I saw no detailed description on how to change a Thermostat on a 1999
Monte Carlo with a 3.1 engine. I finally figured out how to do it.
Here are the instructions
Pliers – to remove hose clamps
13 mm socket
13 mm open end \ box end wrench
Slotted Screw driver – to remove screw hose clamps
1) Remove Air Cleaner canister assembly
2) Remove flexi rubber hose going to the Throttle body – disconnect
the two wire cable assemblies from the flexi rubber hose, and the
black plastic breather tube that goes to the back of the
engine……..move the cable assy. And black plastic breather tube out of
3) Disconnect throttle cables from throttle body.
a. Outside cable clips onto throttle body housing, very carefully push
cable end off of post
b. Either have someone press down on the gas pedal, or twist the
throttle body spring assembly to get some slack in the cable. Pull
the cable up and out of the groove that it is in.
4) Remove the throttle cable bracket assembly from the top of the
throttle body housing. There are two 10mm bolts.
5) Remove the throttle body.
a. Disconnect the wiring cables going to the Throttle body
b. Disconnect the two hoses from the coolant bypass assy.
c. There are two bolts holding the throttle body in place.
i. The upper bold is a 13 mm head, remove it.
ii. The bottom bolt has a threaded end with a 13 mm nut attached.
This nut holds the coolant bypass assy. Remove the nut. Once removed,
VERY CAREFULLY pull the coolant bypass assembly toward the
fender……..it’s only going to move about .5 inches…..just enough for
you to get a 13mm open end wrench on the hex head in the middle of the
bolt. Remove this bolt……….you’re only going to be able to move it
about a quarter turn…….be patient. You can continue to do a quarter
turn to remove the bolt, or, once the bolt is loose, VERY CAREFULLY
attach a needle nose vise grips to the threaded end of the bolt and
turn it out the rest of the way……..BE CAREFULL, the bolt has to be
absolutely loose, or you’ll strip the threaded end.
d. Very carefully pry the Throttle Body assembly away from the
engine. Set it aside, since it’s off the engine you should clean all
the carbon off of it. Also, you should replace the Throttle Body
6) Now that the Throttle Body Assembly is removed, you’ll have better
access to the Thermostat housing
7) Remove the radiator Hose from the Thermostat housing pipe. Put a
catch basin under the car because you’re going to loose coolant.
8) The Thermostat housing is attached to the engine with two 13 mm
head bolts. The lower Thermostat Housing bolt hole is actually
slotted, for easier removal.
9) Remove the upper Thermostat Housing Bolt
10) Now the fun starts, using a Box end 13 mm wrench loosen the lower
bolt. Note that you should have a magnetic pickup tool because you’re
going to drop the wrench many times during this process.
11) It took me over an hour to finally position the wrench and get
enough leverage to loosen the bolt…………keep trying, it does finally
come off. Remove the bolt. It'll take you a while, I could only do a
quarter turn at a time.
12) Carefully remove the thermostat housing \ thermostat
13) Clean surface of thermostat housing and replace with a new one.
14) Assemble and you’re done.
This project took me 4 hours. Good luck