Hi Richard,
When you say steam do you mean steam clean or pressure wash ? Years ago in the UK when I had a DeVille I used to steam clean but that was using a professional model which had a variable pressure and gallons per minute. A proper steam cleaner will degrease quite effectively but as you know it is very tight around the engine compartment and the water goes everywhere. Also if you have Waxoyl it will MELT it. Most cleaners you get from Argos,Halfords,B&Q are fairly low pressureand not steam machines. I pressure wash the Puttputt every month with a 1500 psi cleaner but I do not go near the engine just around the wheel arches chassis etc.
Hope of some use,
The Laird