Engine flush is an engine cleaning service used to remove the accumulated sludge, deposit and other gunk inside the engine. It is achieved by adding an aftermarket chemical into the engine oil port, idling the engine for 10-15 minutes, and then doing a regular oil and filter change.
It is NOT recommended for:
Vehicles whose odometer > 10 0000 km have never had the engine flushed: The sludge and deposit may be the only seal for the tiny cracks. Having an engine flush would reveal these problems and may result in an expensive repair cost.
Vehicles whose oil is changed on schedule
It is recommended for:
Vehicles undergo lots of short trips (for example, 5 minutes drive between the office and home) and lots of stop-and-go (for example, heavy traffic)
Vehicles whose oil is changed way behind the schedule
As the fuel and air explode in the engine, deposit precursor species form on the cylinder, piston and head surfaces. Engine oil rinses off these particles and brings them back to the oil pan. Usually, an engine oil filter could prevent these particles from re-entering the engine system. However, when the engine oil is cold, it doesn't present a good flowability. It would go through another route that does not contain a filter. Without filtering these grimes, they are brought back inside the engine again. Five minutes of driving would not be enough to heat the car. Hence, vehicles undergo lots of short trips, and stop-and-go is recommended to have an engine flush service.
Oil filter could only hold a certain amount of grimes. Once the filter reaches its maximum capacity, engine oil is not going through the filter anymore. All the particles re-enter, circulate and build up through the engine system.
Are deposits, sludge and other gunk that bad?
Sludge is an acidic gel form of engine oil and grimes mixture. The acidic sludge could cause corrosion to the rubber seal and cause oil leakage. Other than seals, there are also plenty of rubber components under the hood, such as vacuum and coolant hoses and drive belts. The leaked oil would continue to cause damage to these rubber components and eventually lead to a replacement.
Other than that, sludge acts as an insulator and stores heat. The presence of sludge, therefore, poses more stress to the radiator and cooling system.
As O'Brien and Christopher wrote, "the presence of these increase engine-out NOx emissions, increase octane requirement, changes in flame speed and thermal efficiency" [O'Brien, Christopher J. "Formation mechanisms of combustion chamber deposits"]. Stated, the presence of these gunk increases your emission, decreases your fuel efficiency and costs you more money.
All of these problems that sludge and deposits bring can be fixed by having an engine flush.