Replacing engine thermostat: 1998 Saturn SL2

Thermostat and gaugeAfter purchasing a used 1998 Saturn SL2, one of the first things I noticed was that once the car had warmed up the dashboard temperature gauge was always pointing slightly less than the 1/4 mark and never went any higher. The coolant temperature sensor was malfunctioning, so I replaced it. However, whenever replacing the temperature sensor it is also a good idea to replace the engine thermostat. It is a cheap part and is very easy to replace. Since you have to drain the coolant for both repairs, you may as well get both parts replaced at the same time to save some time and money. After replacing the temperature sensor and thermostat, the dashboard temperature gauge was indicating about halfway between 1/4 and 1/2 (i.e., at 3/8), which is right where it is supposed to be. Replacing the thermostat and the engine coolant temperature sensor (ECTS) are some of the easiest repairs that you can do on the SL2.

This post describes how to replace the thermostat. The other post I mentioned describes how to replace the ECTS, here: mike-thomson.com/blog/?p=1338. I recommend replacing both the thermostat and the ECTS at the same time, because they are easy and relatively inexpensive (and since both repairs require draining the coolant). The ECTS was about $5 and the thermostat was about $15 from rockauto.com, or $12 and $17 respectively from amazon.com.

Materials Needed

The following materials are needed for performing this repair:

  • 10mm socket
  • socket wrench
  • fluid drain pan (must hold 2 gallons of coolant)
  • coolant/antifreeze (up to 2 gallons)
  • thermostat (Stant 14279)
  • pliers
  • screwdriver

Furthermore, if you are also going to replace the engine coolant temperature sensor (ECTS) at the same time as the thermostat, then you will also need the following:

Although I think my thermostat was still functional, I decided to replace it at the same time that I replaced my ECTS to avoid the repair in the future. The thermostat part number I used was 14279 by Stant, purchased from RockAuto. The thermostat I purchased included the thermostat, an o-ring seal, and a "tool" for removing & installing the thermostat. I say "tool", but it really is just a small, thick cardboard cylinder that looks like packaging material. Nevertheless, it is needed for the repair, so do not throw it away!

New thermostat in box
The new thermostat. Note that it includes the thermostat, an o-ring seal, and a cardboard insertion/removal “tool”. Click image to enlarge.

Location of Engine Components

Refer to the three pictures below to see where the engine thermostat, engine coolant temperature sensor (ECTS), coolant reservoir, engine block coolant drain plug and radiator drain plug are all located.

Component locations
The locations for all of the components referenced in this guide. Note that only the general vicinity of the engine block and radiator drain plugs are shown in this image – their specific locations are further depicted in the next two pictures below. Click image to enlarge.

Thermostat and drain plugRadiator drain plug
The locations of the thermostat and the coolant drain plugs on the engine block and the radiator. All three are right behind the radiator, directly below the air conditioning compressor. Click images to enlarge.

