I should really write this up for CAFCNA Magazine but I've been cleaning engines this way for over ten years and it works 100%.
1) Drive your Avalanche around until your engine temperature is running at normal, around 190 degrees. This works best if you do this during warm weather.
2) Go to a "do it yourself" carwash. You'll need quarters for two squirts.
3) Leave the engine running and open the hood. Using the pressure hose lightly mist (not full pressure) the entire engine bay while it is running. This should only take about a minute. Then go to the wheel cleaner if that is an option and spray the engine bay with the wheel cleaner. Be careful not to get the cleaner on the body panels. Than spray the inside part of the hood with the soap. If you still have time mist the engine again so that the components are wet. Make sure to walk around and get at different angles, lots of nooks and crannies in the Avalanche engine bay.
4) Take a bottle of Simply Green and spray down the engine bay. Spray everything, including the hood liner. After that close the hood - leave the engine running.
5) Sit for about five minutes. The engine should have steam coming off of it and you should smell the Simply Green - don't sit so long that the combination dries out.
6) Go ahead and load up on quarters again. Put the setting on rinse water. Spray down the engine bay at pressure. Stand back so you don't damage components, don't blast the alternator to see how much it can take and watch the air intake. After a quick blast of the engine bay do the inside of the hood first (because the rinse will drip down into the engine) and then do the rest of the engine bay. Again be sure to get any nook and cranny you can find, walk around at different angles.
7) Take a towel and dry down the edges of the hood, the fan shroud, the air cleaner and the top of the fuse junction under the hood. Be careful not to get the towel caught in spinning engine components.
Drive home.
9) Let the engine bay cool down, about an hour is good. It helps to leave the hood open. Take your favorite protectorant (I've found EagleOne spray works GREAT) and treat the plastic/rubber components under your hood (not the belts!). Wipe down any water spots, use a detailing mist if your anal about scratches. If you leave the hood open your hood liner shouldn't water mark. If it does it dried to fast or you left too much soap in it.
Although this sounds long it really isn't. The whole process at the car wash shouldn't last any more than 12 to 15 minutes. The final wipe down unless you really detail the engine bay will only take another 10 to 15. I'm doing my Avalanche once every six months and I'm planning on doing my wife's Thunderbird this weekend.