I found this on the web.
Subject: [uuc] Re: awesome engine bay appearance
Ok, not everyone will agree with this, but I have been doing it for 15+ years without any negative side effects, except for the current Z3 coupe only because I have gotten lazy with age :-), but it's day is coming soon....but before you jump to conclusions hear the whole thing out:
Clean excess grease and dirt from engine bay using automotive degreaser, and a soft scrub brush where needed; the cleaner, the better. Do not bother to wash rest of car yet. Rinse off engine bay cleaner and crank up engine to dry. Buy the big economy size jug of Armor-All and a spray bottle. After engine bay is thoroughly dry, spray down entire engine bay excessively with Armor-All. The reason we didn't finish washing the car before is because if there is any breeze the Armor-All will get on the fender paint and windshield; not to fret, we will clean it all off later. You may want to put something under the car first if dripping Armor-All onto the ground is an issue. Once everything is dripping in Armor-All, allow to sit overnight to soak in; it is OK to drive car during this process, IMO the heat helps, but externally it will look messy. Next day, take a garden hose with spray nozzle and blast-rinse engine bay to remove any excess Armor-All. Fill wash bucket with preferred car soap and wash car. Empty wash bucket, then repeat wash cycle. My experience is this will remove any excessive Armor-All from areas outside engine bay. Dry car to prevent water spots in usual manner. Then take old rags and dry/wipe engine bay down as well as possible, then take for a drive to heat everything up good for final drying.
End result is the most awesome F*ing engine bay appearance you will ever see. Repeat every 4 - 6 months as required. The blast-rinse removes excessive Armorall enough to prevent dirt/dust build-up , but it's an engine bay so eventually it will get dirty if you drive daily on the street. Also, I realize there will be concern about getting Armor-All on drive belt and pulley surfaces, but again I have never seen an issue from it using the procedure as described, nor have I ever experienced any engine bay paint issues. The only negative, and a minor one, is for a couple of days the Armor-All on the hot engine/exhaust will have a unique, but not disturbing, odor, but will soon disappear.
There will be nay-sayers (OK Brett, let me have it :-), afterall this *is* the Internet, but it's easier to discount something rather than try it first... so try it and be amazed.
M Sipe