Replacing the Engine Thermostat

  1. Let the car cool down. The car has cooled sufficiently if the metal of the engine block is cool to the touch. The last you want is scalding hot coolant pouring on you when you replace these parts.
  2. Open the hood. Become familiar with where the engine thermostat, engine coolant temperature sensor (ECTS), coolant reservoir, engine block coolant drain plug and radiator drain plug are all located. See the previous section ("Location of Engine Components") for pictures of these locations.
  3. resevoir cap offRemove the coolant reservoir’s cap. This will allow the coolant to fully drain from the system when you remove the drain plugs.
  4. Engine drain plugDrain the engine block coolant into the drain pan. The first drain plug is on the engine block, directly below the thermostat (see picture). It is accessible from underneath the vehicle using a 10 mm socket. Back out the bolt most of the way with the socket wrench, and then unscrew it the last few threads by hand so that you can avoid getting engine coolant all over your tools.
  5. Radiator drain plugDrain the radiator coolant into the drain pan. Note that the engine block drain plug in the previous step and the radiator drain plug are very close to each other, so if your drain pan has a large opening then you can remove both plugs and have the pan catch both streams of coolant as they drain. Most radiator drain plugs I have seen have a tab on them and are removable by hand without any tools. However, my radiator drain plug had a very unusual 1/4-inch ratchet-type connector on it and I could not remove it. If this is the case for you, too, then you really don’t need to drain the radiator from this plug, just continue on with the steps below.
  6. Are you also going to replace the ECTS? If you are, then now is the time to do that. Go read my other post that explains how to do that here: mike-thomson.com/blog/?p=1338.
  7. Thermostat hoseRemove the hose from the thermostat housing. As you do this, keep the fluid drain pan below the end of the hose because more coolant will come out. If you weren’t able to remove the radiator drain plug in the step previously described, then a lot of coolant will drain out of the radiator through the hose, so just slowly work the hose off of the thermostat housing a tiny bit at a time so that you can contain the stream of draining coolant. To remove the hose, you first must loosen the metal hose clamp that is around it, by unscrewing the screw on the side of the clamp enough so that the clamp can slide back away from the engine about 5-6 inches. Once the coolant is finished draining from the hose, prop the hose up and out of the way so that you can get to the thermostat housing.
  8. Thermostat housingRemove the thermostat housing. Use a 10 mm socket to remove the housing bolts. You might also find it helpful to use a 3-inch socket extension to get at the bolts easier. When you remove the housing, you will see the the thermostat is part of the housing and looks like a big spring along with a metal capsule.
  9. Remove old thermostatRemove the thermostat from the housing. This is when you use the cardboard cylinder "tool" that came with the new thermostat. Place the tool at the end of the thermostat, completely over the circular washer that matches the tool’s inner diameter. Then push down against the spring and turn the tool, which will release two tabs on the thermostat from the housing. The thermostat is now free to be removed.
  10. Remove old sealRemove the old o-ring rubber seal from the housing. Mine was stuck pretty well, so I couldn’t pull it off by hand or even get a screwdriver underneath it. It worked best to put the corner of the flat-head screwdriver in the middle of the seal, and the push in against it to rip the rubber seal at that point. After doing that, the rubber seal can then be pulled off.
  11. Install new thermostatInstall the new thermostat into the housing. Just do this backwards from how you removed the old one. Place the thermostat in the housing, place the cardboard tool around the circular washer that matches the tool’s diameter, push in against the spring until the thermostat side-tabs clear the housing’s tabs, and then turn the tool to get the thermostat and housing tabs to align and lock in place. Make sure that tabs are fully aligned and seated properly, and not partially askew.
  12. Install o-ring sealInstall the new o-ring rubber seal. This is a little tricky. You need to make sure that the seal is fully seated in the groove on the housing, otherwise you can end up having a leak. The o-ring didn’t want to stay in the groove very well for me – you will need to have a light touch as you move the assembled housing back to the engine for installation so that the o-ring doesn’t fall off or come out of its groove at all.
  13. Install thermostat housingInstall the housing back onto the engine. Once again, using a 10 mm socket. I suggest to start threading the bolts on by hand and keep tightening them by hand as much as you can so that the o-ring seal has a better chance of staying in its groove. Once you have hand-tightened the bolts, use the socket wrench to tighten them up the rest of the way.
  14. Thermostat hose installedRe-install the hose and the hose clamp. IMPORTANT: make sure you align the clamp with any depressions it previously made in the hose. I didn’t do that and I ended up having a coolant leak out of that hose connection later. Lining up the clamp as it was originally and tightening it all the way got rid of the leak.
  15. Filling the coolant reservoirRe-install the engine and radiator drain plugs and fill up coolant reservoir to the max cold line. Start your engine. Turn the heater on full-blast. Let the engine warm up to normal operating temperatures, and add more coolant if the level goes down. You might want to "burp" it to get excess air out of the system – do this by repeatedly squeezing the large coolant hoses that go to/from the radiator. This will help move air out of the hoses and back up to the reservoir.
  16. Re-install the coolant reservoir cap.
  17. Take the car for a test drive. Drive it around for several minutes – you should see the temperature gauge go up to about half way between the 1/4 and 1/2 marks (i.e., 3/8). After you are done test driving the car, let the engine cool off again and then check the coolant level in the reservoir. Add some coolant if needed to get it back to the max cold line.

Periodically Check Coolant Level

Periodically check your coolant level in the reservoir and keep your eye on your dashboard warning lights for the next few days. If you have a coolant leak somewhere, then your reservoir will detect when the level gets dangerously low and will start blinking a coolant/radiator light on your dashboard. If that happens, then check if you need to fill your reservoir back up and check for leaks.

6 thoughts on “Replacing engine thermostat: 1998 Saturn SL2”

  1. Thank you so much for taking the time to document this. I replaced my thermostat in my SL1 and also the ECTS. Replacing the ECTS was done only because of your suggestion. I think the ECTS was the culprit in my case. Your blog made this whole project straight forward and I knew what to expect every step of the way. I am so grateful for people like you who take the time to write such detailed instructions.

  2. Hi, Mike.
    Thanks for writing this detailed article. This was great! Answered my confusion about the way my car was acting. Replaced both items. 1998 Satrun has only 108K miles. The temp sensor was not cracked. However, the temp gauge now reads at about 1/3 instead of 1/4. The car also warms up very quickly now, compared with before. I believe in my case the thermostat was stuck wide open.

  3. I noticed that my 96 toyota tercell 1.5 did not have the thermostat i installed it and the electrical temperature gauge needle went all the way hot (the engine was not over heating), the water sensor is new , I then removed the thermastat and the needle returned to the half way mark. I have also notice that on long trips an hour or so the needle rises a little more then half. What do you think

  4. I was able to drain the radiator with a Allen key.
    I had to stick my phone down there and make a video, and then I saw that it actually had a socket for a Allen key.
    But my drain plug looks just like the one in the pic.

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