Originally Posted by SouthArk370Z

In my experience, these types of products seldom do any good and often end up causing more problems. If the equipment is dirty enough that cleaning the engine would be beneficial, there is a high probability of dislodging pieces big enough to plug oil galleys and damage bearing surfaces. If it's not that dirty, then why use a cleaner? Keeping it clean is a whole lot cheaper and easier than trying to clean a dirty engine.

Then there's possible compatibility problems with sensors, plastic/rubber components, and even the metals used in modern engines.

Not sure it's true, but wstar's theory about messing with the O2 readings and hurting the engine sounds very plausible.

There may be a situation where an engine cleaner would do some good, but, for most people, they are a waste of time, effort, and money. If the engine is dirty enough to need cleaning, then it needs to be torn down. If you do the normal maintenance on you car, you don't need to clean the engine.