So your car has been overheating and you've been told to try changing your thermostat, but you don't know where it is. Let me say first that this advice is usually given because thermostat replacement is an inexpensive repair that you can do yourself. Few of us that give such advice can guarantee that this repair will solve your problem, but it's worth a try.
I want to give you some words of wisdom here, but for those that just want the bottom line, let's get it over with:
The thermostat is usually mounted on the engine near the front top end, and it is usually at the other end of the upper radiator hose. So follow that upper hose to the engine, and you most likely will see at the end a small housing that is attached to the engine with 2 or 3 bolts. The elusive thermostat lies within that housing. There are a few engines where this isn't true. In those cases, look around the engine for a housing that is a few inches across and has smaller hoses attached, or ask FixYa.
Removal and replacement procedure:
Tools needed are a socket set, a screwdriver, plyers, and a gasket or paint scraper.
Materials needed are a new thermostat (preferably of the operating temperature that is recommended for your car), a matching gasket (may extend beyond the area needed but must cover the mating surface), and a tube of gasket sealer.
Most professional manuals will advise you to drain the coolant from the radiator and engine. Fine, that's good, but I don't do that. If you want to, there's usually a drainport we call the petcock and the base of the radiator. Some petcocks are reverse threaded, so be careful not to break it off if you turn it the wrong direction. Drain the coolant into a pan that can hold at least 2 or 3 gallons. If the coolant is relatively new, it can be reused. Keep the coolant away from pets, as it tastes good but is poisonous.
If you have not drained the coolant from the car, place a drainpan under the the car below the thermostat.
Now remove the radiator hose from the thermostat housing. It may have a screw clamp or a spring clamp. Loosen the clamp and slide it down the hose a few inches. Loosen the hose from the housing by twisting and then pull it off the housing. Coolant may drain into the drainpan. Some engines may also have a second smaller hose attached to the housing, and there may be a sensor on it as well. Disconnect any additional hoses and connectors, but do not remove any sensor, as this is not necessary.
There may be other accessories blocking your access to the bolts that hold the housing to the engine. You will have to remove any parts that are blocking your access, but I cannot cover all of the possibilities here.
Assuming you now can access the bolts, remove the bolts holding the housing to the engine. The housing may now come loose or you may have to pry it a bit. If you pry, be careful not to damage the mating surfaces.
Remove the housing and thermostat and clean the old gasket material from the mating surfaces on the housing and the engine using a scraper.
Insert the new thermostat into the engine with the copper end first and the pointy end sticking out. Some cars also require the little hole in the disk to be on top.
Coat both mating surfaces with gasket sealer and stick the gasket on one or the other.
Grab a bolt and mate the housing to the engine, screwing the bolt in to hold it. Screw in the other bolts by hand, making sure they pass thru the holes in the gasket.
Tighten the bolts. The torque may be different for every engine, and most of you will not have a torque wrench anyway, so just get them in tight but not so tight as to snap them or strip the threads. If you use a 1/4 inch socket set and turn it hard, you should have it about right.
Reattach the radiator hose and any other hoses or connectors you took off.
Replace any other parts that you had to remove to gain access to the thermostat housing.
Refill the radiator with the recommended coolant for your car. Use the cap on the radiator if it has one.
Start the car and check for leaks around the housing and hoses.
Allow the car to idle up to operating temperture and keep the radiator full as air bubbles out of the system. If the system has bleeder ports (common on Hondas and Toyotas) along the coolant hoses or on the thermostat housing, open the ports and allow air out until only coolant bleeds out. Make sure the engine is at operating tempertaure before you declare it to be fully bled of air, then fill the coolant reservoir and cap the radiator.
