I recently purchased a used 1998 Saturn SL2, and 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. I thought this was a bit odd, since all my previous vehicles would usually stay just below the 1/2 mark, but then I figured that maybe the SL2 engine just stays cooler. Not so. Doing a quick search online uncovers the fact that one of the most common problems with the SL2 is a cracked coolant temperature sensor and/or a stuck-open engine thermostat. Either one of those will cause the dash gauge to stay at the 1/4 mark pretty much all the time after warming up. A correctly-functioning SL2 should stay about halfway between 1/4 and 1/2 (i.e., at 3/8). 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 engine coolant temperature sensor (ECTS). If you want instructions on how to replace the engine thermostat, then view my other post here: mike-thomson.com/blog/?p=1335. I recommend replacing both the thermostat and the ECTS at the same time, because they are easy and relatively inexpensive. Furthermore, replacing either part requires draining the coolant from the engine and radiator – so you may as well replace both since the coolant is going to be drained. The ECTS was about $5 and the thermostat was about $15 from rockauto.com, or $12 and $17 respectively from amazon.com.
- 13mm deep socket
(for the ECTS, I have the Tekton 1230 deep socket set)
- 10mm socket (for everything else)
- socket wrench
- fluid drain pan
(must hold 2 gallons of coolant, here’s mine: Hopkins 11845)
- coolant/antifreeze (up to 2 gallons)
- ECTS, brass tipped (Tru-Tech TX73T)
- thermostat, if you are planning to change this at the same time (Stant 14279)
First of all, the Saturn coolant temperature sensors are notorious for going bad because they are made out of a material that cracks, allowing coolant inside the sensor and causing it to malfunction. When you purchase a new ECTS, make sure that it is the brass-tipped version so that it will not crack again. See the picture below to see the difference between the cracked ceramic ECTS and the updated brass-tipped ECTS. Let me repeat – both are still sold in stores, so make sure you are getting the brass-tipped version. I used part number TX73T (the manufacturer is Standard Motor Products), purchased from RockAuto.
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.
Replacing the Engine Coolant Temperature Sensor (ECTS)
- 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.
- 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.
- Remove the coolant reservoir’s cap. This will allow the coolant to fully drain from the system when you remove the drain plugs.
- Drain 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.
- Drain the radiator coolant into the drain pan. If you are only changing the ECTS and not the thermostat, then you can skip this step. If you are replacing both the ECTS and thermostat, then 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.
- Remove the air intake duct assembly. This is very simple for the SL2, since it only requires unsnapping the 4 air filter brackets (by hand), pulling off the small hose going to the DOHC valve cover (by hand) and using pliers to release and remove the ducting at the throttle body (see picture for locations). The throttle body is where the duct connects to the engine – use some pliers to squeeze the circular metal fastener’s tabs together while simultaneously pulling the duct away from the engine. Also remove the air filter.
- Find the ECTS. It is located about 9 inches beneath where the spark plug wire are connected to the DOHC valve cover. See picture.
- Remove the connector from the ECTS. It does NOT simply pull off. You need to use pliers to grab the two shorter edges of the connector (see picture), and then squeeze the sides together while gently pulling the connector with the pliers. It should take very little force to pull it off. If that doesn’t work well, then try getting your finger back behind the longer edges of the connector and lightly pry one and then the other over the small molded ridge that is there on the end of the ECTS. You should only pull on the connector casing, never on the wires. The pictures to the right show removing the connector with pliers, and the ECTS with the connector removed.
- Remove the ECTS with a 13 mm deep socket. You need to use a deep socket to do this because a standard socket will not fit over the ECTS body and a standard wrench will not fit into the small space where the ECTS is located. I also had to use a 1.5-inch socket extension to be able to comfortably access the ECTS for removal.
- Inspect the old ECTS. Look for a crack in it, like mine had (see picture at right). You might also notice some green sludge in the 2-pin connector mating area from the coolant seeping through the cracked sensor.
- Add Loctite or Teflon tape to the new ECTS (unless the ECTS you purchased comes with it already applied). You are supposed to use blue Loctite (or equivalent), but I’ve read online that many people use plumber’s Teflon tape instead with good results. However, note that some temperature sensors rely on an electrical connection through the threads, in which case Teflon tape should be avoided (thanks for the tip, Jim Z). I used Loctite 6-ml Threadlocker 242. Note that when using Loctite, you only need to put a small amount on – barely enough to cover a few threads (it will spread itself out when you screw it in). You are supposed to add this to the threads for two reasons: to help keep the threads from corroding and to secure the part once installed so that it does not come loose from the engine’s vibrations.
- Install the new ECTS. The 13 mm deep socket wrench is actually probably a bit TOO deep, so you should start installing it with just your fingers at first, and then switch over to your socket once it is part way in.
- Plug the connector back in. Make sure that it clicks into place. It should stay in place even with a gentle tug. To get mine to click into place, I had to push the connector on, and then press the longer edges of the connector toward the body of the ECTS so that their small notches would click past the ECTS’s ridge.
- Install the air intake assembly. Make sure to put the air filter back in first, and then reconnect the duct to the throttle body, push the small hose back onto the valve cover, and clip the 4 air filter housing clips back into place.
- Are you also going to replace the thermostat? 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=1335.
- Wait 20 minutes if you used liquid Loctite on the ECTS threads. If you used Loctite, then it takes 20 minutes for it to partially cure before you can start the engine or fill your coolant reservoir back up. If you used Teflon tape, then no waiting is required.
- Re-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. Note that you might want to help "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. When you do this you will probably notice some air bubbles coming up in the reservoir.
- Re-install the coolant reservoir cap. You don’t want to accidentally drive off without having this in place!
- 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. After you are done test driving the car, let the engine cool off again and then check the coolant level in the reservoir and add some coolant if needed to get it back to the max cold line.
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.
As previously mentioned, it is not necessary but highly recommended to replace your thermostat at the same time as replacing the ECTS. For instructions on replacing the thermostat, see my other blog post that describes that (mike-thomson.com/blog/?p=1335).