Steam cleaning under the hood can significantly improve the appearance of the vehicle and may have other benefits as well. If your engine has an oil leak, steam cleaning may help with the diagnosis by removing the old oil and stuck-on debris, allowing a fresh look at the area of the leak. If you're not careful, you run the risk of engine damage.
Electronic Control Systems
Modern automobile electronic engine controls, such as computers and sensors, are vulnerable to high-pressure steam and hot water. When cleaning an engine, you risk forcing water into these components, causing damage. Often, this can only be repaired by replacing the component, and that is probably not covered by your warranty or extended service contract. Electrical connectors may have water forced into them by steam cleaning, causing connector damage or corrosion. Electrical connector problems may be difficult to diagnose and repair.
The ignition system, consisting of the ignition coils and cables and the spark plugs, may be sensitive to water. This is particularly true in older vehicles, which often have longer ignition cables and a distributor cap, which is notorious for letting water in during an engine cleaning. If the ignition cables are affected by the cleaning, they are probably damaged or worn out, and due for replacement.
The air-intake system is generally resistant to steam cleaning or high-pressure washing. It is designed to deliver air to the engine for operation in all types of weather conditions, including rain. Any cracked or broken air ductwork under the hood can potentially let water in the engine, and the ductwork can get disconnected when impacted by high-pressure steam or water. The mass air flow sensor usually located within the air intake ductwork is also vulnerable to damage from steam cleaning.
If you must steam clean your engine, take proper precautions. Do not direct high-pressure steam or water directly toward any engine control modules or other electrical connections. Make sure that your air-intake ductwork is installed correctly and keep steam away from its inlet. Keep water and steam away from any under hood fuse panels or electrical systems. Consider covering sensitive areas with plastic, such as a zip-top bag, to give extra protection from splashing. Consider having your engine cleaned by a professional, who has experience and may be better equipped to prevent damage.