Carbon Build Up On Pistons.

Engines that consume a steady diet of oil, can build up a significant amount of carbon and other deposits on the piston tops. All engines will form some deposits on the pistons, that is just a function of an internal combustion engine. Completely normal process, eventually it will get to a point where it needs to be polished off (ie, when you rebuild the engine). With most well maintained, modern engines, we are talking about about a significant amount of mileage on the vehicle - 250K+ miles.

To keep it at controllable levels is as simple as making sure you stay on top of the maintain for your car: regular fluid and filter changes, run top-tier gasoline, replace plugs when they are worn, etc.

They do sell "de-carbonization" services, basically run a special solvent through the induction system of the car, also lots of aftermarket fluids and additives that could also be added to the car. Most don't do much of anything, some actually cause more damage than not. To effectively "remove" carbon deposits on the piston tops, combustion bowl area on the head - you have to pull the engine apart and mechanically remove those deposits. No solvent can completely remove those deposits, they can just remove the easily loosened stuff. You could do the same thing by running the car down the highway briskly, with some WOT applications, and some lower gear engine braking.

Depends on where the carbon is building up, Typically, we are talking about normal carbon build-up during the combustion process - involves the piston crown or tops of the piston. It is the solid residue that forms on the crown, as a function of combustion, a normal process. In that case, then no, carbon deposits are not responsible for oil consumption. Usually, heavy deposits on the pistons can lead to hotspots on the piston, that can then lead to engine pinging/detonation.

In the case of "stuck" piston rings and plugged oil return holes - those are carbon deposits formed from the oil coking. This is where the oil is literally "cooked" down to a coke deposit. It is the solid residue left from severely oxidized and thermally broken down motor oil. In this case, these deposits will cause oil consumption.

Problems with most "decarbonization" services and additives - they generally only attack the former type of carbon deposits, the ones that form normally on the piston crown. To attack the carbon that sits behind and around the piston rings and the ones that clog the oil return holes - you'll have to do successive, long term piston soaks and/or blow the engine apart to remove those deposits manually.

When the oil is not properly serviced or not appropriate for use in the car, ie. too long of an interval, poor quality oil, poor filtering, extreme driving environment for oil, etc. - then the oil can see extreme thermal breakdown and oxidation, causing it to form significant deposits around the rings and through the oil drain holes.

The actual specifics of "how" or "why" this happens on some cars and not on others is still up for debate. So far, the overall concensus is sticking with a conservative oil change interval with a quality oil and filter will help reduce the likelyhood of this happening. But it doesn't seem to be completely eliminating the possibility either.

Correct, coking is considered carbon deposits. The level of deposits depends on the type of oil and how long this condition was happening. In the case of oil return holes, coking will eventually plug the holes. IN the case of stuck rings, coking deposits will form around the ring, on the ring lands - between the ring and the ring groove, causing it to not seal correctly against the cylinder bore.

Sometimes - depends on how badly it is consuming oil. If you search for piston soak, you'll run across lots of possible solvents to use. It is hit or miss if they can stop oil consumption or slow it down. Each car is a little different - some saw oil consumption drop, some saw no change, a few actually saw oil consumption get worse.

Piston soak is usually one of the first things owners try to slow down oil consumption - if it works, great - if not, no big loss.

^^^ That's my favorite tune up!

Tons of discussion on this online and on Toyota Nation.

As you know, piston soaking is a stop-gap method to try and loosen hardened carbon deposits that could lead to excessive oil consumption. Usually considered a first attack at the problem, see if you can slow down the oil consumption. There are a number of solvents that have been used - some use ATF oil, Seafoam, MMO, Kreen, BK products, etc. Some have shown some results - both good and bad, many don't do anything as the carbon buildup is usually beyond chemical treatment at that point.

Not fully understanding what he means by "piston soak from the intake side" - unless he plans to pull the solvent in via engine vacuum. Some solvents, like Seafoam, recommend to pull in from a vacuum source to be pulled past the intake valves (brake booster is a common vacuum source). Almost all the vacuum sources on the engine will eventually pull it to the intake side, just the way an internal combustion engine worse. Problem is, if done incorrectly or if the solvent itself leaves deposits - you could make the problem much, much worse.

Only thing I'd run on the intake side would be a pressurized injector cleaner that is plumbed directly to the fuel injector log. Those types, the car actually runs off the solvent, during the cleaning process. Even then, I'd have to worry about breaking down the oil film that is already there and/or loosening too much of the deposits at once. I've also use just plain water on my older carbureted muscle cars - to loosen soft deposits.