I have a 2003 Mercruiser Drive with water intakes on the nose cone and on the side of the drive. Can i run the engine with just a muff on the side intake and tape the nose cone off?

Yes you can but you ned to make sure you seal the low water pickup well or it willsuck air and take out the impeller. Merc sells a muzzle that goes over the low pickup for just this reason.

I have a Bravo X with my 496HO. When I ran the motor on the hose with ear muffs I had the nose pickups covered. I then tried it with the nose not covered. The water pressure from the hose is greater than the pressure impeller sucks in. So I found when I run it this way some water is actually shooting out the nose pick ups, vice sucking in air. I now don't even cover the nose and just use the muffs when I flush my motor. My temp has never been an issue nor has it been detrimental to my impeller. This is just my experiance with a 496HO and a BX outdrive. Hope it helps.

It all depends on what you water pressure is at your hose bib. Beware of too high of pressure to, yo can pop a headgasket that way.

How so? I may be mistaken but sea water inlet ---> sea water pump ---> heat exchanger ---> exhaust....overboard. How would the intake psi effect my headgasket? The only thing the higher garden hose psi may cause is the sea water inlet hose to collapse cutting off water supply, but like I said I have not had any flow issues or lost water supply. Now without closed cooling may be a different story.

my bad. the high water pressure thing is only an issue with raw water cooled engines.

I would think high pressure would just blow past the muffs before popping a head gasket.

It will if you just ude the u shaped muffs, if you use the ones with the pin that goes through the pickup, it can over a period of time detererate the head gasket.