Take a remote 6 gallon fuel tank put fresh gas in it 93 Octane preferably. Add 1 pint of a quality two stroke oil to the fuel, Optimax or Yamalube etc. Then add the appropriate amount of Stabil fuel treatment to the mix. Hook up this mixture after having given it a good shake and run it through at or around 1300-1500rpm for approximately 5 minutes.

Do not run the fuel system dry. Shut off motor and spray any exposed components with a good fooging oil. Disconnect battery ground terminal and isolate it. Put boat in garage.

If you don't fog the engine, don't you risk rust formation on the cylinder walls and valve seats?

Here's Mercury's Service Bulletin 2001-15 describing the procedure.

New Gasoline EFI/MPI Engine Fogging Procedure

Models:
All MerCruiser gasoline EFI and MPI engines produced in Stillwater.
NOTE: This change does not apply to carbureted engines produced in Stillwater or Hi-Performance
engines produced by Mercury Racing.

Change:
Follow this new fogging procedure when laying an engine up for storage.

Fogging Mixture:
In a 6 US gal (23 L) remote outboard fuel tank, mix:
5 US gal (19 L) regular unleaded 87 octane (90 RON) gasoline.
64 fl oz. (1.89 L) Mercury or Quicksilver Premium Plus 2-Cycle TC-W3 Outboard oil.
5 fl oz. (150 mL) Mercury or Quicksilver Fuel System Treatment and Stabilizer, OR
1 fl oz. (30 mL) Mercury or Quicksilver Fuel System Treatment and Stabilizer
Concentrate.

New Fogging Procedure
1. Disconnect and plug boat’s fuel line from Water Separating Fuel Filter inlet.
2. Connect remote outboard fuel tank (with the fogging mixture) to the inlet of the Water
Separating Fuel Filter.
IMPORTANT: If the boat is out of the water, follow instructions for running engine on
flush device as found in service manual for the engine being fogged.
3. a. Cool Fuel models: Start and run engine at 1300 rpm for 5 minutes.
b. VST models: Start and run engine at 1300 rpm for 10 minutes.
4. After specified running time is complete, slowly return throttle to idle rpm and shut engine
off.
IMPORTANT: Do not run engine’s fuel system dry of this fogging mixture in the 6 US
gal (23 L) remote outboard fuel tank.

My #30 Service manual for 496cid/8.1 engines states:

Prepare fuel system for extended storage as follows.
a. Remove water separating fuel filter.
b. Pour out a small amount of fuel into a suitable container, then add approximately 2 fluid ounces of Quicksilver 2-cycle outboard oil to the water spearating fuel filter.
d. Reinstall water separating fuel filter.
e. Disconnect harness connections from fuel pump and fuel boost pumps.
f. Start and operate engine at idle speed until the water separating fuel filter and fuel injection system are empty and engine stops.
g. Remove and discard water separating fuel filter.
h. Install new water seaprating fuel filter.

I know I've read in another manual (not currently handy) that states not to fog a fuel injected engine, only carburated.

Guys ,I'm familiar with the service manuals and presently have a 525 manual open to the extended storage page. No where does it say not to fog. This is the preferred way I know. My question to you service guys out there is. Have you ever personally experienced a problem with fogging with spray fluid? Have you ever changed a sensor you know was damaged by fog fluid? I curious.
Jim

I also am very curious....I have my service manual open #6 for my efi500 and it says to fog for winter lay up..Did they change the procedure from 2003??

A new sensor is cheaper then dropping a valve seat and taking out a piston.

Just a question guy , i am a 2 stroke motocross man , and i bought the best oil for my bike (Motul 800 2T) and the only difference on the sticker of Mercruiser 2-Cycle TC-W3 Outboard oil , is the TC-W3 ( i know it stand for two-cycle water cooled engine) can i hurt my 525 with my Motul ? , because the Motul is the best of is class in two-stroke period

hope my question is clear , i just want to use the best oil for my 525

Thanks in advance

Highly doubt you will hurt the motor with any 2 stroke oil since you are not using it for lubrication purposes. I always thought fogging my 2.5 outy was a waste of time because I was already running it @ 32:1.

However...the MAP sensor issue does concern me.

We have fogged hundreds of EFI engines at the shop and never had a problem. We deal in stock Merc power only and we follow their service instructions to the T. They have spent more time and money researching this then any other person or shop possibly could have, Merc is pretty good about revisions and service bulletins. If there had been a trend they would have altered the storage procedure, they don't want to pay warranty claims on newer stuff that was damaged by THEIR OWN procedure.

Mercury Service Bulletin.

http://www.boatfix.com/merc/Bullet/01/01_15.pdf