When talking about rotary engines, carbon buildup is a common problem brought up. I have a writeup elsewhere in the tech section of internal engine damage pics, showing the effects of carbon buildup. A good way to combat this harmful effect is to inject water into a running engine as a matter of maintenance (note that I am not referring to water injection as a method to assist tuning and combat detonation, but rather as a maintenance procedure). By allowing an idling engine to injest a small amount of water, it hits the rotor faces and turns to steam, cleaning off carbon as it does. The more regularly you perform the maintenance, the cleaner your engine will remain. The procedure (I believe) was originally brought to light by Rob @ Pineapple racing, so please note that I’m not trying to take credit for this, only spreading the information for use by individuals.
Note that this should in no way harm an engine in good health. Water doesn’t combust, but it doesn’t harm any internals or sensors such as plugs or 02 sensors either. The engine will misfire and stumble during the time water is injected, this is normal. You will have to apply throttle to keep the engine above 3000rpm during the injection.
To perform this, I use a gallon jug of water and a long vacuum hose, say 3 feet. Though this is different for nearly every year and model, the underlying goal is to find 1 or 2 vacuum nipples on the intake manifolds after throttle body, preferably on the lower intake manifold so that the water can run straight down into the block. You want to feed both front and rear rotors evenly…generally you have 4 intake runners, 2 for the front, 2 for the back. Some engines have one nipple that can feed both (s4 turbos, for example, have one above the BAC valve that is evenly split between F and R primary runners). Some engines (fd’s and s5 na’s for example) have 2 separate nipples that can be teed together externally (vacuum hose and tee) to evenly feed both rotors.
With the engine running, remove the vacuum cap or lines that used to cover the nipple you’re using. You might have to apply throttle, so the engine doesn’t die due to a vacuum leak. Insert your hose(s) onto the nipples, and bend the hose somewhere in the middle by hand so that it seals off the air intake. Now, dip the end of the hose into the water at the bottom of the jug. Either grab the throttle linkage and rev the engine up, or have an assistant hold the throttle for you, above 3500 or so. Release your crimp on the hose, letting the engine vacuum suck water in. The engine will begin to shake and misfire, apply throttle as necessary to hold 3-4krpm and keep the engine from dying.
Expect a lot of steam from the exhaust. This is normal. I let the engine drink the whole gallon at once. When it’s done, it’ll begin to clear up slowly, and you can gradually let off the throttle and replace the original vacuum hoses/caps.