How do you change a thermostat in a 1991 Chevrolet Lumina and where is it?
Locate your two large coolant hoses. One connects to the lower part of the radiator and runs to your water pump. The large hose that connects at the top of the radiator is the map leading to the treasure. Trace the upper hose until it connects to the engine. The hose should be connected to a dome shaped metal housing. There should be two or three bolts that fasten the dome housing to the engine. Remove those bolts, but expect to have some coolant spill out(good idea to have a catch pan under vehicle). The thermostat is located in the dome housing. Take note as to which way the thermostat is facing so as to not install the new one in backwards. clean the old gasket and silicone from the two mating surfaces(a razor blade and gasket remover spray work well). I'll empasize that you must clean the surfaces thuroughly or you will have a leak when you are done. After cleaning place the new thermostat in the engine opening the same way you found the old one. Place gasket making silicon on the new paper gasket(gasket normally supplied w/ new thermostat). Don't use an excessive amount of silicone, but make sure both sides of the paper gasket surface are well covered. Place the dome shaped housing back on the engine opening making sure the thermostat stays in place. Also make sure that the gasket with silicon on it is between the engine opening and dome housing. Line up the bolt holes and start threading them by hand. Tighten the bolts with a wrench, alternating back and forth until they are snug. You want the bolts to be tight enough to make a good seal and not viberate out. However, do not overtighten them to the point of breaking something. Have fun! Chris
**Another point of view.
Typical of automotive engineers, they designed a 3/4 inch tube to block the removal of the flange to the thermostat. Instead of using a simple bolt and spacer, they have a one piece bolt and stud holding the upper radiator hose connection and a brace for the 3/4 inch tube. The tube can only be properly removed if the exhaust manifold is removed, as the exhaust manifold was installed over the attachment point of the tube. Not real bright on the engineer's part. The easy way to replace the thermostat is to remove the nut holding the 3/4" tube from the flange stud. Next remove the bolt/stud holding the bottom of the thermostat flange. You may have to grind a cheap 13mm open end wrench thinner to fit the tight space. I used a cheap stubby 13mm open end ground thin. In the process of removing that bolt/stud, the small attachment bracket for the 3/4" tube will be bent. No problem...just bend it straight later. Remove the other bolt holding the flange covering the thermostat. Clean the old rubber gasket from the counter bored thermostat seat.
You don't need silicone to seal anything. The thermostat comes with a rubber gasket, and also a paper gasket for spacing. If you don't use the paper gasket, the flange will be bent when you tighten down on the bolts because the rubber gasket is thick. Instead of reusing the original lower bolt/stud combination, I used an 8mm-1.25 X 45mm bolt, nut, a flat washers, and a spring type split lock washer . Position the thermostat with the rubber and paper gasket, the upper radiator piping flange over the thermostat, and install the upper bolt finger tight. Straighten the previously bent attachment flange on the 3/4" tube. Put the lock washer and the flat washers onto the bolt. Now holding the 8mm nut between the thermostat flange and the flange for the 3/4" tube, screw the 8mm bolt through the tubing flange and the nut you're holding and into the flange for the thermostat. It may take two wrenches: One to hold the nut, and the other to turn the bolt. You will simply have to tighten the bold in such a manner that the nut and the bolt tighten down simultaneously to where the thermostat flange and the 3/4" tubing flange are tightened snug. It is more important that the nut holding the thermostat flange is tight than the bolt holding the 3/4" tubing flange. The 3/4" tubing only has to be secure in that it won't have any looseness. Note: Do not use a bolt any longer than the 45mm length. If you do, you'll probably crack the thermostat mount or punch through and hit coolant. Of course you can always just reuse the original bolt/stud if you don't mind wrestling with it. This explanation sounds difficult to understand, but once you get into it, it will all make sense. You can use two or three flat washers to make sure that you don't go too deep with the bolt.