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I have used a Binks #7 paint gun and shop air and water in the gun. Clamped the throttle linkage at 2k RPM and set the paint gun to flow into the intake with the air filter removed. This was done to 81-83 Datsun 280 ZXs which developed a carbon knock that sounder like terminal engine damage. The flat top piston (only used those 3 years) was only about a millimeter from about 40% of the cylinder head surface, a little more than the thickness of the head gasket. When enough carbon accumulated to fill that gap you got a knock that sounded like a rod knock.
That pocess was developed after many heads were removed for decarbonizing to get rid of the knock, which was in turn developed after several engine tear downs with no obvious damage except the flattened carbon deposit where contact was being made.
Those old engines used flap potentiometer air flow meters and were the first oxygen sensor 3 way feedback systems used by Nissan in their cars in the US. Instead of charging many hours labor to tear down and inspect an engine that had no other issues we developed the spray gun technique that took less than an hour and required no disassembly other than the air cleaner assembly.
Looking into the cylinders with a fiber optic light showed the carbon gone from the piston crown, and the repair lasted a long time and could be done many times, but we ususally never had to do it twice and it was usually close to 100 k miles when we had to do that cleaning.
I hesitate to do that anymore, not sure what the sensors might suffer as damage from the process. The hot wire mass air flow sensors and different cylinder heads in the V6s that Nissan put in the 84 and later 300ZXs never had the problem. The earlier 280s did not have the flat topped pistons so they would not develop the same carbon knock.
It cleaned the pistons to almost the same level as a blown head gasket did when the coolant was leaking into that cylinder, practically spotless.
regards
Mech
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