Don't pat yourself on the back yet. Check the operating temperature of the old thermostat and compare it to the new one. If it is different, expect your gage to settle on a new point compared to where it "lived" before your overheating troubles started. You may also want to boil the old thermostat for curiosity purposes to see if it was really bad or if it opens as designed. If it does, your woes are not likely over. If it doesn't open in boiling water, you may be in the clear. Either way, I recommend a few trips around the neighborhood to see how the gage behaves. If it behaves well, grab a cold one and call it a night.
If it is still overheating, have a seat and finish reading my tip.
So now you have to consider some other possible reasons your car is overheating. Here are the ones I've seen over the years:
1. Leaks
2. Bad fan or fan circuit
3. Bad water pump
4. Clogged radiator
5. Blown head gasket or cracked head (or block)
Let's troubleshoot these one at a time.
1. If your car has been losing coolant, you know you have a leak. If it isn't leaking out the bottom, it may be going into the oil pan or into the combustion chambers, but let's leave that horrible possibility for last. Some common leaks are the water pump weep hole (small hole at pump shaft bushing that leaking when bushing wears out), radiator hoses, other coolant hoses, radiator, heater core, and my personal favorite--rotten freeze plugs. If you have any of these leaks, you car will habitually be running on less coolant than it is designed for, and the coolant will boil at a lower temperature because of reduced system pressure. I'm not going to cover how to repair all of these but rather ask you to get back to us if you need help with them.
2. If your electric fan(s) is not coming on when the engine is hot, first check the fuse, usually in the fusebox under the hood. Then, with the engine hot and running, pull out the fan relay (usually in the same box as the fuse). If it's working, it should click when removed and again when replaced in its socket. If it doesn't, suspect the relay or sensor has failed. See here on how to troubleshoot the relay socket:http://schematicsdiagram.blogspot.com/2011/06/how-to-troubleshoot-relay.html. If your gage runs off the same sensor and works, it's probably fine. If you're good with electronics, you can also test the sensor. If these are all good, either the wire to the fan is bad or the fan itself is bad. You can test the wire for voltage with a test light hooked to the fan connector, and you can test the fan by hotwiring it to the battery.
3. As I have already mentioned, the water pump bushing can fail. If it is leaking or if the belt comes off because the pulley wobbles, replace the pump. Some water pumps (notably Honda and many Toyotas), are located behind the timing cover and are difficult to observe. Fortunately, these are generally also more reliable. The other way in which a water pump can typically fail is by broken/eroded impellers. This is rare in the case of steel impellers and most common with plastic impellers. If you know what kind you have, you can use that as a factor in deciding wherther or not you want to replace it. Here are some troubleshooting steps you can also take: assuming the engine is warm and the thermostat has opened, and if you can see in the radiator through the filler cap (be careful opening the cap with the engine hot), you may be able to see the coolant flowrate. If you can't see into the radiator, try squeezing (with oven mits on) the upper radiator hose to see if you can feel/hear the coolant flowrate. A good flowrate indicates your waterpump is working and your radiator is not clogged.
4. If you have a poor flowrate, and assuming your thermostat is opening, it could be caused by the water pump or a clogged radiator. If you can see extensive deposits inside the radiator, either backflush it, have it cleaned professionally, or replace it. Otherwise, the water pump is your problem.
5. If none of the above are the cause of overheating, we must consider some more serious causes. A blown head gasket or cracked head/block can allow oil into the coolant or combustion gases into the cooling system (among other things that do not cause overheating). This latter case can cause the engine to overheat within a few minutes. If you car overheats this quicky, a blown head gasket is likely (but I would still advise someone to check the thermostat before pulling a head). This condition can be verified by removing the radiator cap and running the engine until hot. Bubbles of combustion products will be seen coming up in the radiator or into the coolant reservoir. As mentioned under bleeding after thermostat replacement, it is normal to have air come out during bleeding of the system, but if the bubble continue indefinitley, you are getting combustion products.
Oil in the coolant can also cause overheating as the oil mixes and thickens or "foams" the coolant until it cannot flow or transfer heat sufficiently. This contamination is easy to observe.
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