My first reaction to such things as grimy engines is to seal the engine up real good, toss it in the back of the truck, and head for the quarter-car-wash. If I had a pressure washer, and for the life of me* I can not understand why I do not own a pressure washer, but if I had one I'd blast the road-crap off of it myself.
* what makes this double-perplexing is that my wife has suggested to me several times that we need one; generally I don't need to be told twice to go buy a power tool of any kind.