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The thermostat is mounted under the throttle body behind the water outlet to the upper radiator hose. Find your upper hose on the drivers side front and follow it down below the throttle body to the end of the hose. That is the thermostat housing.It is very helpful to remove the throttle body first, then removing the upper water outlet bolt completely, only remove the lower bolt 85% of the way. The bottom hole is slotted. Remove the water outlet and the old tstat and sealing gasket. Install the new tstat which should come with a new seal. Slip the outlet down onto the still installed lower bolt, and then install the upper bolt Tighten the lower bolt. Reinstall the throttle body. There is a special fill procedure to follow for this engines cooling system. Open the bleeder screw on the water outlet by the thermostat, and above the water pump. Fill the cooling system with a 50/50 mix of Dexcool and water. Add the mix until only the coolant and water mix comes out of the bleeders and not air. Close the bleeders and then start the engine.

Answer from a 1999 Buick Century owner: Having just done this to my 99, it's complicated. 1st, make sure you have your new thermostat, coolant, and a throttle body gasket. Yes you have to remove the throttle body, and replace the gasket. Also, a wide, short standard screwdriver, socket set with 8mm, 10mm, 12mm, 14mm, and 1/2 sockets, and a 1/2" crescent wrench with the round end. I'll explain later. Next thing you need to do is unhook your negative battery cable, this will ensure the fan wont kick on while your fingers are in it, and jack up the car a few inches for a drain pan. Then you need to drain the radiator. To do that, open the radiator cap near the battery and grab a wide standard screwdriver, I used a 4" stubby. The drain plug is at the bottom corner of the radiator on the drivers side. You can see it from the top. I removed the chin spoiler to make it easier to access the plug. Insert the screwdriver and turn the plug 1/4 counterclockwise, and pull/pry off. Should come off easily. While that spills everywhere, head up top and begin by removing the sensors. There are 5 of them, they are color coded, write it down. They kinda fall back into place easily, and remove the intake hose from the filter box and the throttle body. Easy stuff is done, now you have to remove the throttle body. There are 2 bolts to remove, the 1st in top front, under the black bar holding the throttle cables. The 2nd is lower rear and has 2 nuts, the 1st nut unhooks the coolant line (The black pipe with the 2 rubber hoses leading into the t-body.) The 2nd is a pain to get too. You need a deep well socket and a small prybar. Use some ingenuity with the prybar and you'll get it off. Then, remove the 2 hoses from the coolant line. Not easy. The t-body "should" come right off. With that out of the way you can now see the therm housing. It also has 2 bolts, similar locations. The top is easy to remove, the bottom isn't. It;s wedged between the block and the exhaust with minimal clearance. The only way I could get it off was to use a 1/2" crescent wrench, with the round end, and use the round end to ever so slowly, loosen the nut. You're stuck under the fuel lines, and other junk so be patient. When you get that off, the therm comes right out, replace and work backwards. Good luck, it took me over 4hrs.

Above instructions are right on and saved me a ton of time and work....thanks to the poster. The only added comment is on the lower T-stat bolt, I was able to use a short (4") 1/2" box end wrench to remove the bolt. You have to be patient and be willing to contort your hands to reach the bolts, but it is rewarding to complete and save some labor charge since the T-stat itself is about $7. Good luck.

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Q: How do you replace a thermostat on 1999 Buick century?
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