Something to read, has some interesting points in the Read, borrowed from another Forum (not my own work)
While having a sparkling clean engine is
a) appealing to the eyes
b) adds immediate resale value to your vehicle
c) makes it easier to spot leaks while promoting a cooler-running engine
d) You don't get dirty while playing under the hood on a Clean engine
These added benefits do not come without a certain degree of risk. Now I know what you might say, "yes, but my car is new and is sealed perfectly" - and well, you're quite right! I must note however, that there is always a risk of something happening whether it is a matter of soaking the wrong wires or perhaps a wire with a cut in it or worse, some type of manufacture defect on the seal of a plug that missed QC, or getting water in a spark plug boot and having a misfire condition upon start-up. The proper way to go about it is either go to a professional if you are not 100% at ease with performing this task yourself, OR if you are perfectly fine with doing this yourself, by all means fill your boots!
Depending on the type of vehicle you have you may need to cover more areas than others - such as someone mentioned previously. The best course of action is to use plastic bags to cover important areas, i.e. fuse block, exposed cold air intakes, any modules/clusters of wires, spark plug boots/areas that may easily be disturbed if water enters at the improper angle, and so on. Oftentimes you can revisit these areas post-engine shampoo and just clean them by hand with your favorite all-purpose cleaner, a tooth or small (engine only) detail brush, and an older rag that will presumably be garbage material once you finish.
As far as products are concerned, yes there are a plethora of products that will work just fine. My personal preference is Meguiar's all-purpose cleaner because we generally dilute it to the strength we need given the task at hand. I usually use it at 25% strength instead of full when doing engines and it seems to do the trick. The only other cleaner I would recommend is AC Delco Engine Shampoo. This stuff is simply FANTASTIC! It melts away the oil/grease on contact with little rubbing. The only difference between the two is that the all-purpose is biodegradable and non-flammable, and the AC Delco (and A LOT of the engine cleaners on the market) ARE flammable and non-biodegradable. If you go the AC Delco route, just make sure rinse the cleaner off fairly well (while not SOAKING the engine) as you would not want to have a fair amount of residue left over that could become a hazard with a hot-running engine, etc. Simple green is a good product, however many people have noted that it does not fare well with metal parts, i.e. it will cause a bit of tarnishing and the like. As far as I know Spray Nine should be able to handle the task in the same way that the Meguiar's All-Purpose does but I cannot confirm this at this time as I have never used that particular product on engines.
When actually going to clean the engine make sure it is cold, but let it run for two minutes or so just so that is is warm enough to allow the product to be more effective. Next, cover the areas that you may think might cause an issue if they get wet so you do not have any surprises come time to start the engine. From there, I would recommend rinsing your windshield, front fenders, bumper, and wheels - then soap them down with a dedicated car wash soap as you normally would when washing the car. The reason I suggest this is simply due to the fact that you would not want the all-purpose or engine cleaner to come in direct contact with the paint as it may spot it, or quite possibly strip the wax. By adding the layer of soapy solution to the painted areas, there is less of a risk of encountering such a situation. I will say this though, make sure you do all of this in a fairly cool area, and it is always best to wax the vehicle as you normally would post-engine cleaning, just to keep the paint completely protected. Next, spray your engine cleaner on the engine, on the hood, hood liner, firewall, etc. Let it sit for a few moments, and then rinse with the pressure washer, keeping in mind to spray with the pressure washer a fair distance away from the engine. For example, do not point the nozzle an inch away from a spark plug boot or painted surface, electrical, etc. I generally prefer to keep the nozzle about a foot away from sensitive components just in case. From there, I would strongly suggest using the air from an air compressor to dry everything up - it makes everything SOOOOOO much easier and certainly faster. Next, uncover your previously covered areas and clean those by hand with some cleaner and a dedicated rag/tool to remove the grease and oil, etc. For the water that still remains again, use a fairly older towel/rag to absorb the remaining water and what not. For a final touch, I usually use Meguiar's #40 vinyl/rubber cleaner and conditioner on the plastic components by spraying it on the component or the cloth I use (specifically for the engine area only) to help ensure the plastics, hoses, etc are protected. I would NOT recommend just spraying the product wildly over the engine as silicone-based products do not work well if they come in contact with belts and what not. From there, you might want to start'er up and see if everything still works...
... and if it does great! - finish the car off. If not, well - you'll be in a tough spot.
Afterwards, finish things off by rinsing the vehicle down, giving it a thorough wash/wax, etc and pat yourself on the back for a job well done!
I will make this comment however, as I think it should be mentioned. I'm not a fan of engine shampoos nor do I perform this service on ANY of my clients' vehicles. Its not that I do not want to offer them the service, but rather if I were to clean an engine and if something were to go wrong.. well it would certainly not create a positive experience in the clients' mind and potentially cost future business, referrals, etc. I do however clean the engines on my OWN vehicles, but it is a bit different as I know how they have been maintained, what trouble areas to cover, etc so that there are no issues. I am also not responsible if you decide to follow these directions and something goes wrong after cleaning the engine. As I had said, if you are in doubt have your dealership clean the engine and if something happens when they do it, well - thats their situation to handle.
Hope this helps shed a bit of light on the topic for everyone!!