Replace Thermostat in 1995-98 Mazda Protege

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If your 1995-98 Mazda Protege is overheating, you may need to replace the thermostat.

If your 1995-98 Mazda Protege is overheating, there is a good chance you need to replace the thermostat. This is not a major repair and can be accomplished in your garage or driveway with some common tools and a medium level of mechanical knowledge. It will take about 1 hour, perhaps longer, depending on the condition of the thermostat gasket.

The tools you will need are: a small ratchet with 12mm socket and one 4-inch long socket extension. Also, you will need a 14mm box wrench, small needle-nose pliers, new thermostat and new thermostat gasket, a can of WD40 or similar product, a drain pan (the same type used to drain motor oil into), a flash light or better still, an attachable mechanic's light and a few clean rags.

First, park the car on a flat surface and let the engine cool down to the point you can comfortably touch the radiator cap - or about an hour in 70 degree weather. Begin by putting the drain pan below the radiator drain-plug. Undo the plug and allow approximately 2 liters of coolant to drain from the radiator into your pan. Replace the drain-plug and make sure it's secure but don't tighten it too much. They are easy to damage.

To locate the thermostat housing, follow the large rubber hose that comes off the top right hand side of the radiator (as you face the engine). It will lead to the aluminum thermostat housing, which is located on the left end of the cylinder head (as you face the engine), just below and to the rear of the distributor. Some mechanics like to first remove the distributor for easier access to the thermostat housing but this is not absolutely necessary.

The thermostat housing will be attached to the end of the large rubber hose coming from the radiator. The housing is dome-shaped and is attached to the cylinder head with one 12mm nut and one 12mm bolt. The 12mm nut is the one toward the rear of the engine. The 12mm bolt is partially beneath the distributor and is toward the front of the engine.

Before attempting to remove the nut and the bolt, spray them liberally with your can of WD40 or similar product. They may have been in place since the car was manufactured and may be stuck in place with rust or corrosion. After spraying the nut and the bolt with WD40, wait several minutes for the product to penetrate and loosen them.

Now stuff a clean rag under the thermostat housing to catch any little bit of coolant that may dribble out when you remove the thermostat housing. Next, apply your small ratchet with the 12mm socket to the thermostat housing nut at the rear. Turn it counter-clockwise to remove it and put it in a safe place. Now, attach the 4-inch long socket extension to your ratchet and re-attach the 12mm socket. You will have to use the ratchet under the distributor to reach the 12mm bolt at the front of the thermostat housing. Once again, carefully remove the 12mm bolt and put it safely aside.

If the housing remains fixed to the cylinder head once the nut and the bolt are removed, tap on the housing with a small rubber mallet to loosen it - VERY CAREFULLY. DO NOT hammer on the housing. It is made of soft aluminum and is easy to crack. If the housing is still stuck, using a very thin screwdriver blade and apply it to the gasket that visible behind the thermostat housing. Tap on the screwdriver carefully and the thermostat housing will pop off. Be patient. Once the housing comes loose, a little bit of coolant may dribble out. Do not be alarmed, that's why you've got that clean rag stuffed underneath. The old thermostat should fall free of the cylinder head but if it is stuck, use your needle-nose pliers to grasp it by its metal frame and pull it loose. You won't need very much force to do this. Pull the thermostat housing back out far enough to be able to remove the old thermostat.

Hopefully the old gasket won't be badly stuck to the thermostat housing but they often are. DO NOT be tempted to reuse the old gasket, even if it looks perfect. It's not. It will have small imperfections perhaps not visible to the naked eye but enough to allow coolant to leak out. You must remove the old gasket. If it is truly stuck to the thermostat housing, begin by making very fine cuts with an exacto-type blade. Not deep enough to touch the thermostat housing itself but to break the surface of the gasket. Now spray it very liberally with WD40 or similar product. The blade cuts will allow the penetrating oil to seep into the old gasket and make it easier to remove. Be patient at this stage. You don't want to damage the soft aluminum of the housing. Carefully work with your exacto-type blade to take off the bigger pieces of the old gasket. If there is a final layer stuck to the housing, you can use fine grit sandpaper to wear it down and as a final step, using a green scrubber pad should do the trick. In the end, you want to have a nice, clean thermostat housing surface for the new gasket. Any old material remaining and you will likely have a leak.

In order to thoroughly remove the old gasket, it may be easier to pull the housing, while still attached to the hose, under the distributor and bring it free and clear to the front of the radiator. This will give you more room to work. But be very careful while pulling the housing under the distributor so as not to damage either unit or the radiator hose attached to the housing. Be just as careful when you are sliding the housing back under the distributor and back into place.

Once you have a clean thermostat housing and its back in place, take your new thermostat from its package and check its exterior diameter against your old thermostat to make sure they are a match. Now slide the new thermostat into place against the cylinder head. There will be a round recess where the thermostat rests and should sit in there without falling back out.

Next, VERY CAREFULLY slide the new gasket over the bolt head at the rear of the cylinder head and hold it up from the bottom so it doesn't fall downward. With your other hand, carefully slide the thermostat housing over the bolt head at the rear of the cylinder head and put the nut back on and make it finger tight. Once again holding up the gasket so it doesn't fall, use your ratchet and extension, with 12mm socket to replace the 12mm bolt at the front of the thermostat housing. Now do this very slow and carefully, making sure that the bolt is going through the correct hole in the gasket. If you push the bolt in very slow and feel any resistance, you are hitting the gasket surface and not the proper gasket hole. Make sure the gasket is lined up correctly with the thermostat housing. Once you have the bolt in properly, tighten it snuggly but not too much.

Next, make sure the gasket is showing a little bit above the thermostat housing around the top half and that there are no crinkles or folds in the gasket. Once you believe it is correctly in place, THEN tighten the 12mm nut at the rear of the housing and the 12mm bolt and the front of the housing. Alternate tightening each of them so you spread out pressure on the gasket evenly until both are tight. BUT DO NOT TIGHTEN THE 12MM NUT AND 12MM BOLT TOO MUCH. The thermostat housing is fragile and cracks easily. You do not need to reef on the nut and the bolt. Internal engine pressure will create a final seal. Remove the rag you've stuffed below the thermostat housing.

Once the thermostat housing is back on, pour 1 liter of new coolant into your radiator. Not through the reservoir but remove the radiator cap and pour the coolant in through the opening. Next, start the car and check for any leaks around the thermostat, leaving your drain pan under the car to catch anything that might drip. If there is a leak, turn off the car and check the tightness of the 12mm nut and 12mm bolt that attach the thermostat housing to the cylinder head. Make sure they are correctly attached - and I hope you used that new gasket, like we said.

If there are no leaks, allow the car to idle and pour in the remain 1 liter of coolant as the car is running. This allows the cooling system to purge any air pockets that have formed. Once the radiator is topped up, about a half inch below the filler neck and remains at that level, turn off the car. Once again check for any leaks. If there are none, remove the drain pan and re-check the radiator’s coolant level, topping it up if necessary. Also, check radiator's plastic reservoir and fill it up to the FULL mark. Replace the radiator cap. Now start the car and allow the needle in your temperature gauge to reach its maximum operating temperature. In the Mazda Protege, normal temperature level is usually just below the half way mark on the gauge. Once you've check your temperature level and it reads properly, give yourself a pat on the back. You've just saved yourself some money and learned how to affect an important repair on your car.