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 ¼” hose clamps
This is facing the rear of the motor with the Port on your left and Starboard on your right.
I removed the Fuel Pressure Nozzle Caps before working to prevent cracking them when working in the area.
Starting with the Port side (Your Left) remove the 2 bolts using the 5/16" ratchet. Pull away the cover as shown.
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.
Using 2 Crescent Wrench's (I only had one) Remove the threaded fitting from the cover.
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.
Moving to the right side remove the Fuel Pressure Nozzle Cap.
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.
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.
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.
Use the gasket scraper and remove all remaining gasket from the surface
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.
Insert the new Thermostats.
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.
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.
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.
Reinsert the Compressor hose back into the fuel rail. It should sit flush.
Re install the hold down and the 2 small bolts.
Repeat the steps for installing the Port side cover. Installation is now complete.