Originally Posted by TOGWT

To state that an engine compartment and components are weatherproof because they are in semi-open environment is a naive excuse for steam or high pressure washing (usually used to get the job done quickly as opposed to safely). The electrical and other engine components are made to be water repellent (very different from waterproof) providing the water is from a low pressure source; i.e. water migration from going through a puddle of water.

Engine Compartment Steam Cleaning:
High-pressure steam cleaning can force solutions into the engine harness sheath and associated cables, causing unusual problems that would not have occurred otherwise. Steam will subject the engine’s electrical components to a higher pressure and heat conditions than they were designed or specified for. Once steam is applied to a cold metal surface it condenses and becomes water, which may migrate to places that it cannot be easily removed from.

Modern vehicle engine harness multi-plugs and related sealing measures are not designed for the discharge velocity and force of typical commercial steam cleaning units. Further, cleaning solutions with the high-pressure discharge can leave conductive and often corrosive residues deep within the harness and cables.

Most engines are rarely so encrusted with automotive residues and dirt that normal, low-pressure degreasing methods fail to provide proper, efficient cleaning. High-pressure engine water / steam cleaning are unnecessary and should be restricted to extreme cleaning jobs, such as commercial trucks, off-road vehicles, etc.

I would recommend wearing a pair of latex cloves while working on the engine compartment. Road grime, tar and grease are very difficult to remove from underneath your fingernails. Outside of the garage your hands shouldn't look like you need lessons in personal grooming

Do not use a concentrated solvent based product for engine compartment detailing as solvents have a low flash point and could start a flash fire in the engine, solvents also deteriorates and expands rubber, which may have a detrimental effect on belts and hoses.

Recommended Cleaning Methodology:
•Remove any excess debris, leaves and etc from the hood, grille or air-vent openings
•Spray electrical connectors with a water deterrent (WD40) which repels moisture (avoid spraying on belts)
•Use cling-film or Saran wrap elastic covers to cover any sensors, electrical black boxes, electrical devises that could cause an electrical short-circuits if subjected to water spray.
•This will not guarantee that you won’t cause a problem when cleaning an engine bay but it will greatly reduce the risks.
•Start and run the engine to get it warm, not hot, this will enable the de-greaser to work more efficiently on a warm surface.
•Apply an engine de-greaser (Autoglym Engine Cleaner) or an all purpose cleaner (APC) without butyl cellosolve, Omni APC 10:1 concentrate – http//:Automotive International - Valugard Product Line a light engine cleaning, or a solvent free, water based cleaner (Simple Green® Aircraft & Precision Cleaner) - Extreme Simple Green® Aircraft & Precision Cleaner - More Info by lightly spraying the lower parts of the engine first and then working upwards.
•Enable the cleaner to work by allowing dwell time (but do not allow to completely dry) or to remain on painted surfaces, and be sure to dilute with water before drying with a cloth.
•For heavily soiled areas agitate with a nylon engine cleaning brush and a diluted citrus-based cleaner (P21S® Total Auto Wash)
•When the grease and grime is dissolved and you're ready to remove it, spray with a light (garden type) pressure water hose.
•To shine aluminium engine block / parts use # 0000 synthetic bronze wool - Synthetic Steel Wool2 and a metal polish, English Custom Polishing - Metal Polish & Restorer
•Use the blower side of a vacuum (avoid area that may be sensitive to a forced air flow) or allow the engine compartment to air dry and then thoroughly wipe down all accessible surfaces and then remove cling-film or aluminium foil.
•When everything is dry apply a vinyl/rubber protectant to hoses, shields and wires, and wipe engine parts with CD2 Engine Detailer or 303 Aerospace Protectarant
•Wipe off any excess with a clean cloth and apply a wax/sealant protection to painted surfaces.

Note-
Apply a vinyl dressing to hoses etc (no diethyl silicone fluid based dressings (DS) should be applied in the engine compartment. Use only water based polydimethalsiloxane (PDS) dressings, do not directly spray on to engine parts, spray a cloth and wipe