Few of us ever look inside our engines to see how they’re faring, internally speaking. As the miles pile on, it’s common for ugly carbon deposits to build up, and they can at times get bad enough to rob the engine of performance. But how do you clean carbon deposits inside the cylinders themselves?
As it turns out, there’s an old hot rod trick for doing just that. It involves introducing water into the combustion chamber, which sounds bonkers if you’ve never tried it before.
It’s a popular shadetree hack, passed down from one wrencher to another over the years. It’s also a great way to brick your engine in seconds flat if you do it wrong. Let’s look at what’s going on here.
Water, Water, Everywhere
Let’s start with the why. The process of combustion inside an engine tends to create carbon deposits over time, either from the incomplete combustion of fuel or the burning of oil. As clean as modern cars run, most will still create some carbon deposits over time. If they build up too severely, they can impact the engine in a number of ways. They can interfere with the seating of the valves, they can reduce flow into the engine, and they can also gum up the rings such that they no longer seal properly.
To remove these deposits, the concept is simple. You want to introduce water into the combustion chambers of the engine. But how to do it? Well, carefully is the way if you don’t want to incur terminal engine damage.
Ideally, you want to introduce the water in a relatively fine mist. This can be done by spraying it into the throttle body or carburetor in the absence of an air filter. Alternatively, a vacuum hose can be pulled off to act as a point to introduce water into the intake. The water will be carried along by the air stream, with ideally some of the droplets remaining liquid until they pass the intake valves. The engine ideally run up to operating temperature before introducing the water, to give it the best chance of evaporating the injected fluid.
However, for the sake of ease, many people take a more dangerous route. You can achieve similar effects by simply pouring water from a cup into the intake. Others unhook a vacuum line and use the engine vacuum to try and suck water directly into the engine. However, go via these routes, and you risk tragedy.
See, if you pour too much water, you risk hydrolocking the engine. This is where the piston cannot reach top dead center because the cylinder has filled with too much water, which is incompressible. The water stops the upward travel piston, which is being pushed upwards with great force by the crankshaft. The usual result is a bent or snapped connecting rod and lots of collateral damage. A full engine rebuild is required in this case, if the engine is salvageable at all.
As seen in the video below from Kreosan, you can also expect a great deal of white smoke to come out of the exhaust. But that’s not smoke. It’s steam! Steam from all the water that you squirted into the engine.
Just about every major car channel has done a video on decarbonizing with water in the engine. Sadly, few actually use a borescope to verify whether their work had any effect.
The pouring method can be okay if you’re very careful about how much water you pass into the engine. You want to limit it to teaspoons at a time in most cases, as the average combustion chamber volume is on the order of a small shot glass or less. It’s okay to use a vacuum line as an injection point for water, too, as long as it is used as a port for injecting water, not a tube to suck it up wholesale from a vessel. Controlling the amount of fluid entering the engine is key.
It’s also a good idea to change your oil after this procedure. That’s because it’s likely some of the water you’ve squirted into the engine has gotten into the oil.
But what is the mechanism behind water’s cleaning action in this scenario? Filming in a running engine’s combustion chamber isn’t really possible, so it’s hard to say for sure. But the leading theory is simple. Water entering the cylinder is absorbed into the carbon deposits on the piston. In short order, the heat inside the engine causes it to quickly boil into steam. The liquid’s rapid expansion into a gas helps break up the carbon deposits, freeing them to be ejected via the exhaust.
Indeed, it’s worth noting that head gasket failures often result in a similar cleaning effect that is well known to mechanics. Such a failure often releases coolant into the cylinders, where it boils off during the combustion process. Often, upon disassembling an engine with a head gasket failure, the area of failure is obvious. This is because cylinders with coolant ingress typically appear sparkling clean and new, with minimal to no carbon buildup.
Does it really work, though? Well, hunt around on the Internet, and you’ll find results are positive but mixed. Some run the water, and borescope their cylinders, finding much of the carbon has been removed. They’ll swear by it. Others find little result and consider it unimpressive. The method definitely seems to work to some degree. The key seems to be getting the water where it needs to go. If it’s not reaching all the pistons in your engine, then it won’t be able to clean them, either.
In the case of a multi-cylinder engine, this can be more difficult than you might think. If you’re simply pouring water into a random vacuum port, you might be delivering more water to certain cylinders than others. Some might not be getting any at all. This is where an atomized spray of water into the throttle body can be more effective, as the intake after the throttle body is naturally designed to feed air to all cylinders. Still, in some cases, flow effects inside the intake and inside the cylinder might stop denser water particles from getting everywhere you need.
It’s also worth noting that if you’re spraying water in too fine a mist, most of it might be evaporating in the intake charge before the liquid water can get to the carbon deposits. Then it wouldn’t be doing a whole lot to clean off carbon at all.
It’s also worth noting that this method won’t do much for carbon buildup on the intake valves. This is a common problem with direct injection engines. Unlike port injection engines before them, they don’t have a regular squirt of fuel on the back of the valve to wash off any buildup. Thus, they can get covered in sticky carbon which causes all sorts of problems. In any case, the liquid water method doesn’t seem to do much to remove buildup on the intake valves They’re not exposed to the higher temperatures and pressures inside the combustion chamber which helps turn the liquid water to steam.
If your engine is running fine, it’s hard to imagine this being worth the bother, carbon deposits or not. If you’ve got an older car that’s maybe running a little poorly and choking itself up, you might be considering trying this. If so, it’s worth getting a borescope so you can poke it down the spark plug holes and see if it’s actually working for you. Most of all, though, you’ll want to be careful. Slip up with the water, and you can kill your engine in seconds. Maybe best not to fiddle, hmm?
Image credits: via YouTube screenshot