Dont do it!!!
i repair pressure washers, there is very few that will reach steam its just hot water about 80 deg C which is way to much water to be spraying round your engine bay.
if you do get a washer that will do steam it wont clean much dirt off because there isnt no flow of water, steams only good for sterilising.
Anyone using a hot washer to clean wheels or paint keep the thermostat below 50 deg C