I washed my Lincoln under the hood, and the spark plugs got messed up. Also, it damaged the voltage regulator. I was upset. I didn't use a dog gone fire hose. I only used a garden hose. Lincoln/Ford. I don't know!
Washing under the hood of a car would be like taking the lens off a digital SLR camera and using a hose to wash the dust off the digital sensor. At least it would not mess up the spark plugs...
What is wrong with people? Washing under the hood with a garden hose and then blaming the mfgr. Hey: why not "wash the electrical outlets" in their home and see what happens. No wonder people have troubles. They bring it on themselves.
To original poster, you should have had the car running when you washed it under the hood. Unlike what a lot of posters say, you can wash under your hood with no harm being done if it is done correctly. You cannot use a garden hose; tap water is full of corrosion minerals and is bad for a car period. You need filtered water where all minerals are removed. And auto shops carry this foam that you spray over the engine bay area that reduces corrosion and water damage to electronics's and wires, plus it helps clean the dirt and grim.
That's why you check the spark plug wire and make sure it's seated properly before washing.
Cars, of any variety, are NOT designed to be washed under the hood. Lots of electrical components under there. You risk damaging electrical components. Think taking a garden hose to your home electronics. Not a good idea. If you spray cold water on a HOT engine, you risk even more damage, but this time to the engine and connected components. Cold water on a hot engine can cause cracking of the exhaust manifold and engine block.
Before I knew better, I used to spray under the hood with the hose to clean the engine bay. I did this to a Ford Taurus, which caused the car to not start for several hours until the water dried. I also sprayed water under the hood of a Buick Park Avenue, causing the dash instrumentation to short out.
If you insist on cleaning under the hood, do it by hand with proper cleaning products.
OK, OK! Wow, yes everybody is right, you shouldn't wash under the hood on a Ford with a 4.6/5.5/6.8L engine. Why? Because the spark plugs/coils are next to the intake manifold on top of the engine. I had a Crown Vic and learned the hard way myself. I had to spend an hour removing all the ignition coils, and then blowing out the spark plug wells with compressed air before the engine would run without missing.
I also had a Camaro, which I regularly washed the engine and never had a problem. The spark plugs were under the exhaust manifold on the bottom of the cylinder heads, and weren't as susceptible to moisture.
The above Ford design is not flawed, as neither of my cars were really designed for the top of the engine getting wet; it's just one liked getting wet more than the other.
I don't think any car built 90 or newer should be washed in the engine compartment. Most cars by then had fuse boxes under the hood, and luxury makes put ECMs, air bag controllers, and ABS controllers etc.. under the hood.
"If you insist on cleaning under the hood, do it by hand with proper cleaning products."
I show my Mustang at car shows, so a clean engine compartment is a must. I always clean everything by hand using a rag and proper cleaning agents. I never spray water under the hood. Yes, it's more effort to hand clean everything, but it is far safer and does a better job. The best bet is to clean often so there is less dirt to remove.