I don't have an aluminum head on my Merc, but I do have other cars with aluminum heads, and have never had trouble pressure washing the engine compartment.

Just steer clear of electrical connections and the intake system. Water in those can be, well, bad. I also never wash down a hot engine. Mildly warm, by all means. If the motor is cold, I fire it up and let it run for a couple of minutes, then give it another couple of minutes off to let the exhaust manifold/s cool down. Once upon a time a cast manifold on my Chevy cracked as soon as i hit it with cold water from the pressure washer. I'm sure it was going to crack anyway, but I'm reasonably confident I sped up the process when I shot cold water on it while it was still blistering hot.

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- K.C.Adams

'77 300D Euro Delivery
OM617 turbo / 4-speed swap
404 Milanbraun Metallic / 134 Dattel MB-Tex

Current status:
* Undergoing body work

My '77 300D progress thread