For the firs time just degreased the turbo diesel engine and compartment with Castrol's Super Clean Tough Task cleaner/degreaser. (spray bottle) - the label says "biodegradeable".
I even sprayed on painted surfaces (but rinsed immediately and thoroughly) and it did a great job with the original paint showing through after the first application. The heavily caked area was loosened with a brass brush and a good soaking for a minute removed the rest without problems.
Used a regular garden hose sprayer to avoid driving to the car wash and heat up the engine.
81 300SD 190,000 miles.