If you are going to use a degreaser, and it happens to be alkali based, such as Alkasteam, do not use this stuff anywhere near unprotected alloy finishes; it will permanently ruin the finish. Use a solvent based degreaser, if you must, provided it does not affect very old rubber seals or bushings.
I use a 3000 psi water blaster in wheel arches and underneath my car, occasionally with a degreaser, but I have never used a water blaster in the engine bay from the top. 3000 psi is the sort of savagery that is capable uf upsetting old water seals that are supposed to protect the electrics. Better to use a mild degreaser and well directed gentle hosing.
It is very common for me when detailing new cars to use a degreaser and a water blaster on to the top of an engine, but strictly in new cars only.