When you have overheating problems in your GMC Sierra, it can lead to check engine codes like P1258 and engine damage. If your Sierra is overheating and the temperature needle is well in the red, knowing how to replace your GMC Sierra thermostat can save your serious money. When your GEN III GM engine is overheating, a good way to see if your thermostat has failed is to carefully touch the lower radiator hose.
Because the thermostat inside your GEN III GM engine is set to open at a specific temperature, when it fails it will cause the engine to overheat. There’s a wax thermal pellet incorporated inside your GM thermostat, and if that’s stuck closed the lower radiator hose will remain cool, even though your temperature meter reads hot.
Because this guide is working on hot components and near your exhaust and cooling fan, if you are not comfortable working on your own vehicle, stop now. Take your vehicle to an experienced mechanic instead, while our how to replace your GMC Sierra thermostat guide is relatively easy and straightforward, it’s still pretty easy to get hurt.
Today I’ll be showing you how to replace your GMC Sierra thermostat in a 2002 Sierra with the 4.8 liter engine in it. Because this guide focuses on replacing your Sierra thermostat, if you are still having overheating problems, check our guide on overheating GEN III engines, and how to troubleshoot overheating.
To replace your 4.8 liter Sierra thermostat, you’ll need coolant as well as a 10mm socket and wrench. The part number for the factory thermostat for your Sierra is GM part number 12562740, 12571261, or 12562740. The temperature rating for these thermostats is 186 degrees, and that is the stock rating for your Sierra.
If you are still having overheating problems, we suggest stepping down to a 160 degree thermostat.
Before beginning our how to replace your GMC Sierra thermostat DIY guide, it’s a good idea to disconnect the negative terminal on your battery, just in case. Because your fan is belt driven, make sure the key is out of the ignition and battery disconnected before you continue. Also it goes without saying that your Sierra being cold or at least not warmed up will go a long way towards you replacing the thermostat.
After you’ve disconnected the battery, drain the coolant from your radiator.
Remove the bolts that secure your Vortec 4.8L air intake box and Mass Air Flow (MAF) housing. Disconnect your MAF sensor by lifting up on the gray lock tab. Once the gray lock tab has been pulled out, you can disconnect the MAF by depressing the clip shown below.
Now with the MAF disconnected, remove your PCV (if applicable) and remove the entire intake tract by undoing the clamp on your throttle body. Now your Sierra should be ready for you to remove the upper fan shroud.
Simply put, this monster of a fan shroud completely blocks your access to your engine as well as your thermostat. To remove the upper portion of the fan shroud, remove the six 10mm bolts that run along either side of the shroud.
Once you have all 10mm bolts removed, lift up and carefully take out your GMC Sierra fan shroud.
Give yourself some extra wiggle room by undoing the bolts that secure the radiator in place.
The thermostat is hidden well behind the fan, you can access the two 10mm bolts that hold the thermostat in place, by rotating your fan. Spin your fan and fit your 10mm socket between the fins in order to remove the GMC thermostat housing.
Once those two 10mm bolts have been removed, pull on the lower radiator hose to completely disconnect the thermostat housing. Remove your old thermostat and discard the unit.
Now use a razor blade to carefully clean the mounting surface of your water pump outlet. This should be free of any debris and clean so that your gasket maker has a good chance to recreate and form a good seal. This is important if you don’t want to repeat this job of how to replace your GMC Sierra thermostat.
Clean and dry this area well, if coolant is left inside the housing, dry this area as well. You don’t want the coolant to interfere with your gasket maker’s sealant.
Prepare the new thermostat by inserting into your old thermostat housing, and apply a thin layer of hi-temp gasket maker.
Carefully push the new thermostat and radiator hose / thermostat housing back into the engine. Be careful aligning the thermostat housing, and when fully installed, insert the 10mm bolts and tighten to specification.
Take a break now and put back your MAF and upper fan shroud. Take some time before pouring in the coolant, give the thermostat housing and the gasket maker a chance to dry and seal. Return after a few hours and fill up your radiator. Test drive your GMC Sierra to make sure that your thermostat is working correctly and routing coolant into your radiator and fans.
You now know how to replace your GMC Sierra thermostat, if you have any questions or comments please leave them for us below.