Steam cleaning your combustion chambers is an effective way to eliminate pinging and spark knock due to carbon build up in vehicles which do not respond to other methods of repair. This is actually an old, proven "fix" and while it is very simple in both concept and implementation there is a real danger that you could . . .
***** DAMAGE YOUR ENGINE *****
The possibility of doing serious damage to your engine comes from the fact that you will be introducing water into the combustion chambers of your running engine. In small amounts it immediately turns to steam, but if too much is introduced too quickly, the combustion chamber could become filled with water and "hydrolocked". I suppose you could compare it to jamming a stick into the spokes of a spinning wheel. Everything comes to an immediate halt with much bending and breaking of metal parts and associated wailing and gnashing of teeth by the vehicle owner.
Don't let that scare you off just yet. If you have the wherewithal to do any other basic vehicle maintenance, I believe you can safely accomplish this procedure as well. It simply takes a bit of common sense and patience. Bottom line however: Proceed at your own risk!
We can virtually eliminate the possibility of hydrolock by keeping the engine running at a fast speed and only letting small "sips" or water be drawn into the intake tract.
First you need to find an appropriate place to introduce the water into the intake tract. On an older carbureted vehicle you would simply dribble it into the top of the carb. On a newer fuel injected vehicle it must be introduced on the engine side of the throttle plate.
On my '99 Ford Windstar van with the 3.8 V-6, the line from the PCV valve turned out to be the perfect spot. The PCV valve line connects through a nice rubber boot that perfectly accepts a piece of 3/16"I.D. X 7/16"O.D. clear plastic tubing. The clear tubing is a good idea since you can visually monitor the water flow though it.
The other end of the tubing goes to your water source, like a soon to be recycled empty milk jug (well rinsed). I filled it nearly full and used almost all of the gallon of water. Don't drop the end in the water just yet! The engine needs to be running and running fairly fast - say around 2500 RPM. Either someone can be in the car with their foot on the throttle and monitoring the engine speed, or . . .
since I discovered I had one hand with nothing better to do, I reached over and held the throttle open with it and kept the engine up to speed "by ear". Keep in mind that those electric radiator fans can come on at any time and probably will while you are working in there. Keep yourself and your clothing clear of them at all times!
Dip the end of the tube into the water for just a second then lift it back out. You should see some water pass through the tube (you did use clear tubing didn't you?) and the engine will slow a bit and run rough for a moment. Congratulations, you've just given the combustion chambers their first cleansing shot of steam. What you want to do is get a feel for how much water you can draw in without slowing the engine too much. Increasing the RPM helps but don't get carried away with either too much water or too much RPMs. Be patient and listen to the engine while it is getting its internal steam cleaning. Being in a hurry or accidentally letting the engine speed drop to an idle at the same time it takes a big gulp of water could be disastrous.
Doing this procedure has eliminated nearly all of the engine pinging I was experiencing and seemed to just smooth out the engine overall. I plan on making this a bi-annual maintenance step.