One trick we use for serious cleaning up underhood and underneath the car is a steam cleaner. This beast cleans off the underbody, blasting crud into oblivion and leaving everything squeaky-clean. It's a must for cleaning up after a leaky oil seal or power steering hose. You'll want to be careful because the spray is very powerful and it could lift the paint if you hold the wand too close. You can rent one of these machines for around 50 bucks.