i've always used full strength purple power, rinsed off my fenders so any purple power overspray wont eat up the paint, spray the entire bay (not covering anything) with purple power, start the vehicle up and clolse the hood let teh car heat up for about 5-10 minutes and spray the fenders again while waiting to spray off any degreaser overspray, pop the hood and let the garden hose at it. only time i had a proble was when my distributor cap had a hairline crack in it and it had a skip..replaced it, car was fine. no need to cover the alternator and all that jazz