I would use a heavy solution of Dawn Concentrate (Original Blue) and water. Use the brushes . . . if really grimy and built up with dirt mixed in, might take several applications, but Dawn will break down any and all grease/oil. If you can't get access for the brushes, then I would use Gunk or Engine Degreaser, let sit a bit and then rinse off with the hose. Do not forget and leave on as it will eat rubber!!!! Sometimes, it takes several applications if the oil has had a chance to bake on.

I have also gone to the car wash with a pocket full of quarters, and two cans of engine cleaner . . .opened the hood, sprayed the crap outta everything then put in the quarters to rinse the car. Some car washes have an "Engine Compartment Option", which gives a high dose of soap . . ..

Guess what you wind up doing is up to you, and based on what you can afford and what's available.

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1983 300D, the "Avocado"
1976 240D, 4-spd the "Pumpkin", SOLD to Pierre
1984 190D, 2.2L, 5-spd, my intro to MBZ diesels, crashed into in 2002