-oil 20W50 in summer, 10W30 in winter. (A lot of experienced drivers swear by synthetic motor oils for high temperature running. The additional expense of synthetics may be offset by their longer drain intervals.)
Drive a while so that your oil and water temperatures rise into the normal zones, then head out to your local four-lane. In light or no traffic, try to run for one or two minutes at 90 to 100 kph (~ 55 to 60 mph) in second gear. The engine will be running (loudly) near the red line.
If you continue to push up into the red you will trigger the Digifant rev limiter which cuts the fuel supply in half. This will feel like a sudden roughness in the engine and a loss of power. The first time you experience this you may think you've blown something. Just back out of the throttle so you don't keep banging into the rev limiter. If your engine is in good shape, this is not dangerous, just disconcerting the first time you experience it.
Again, neglected engines will not be up to the strain. If you really don't know the maintenance history of your Digifant car, or if you've just bought it, do not attempt this!
WARNING #2:
If you blow a head gasket or otherwise detonate your engine by following this procedure, don't say you weren't warned! Because you have been warned.
Now if you feather the throttle back just out of rev limiter range and hold it there for 60 seconds or so, the car will continue to cruise smoothly, albeit loudly. (If you are not used to driving your car at or near the redline, the noise can be disconcerting, especially to your passengers!)
The point of holding the rpms steady under the red line is to get everything really hot from the piston tops and valve faces out to the catalytic converter. Under these conditions the engine will quickly reach peak operating temperatures.
I have this vision of four wonderfully hot whitish blue flames pouring out of each exhaust port into the downpipe. Sort of like getting the mixture just right on a Bunsen burner or an oxyacetylene torch, or how about on a 1650 hp Rolls Royce Merlin V12 at just under "war emergency" power? Nice!
If you have an oil temperature gauge, keep a close eye on it because depending on the ambient air temperature, this should be as high as it will ever get. In my Jetta on a warmish summer day, oil temperature peaks around 110°C (~230° F). That's hot!
Remember, if your car is in basically good shape to begin the red line is fairly conservative. A kilometer (or mile) or two of this is all you need to burn off the harmful deposits from stop and go driving. At 100 kph (~62 mph) and 6,000 rpm, a 60 to 90 second burst should be all you need.
After one or two such treatments, the improvement in low speed driveability and throttle transitions may be quite noticeable, especially after a few months of winter or just city driving.
Remember, the famous "Italian Tune Up" is meant only for drivers who know their cars well and know that their cars are in good shape. If you don't know whether your car is up to this kind of treatment, or if you can't afford to fix it if it blows up, then don't risk it.
After all of this, your car should be running like a champ.
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