Once every 12 to 24 months is enough for most daily drivers. The engine bay does not get the same daily contamination as the exterior or interior, so unless you have an oil leak, drive on heavily salted winter roads, or attend track days, an annual detail keeps everything looking fresh without overdoing it.
Before-sale prep is a clear "yes, do it now" signal — a clean engine bay signals to buyers and inspectors that the car has been maintained, and it pays for itself many times over against private-sale or trade-in value. Pre-purchase inspections are also easier when fluids and component labels are visible without grime in the way.
Skip it if you have an active oil or coolant leak — clean the leak first, then detail. Otherwise the contamination just comes back the same week and the new dressing makes the leak harder to spot until it gets worse.
