Is your Jeep over heating? Heater sucks? You should probably consider changing the thermostat on your Jeep. My Jeep didn’t have one as the previous owner removed it for whatever reason, so I needed to install a new one. This is a pretty simple job that can be done with basic tools by anyone who can turn a wrench, so let’s dive in and show you how to replace your thermostat on your 4.0L Jeep.
Buying a New Thermostat
There are quite a few brands of thermostats, however, I personally don’t have a preference. Generally, you will have an option between a thermostat that opens at 160*, 180*, or 195*. The factory thermostat opens at 195*, however, if you live somewhere hot like we do in Arizona then I would recommend a 180* thermostat to help keep things cool under your hood.
Removing the Old Thermostat
1. You must first remove the coolant hoses attached to the thermostat housing. They both have simple clamps on them and are pretty easy to remove. The hoses might be pretty stuck onto the thermostat so be prepared for a fight.
2. Carefully disconnect the coolant temperature sensor and set the wiring out of the way.
3. Remove the two, 1/2 inch bolts holding the thermostat housing on. The top one is much longer than the bottom one. The serpentine belt might be in the way of the lower bolt. If the belt is in the way, just loosen the tensioner and push the belt out of the way.
4. Remove the thermostat housing from the cylinder head. If the thermostat housing is stuck on with RTV than carefully pry it off with a flat blade screw driver.
Prepping the Housing
Before you install your new thermostat, you must clean the gasket surfaces.
1. Take a razor blade, and carefully scrape off any old gasket material, or RTV. Do this for both the housing gasket surface, as well as the cylinder head gasket surface. Be careful to not put large cuts on the surface.
2. Then wipe both surfaces off with brake cleaner, or alcohol and a rag.
Installing the New Thermostat
Now you can install your new thermostat housing.
1. Apply RTV to both the housing and cylinder head gasket surfaces and smear it so you have a nice even coat all around. If you have a gasket, then you can skip this part.
2. Apply a very small amount of RTV to the spring side surface of the thermostat, this will help keep it in place when installing.
3. Set the thermostat with the little burp valve (as seen in the above picture) facing upwards into the cylinder head. There is a recessed circle in the cylinder head for the thermostat to sit in. Make sure the spring goes on the inside of the head. It should not be visible to you.
4. Bolt the thermostat housing back on. I would recommend starting with the top bolt, leaving it loose, and then the bottom bolt. Tighten both bolts evenly, and don’t over tighten them.
5. Reconnect the heater core hose and the radiator hose, and clamp them down.
6. Allow RTV to dry for the recommended time. This is usually 1-2 hours for the sealant to set and a full 24 hours for the RTV to cure completely. Usually, you can get away with only waiting a couple hours before proceeding to the next step, but the less time you give the RTV to dry the more likely you are to have a leak later on.
7. Check for debris and corrosion before reconnecting the coolant temperature sensor. If corrosion is present clean and lubricate the connection with some electrical grease.
8. Refill cooling system with coolant. Make sure not to mix coolants as red and green coolant don’t mix well and can end up becoming a jelly like substance.
The last step is to start your Jeep up and let it idle. The temperature gauge should climb to 160*, 180*, or 195* depending on what thermostat you just installed. If the temperature continues to climb past that point you may have air in the cooling system or a part of your cooling system may need to be replaced or upgraded.
Replacing the thermostat on your 4.0 Jeep should only take about an hour or less, and is a fairly easy job even for the novice mechanic. If you have any questions or comments about this short tutorial please feel free to leave them in the comments below!