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Thread: Mercury Optimax Thermostat Replacement How To

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    Administrator FisherMan's Avatar
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    Mercury Optimax Thermostat Replacement How To

    Mercury Optimax Thermostat Replacement How To

    I am doing this on a 1999 Mercury Optimax 225. This how-to should cover from 200-250HP motors from 99 on up.
    The reason for the swap is to upgrade it to the 130 degree thermostats recommended by Mercury from the stock 120 degree set. Check your local dealer for the exact part numbers required for your engine.

    Items you will need before you start:
    2-Quicksilver 130 degree thermostats
    2-Quicksilver Thermostat Covers (Housing design changed from the 120 degree stats to fit the 130 degree stats)
    2-Quicksilver Thermostat Housing Gaskets.
    5/16 Socket with a 3 extension
    5/32 Allen Wrench
    1-Gasket Scraper or Tapered Putty Knife
    2-Crescent Wrenches (prefer these over vice grips which I used one)
    RTV Gasket Sealant (Prefer clear, looks more professional)
    Red or Blue Permatex Thread Locker
    2-6 Zip Ties or 2-1 hose clamps

    P1010868.jpg
    This is facing the rear of the motor with the Port on your left and Starboard on your right.
    P1010870.jpg
    I removed the Fuel Pressure Nozzle Caps before working to prevent cracking them when working in the area.
    P1010871.jpg
    Starting with the Port side (Your Left) remove the 2 bolts using the 5/16" ratchet. Pull away the cover as shown.
    P1010872.jpg
    The large hose attached to the fitting threaded into the housing will be held by either a zip tie or a crimp clamp. Mine had zip ties. Cut the zip tie or crimp clamp and pull off the hose. You may need a small flat screw driver for assistance.
    P1010873.jpg
    Using 2 Crescent Wrench's (I only had one) Remove the threaded fitting from the cover.
    P1010874.jpg
    The cover on the left is the new cover for the new thermostats and the one on the right is the one you will have removed.
    P1010875.jpg
    Moving to the right side remove the Fuel Pressure Nozzle Cap.
    P1010876.jpg
    On this side you need to remove the air compressor hose. Use the 5/32" Allen Wrench and remove both screws. They are small so be sure to grab them when they are close to being loose. Remove the metal hold down tab.
    P1010879.jpg
    Gently pull on the air hose and fitting and it will slide right out of the Fuel Rail. Place off to the side or drape it over the top of the above hoses.
    P1010882.jpgP1010883.jpg
    Remove the threaded fitting on this side out of the cover. Install them onto the new covers. Be sure to pay attention as to which way the nozzle of the fitting was pointing. Add some thread locker to the fitting thread before inserting it into the housing.
    P1010884.jpgP1010885.jpg
    Use the gasket scraper and remove all remaining gasket from the surface
    P1010886.jpgP1010887.jpg
    Both surfaces should be gasket clean and smooth. There may be some discoloration left from the old gaskets. Do not scrape too hard and leave grooves on the surface.
    P1010888.jpg
    Insert the new Thermostats.
    P1010891.jpg
    Take the RTV and place a small path of RTV around the thermostat. Place the gasket onto the surface. The RVT not only holds the gasket in place but gives you a little added sealant.
    P1010892.jpg
    Hook the the large hose up and either zip tie it, or use a hose clamp. If you use a hose clamp be sure to put it over the hose before placing it on the fitting.
    P1010889.jpgP1010894.jpg
    Place a small ring of RVT onto the cover face. Place it onto the mount. Add a drop of thread locker to the bolts and insert and tighten snug.
    P1010897.jpg
    Reinsert the Compressor hose back into the fuel rail. It should sit flush.
    P1010900.jpg
    Re install the hold down and the 2 small bolts.
    P1010901.jpg
    Repeat the steps for installing the Port side cover. Installation is now complete.
    Life is too short, Go fishing!

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    Administrator Heavy Duty's Avatar
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    Great How to Fish!!!!!

    5 stars brother!
    Questions, comments or concerns just let me know just CLICK HERE
    Please visit my blog page at http://www.usaoutdoors.org/blogs/heavy-duty/

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    USAO Staff bullet20dc's Avatar
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    ya did good excellent read i see ya fixed it so I deleted the rtv thang
    Last edited by bullet20dc; 04-15-2012 at 05:13 PM.


    Charlie B
    96 Bullet 20DC w/250XS
    Politically Correct Impaired

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    Administrator FisherMan's Avatar
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    Yea yea....
    Life is too short, Go fishing!

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    Thanks for the detailed info. I was wondering if anyone here knows when Mercury changed over to the 130 Thermostat including housing system? I have a 2007 Gen two Opti and was wondering if the 130's would have already been put in.

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    Administrator FisherMan's Avatar
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    I am not 100% sure, but I believe this was on the early optimax's and yours should have the 130 degree stats already. I think they changed over in 2002 ish but do not quote me on that. If I had your serial number on your motor I could check for ya. PM me with it if you would like me to check.

    And welcome!
    Life is too short, Go fishing!

  7. #7
    USAO Staff bullet20dc's Avatar
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    I believe when they went to the higher rev limiter and low octane gas in 2005 the tstat went to 130 mine has 120's in it its a 2004


    Charlie B
    96 Bullet 20DC w/250XS
    Politically Correct Impaired

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    Administrator FisherMan's Avatar
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    Ur not running 130"s Charlie?
    Life is too short, Go fishing!

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    Thanks FisherMan, also checked with dealer and yes it seems Im covered. i\In 2006 the motors were shipping them with the 130's the dealer told me some 05's as well but the major changes to the Optimax were in 2006 when Mercury overhauled the engine to the second generation Optimax. Thanks now I know which ones I have, but I still have a bad thermo somewhere that is throwing the motors out of whack I now have to identify the correct one. There is always something.

    Martyn

  10. #10
    USAO Staff bullet20dc's Avatar
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    Quote Originally Posted by FisherMan View Post
    Ur not running 130"s Charlie?
    nope didn't see a need to change over. maybe some day Ill actually get to working on my boat.


    Charlie B
    96 Bullet 20DC w/250XS
    Politically Correct Impaired

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