I clean the metal parts of the engine with paint thinner, and the rubber parts with WD-40. I save all the old socks in our household and weekly, when I check my tires and fluids, I use one to wipe dirt from areas in the engine compartment, wiping the battery last. I can reach into small areas with the sock on my hand. I wax all painted parts and areas under the hood with non-cleaner wax(No white deposits), even so, the solvents in the wax do clean some soil off.

With each of my cars, I removed the plastic cover on top of the engine and cleaned the area under it with kerosene and a rag or cotton swab where necesary. I pulled the plug wires from their sockets, and examined the plug wells, one of my cars had three plug wells that had what looked like a rusty deposit in them. These wells are very hard to clean. When I put the cover screws back, I put never-sieze on them.
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96 855R, 95 855,854, 90 744 Ti - 310,000 miles driven