theres a few things to be cautious on, mainly is the ignition electrical and the engine block sensors,fuse boxes,alternator, injector connections(if its not covered by something). there are 2 vehicles ive heard of issues with not using any covering(witnessed this due to coworkers lazyness to cover up electronics) those 2 are the new body stile f150's with coil over plug ignition(or any coil over plug ignition) you want to cover up. and a jaguar cam sensor dont like water much. but mainly i cover up all electronics possible using plastic bags, electrical tape, or plastic wrap, i do not like to use foil just incase some wire is some how exposed and could short something out, but thats just an extra percaution. i mainly use steam for most engines but if its horribly nasty and greasy i will use water, when using steam i dont go as far as i do when using straight water into the engine bay as steam quickly evaporates..main thing is electronics and not using strong pressure near them, i only use strong pressure on firewall area or radiator supports or stuff like that IF NEEDED most of the time i will not use high pressure i use little as possible just enough to rinse off the APC/degreaser. of course with that you really need a good bit of brushes dedicated for engines, my main brushes are flagged wheel/tire brushes, small paint brushes, and baby bottle brushes.like most have said just be very carefull with electronics especially the newer cars with coil over plug ignition, if you dont know what that is, here is a sample image.
(the coil over plug ignition is on the valve cover. shown in the image is a 1mz toyota/lexus engine) there is other models like a chevy LT1 where the normal ignition coils are below the exhaust so always look out for hidden ignition parts....
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