Steam cleaning an engine can be more than an aesthetic exercise. Oil and grease that sticks to the engine attracts dirt, which can insulate the engine block and cause overheating, and might catch fire if the exhaust or emissions system experience an overheat. A clean engine also allows for easier maintenance, helps in the detection of leaks and potential problems but caution is advised.
Rent a Cleaner
While steam cleaning is an effective way to clean an engine, don't even think about farming the job out to a local shop. Modern engines have a great number of sensors and sensitive electrical equipment that cause serious problems if the cleaning is done improperly. The best steam cleaners are powered by propane gas, which both runs the engine and provides a source of heat. Though rental fees might be a bit on the steep side, you may be able to offset costs splitting it with a few friends or neighbours and having everyone over to steam clean on the same day.
If steam cleaning has one major drawback, it's that the same high-temperature steam that removes gunk and grime will insinuate itself into your electronics and ruin them. Before one drop of water touches your engine, you are going to need to insulate all of its electrical components. One way do so is to wrap packing cellophane around your electrical connectors, sensors and wiring harnesses, and secure the cellophane tightly with duct tape. Even this isn't a sure bet; if your car uses an engine-bay mounted computer or has electrical components that cannot be insulated, then avoid steam cleaning. You still can reap most of the benefits of steam cleaning by turning your cleaner's temperature down to about 65.6 degrees C and introducing a little bit of solvent to the mix.
Once you have everything wrapped up, spray your engine, suspension and chassis down with a quality degreaser such as DP Engine Degreaser or Engine Brite. Make sure to use one that is plastic and silicone safe, or you risk melting everything that isn't metal. Allow the engine degreaser to sit for the recommended period, and avoid using it in the top of your engine, where it might collect around fuel injectors and sensor equipment.
Steam cleaning is the easiest part of the process. This process is practically identical to using a pressure washer on any other part of your car. Keep in mind that part of steam cleaning's function is that the hot hater causes grease to melt so be patient. It might take a little more direct application than you would normally use to melt and remove grease and caked-on grime.
Stay away from powerful chemicals and solvents in the pressure washer itself; commercial detergents available at your local hardware store will do a fine job. Simple Green, Kaboom! and Griots Garage Engine Cleaner are all popular options. You can use the more powerful cleaning agents on other underbody components, but take it easy on your engine. It is a good idea to dry all electrical connections and sensors with compressed air afterward.