1999-2004 Ford Mustang GT Thermostat Install

Posted by on April 3, 2008 - more

Article Credits

Author: jimmy_beaner

Editor: Brent

This procedure can be used to either change a broken thermostat, or change your stock 190° to a lower temperature thermostat. You will know to suspect a broken thermostat if your car throws a P0125 Service Engine Soon light. I got mine scanned at Auto Zone for free. I also ordered the Motorcraft replacement thermostat from O’Reilly Auto Parts. Be sure to also get a new O Ring.

Things you will need:

  • 10 mm socket for removing thermostat housing and alternator bracket bolts
  • 8 mm socket for removing battery terminal cable
  • 6” extension
  • Ratchet
  • Torque Wrench for re-installing thermostat housing bolts (18 ft/lbs)
  • Replacement Thermostat
  • New O-ring
  • Coolant
  • Bucket for catching coolant
  • Vice Grips (not required, only if you want/need to remove the coolant hose)
  • Stove with pan and distilled water for checking the new thermostat
  • Screwdriver for popping out the old thermostat
  • Funnel

Step 1
Boil distilled water on stove top deep enough to cover thermostat. This checks your thermostat before installing it to make sure it will open. There is no point in installing a bad thermostat as it can damage your engine, and it will make you do the work twice.


When you see the gap as in the picture above, you know that your thermostat has opened. It is now ready to go in your car. You may want to check your o-ring at this point to make sure it is clean.

Step 2
Now you are ready to set the car up to be worked on. Depending on the size of bucket, you will need to jack the car up. My bucket was too tall, so I set the front end on jack stands. You can now get under the car on the passenger side. Be very careful before opening the radiator drain plug as coolant is dangerous and toxic. You will need to be sure to catch all of the coolant and dispose of it properly. Place your bucket under the car and slowly open the drain plug until a stream of coolant is coming out. This is a great time to flush your coolant system.

Step 3
You will need to undo the coolant reservoir cap or the coolant will stop flowing from the drain.

Step 4
You need to wait until the reservoir has emptied. If you take off the thermostat housing before this coolant level has dropped, coolant will fall all over your engine. A lot of coolant will come out, so make sure your bucket is large enough.

Step 5
Here, you should get your 10 mm socket, a 6” extension and your ratchet. Undo the alternator bracket bolts. Remove the bracket.


You will probably notice that I removed my belt for this. I have omitted that step because it isn’t required.

Step 6
Now remove the thermostat housing bolts. They are also 10 mm bolts. For my engine, they were stuck on a little. Some good effort should break these bolts off without too many difficulties. I did this in a little different order. I removed the driver’s side bolt before the passenger side bolt. I would recommend taking off the alternator bracket first.

Step 7
Now pull off the thermostat housing and hose. Some methods say to remove the hose from the housing, but I didn’t find it necessary. If you feel like doing this, use Vice Grips on the wing tabs on the hose to relieve the pressure it applies. You should then be able to pull the hose off the thermostat housing. This step will show you the thermostat.

Step 8
Now use a screwdriver to carefully pull the old thermostat out. You don’t want to indent the housing. Vice Grips may also work well here.

Step 9
Place new thermostat and o-ring into where the old thermostat was. Make sure it is properly seated. If you push down on the thermostat, it will pop into place. Be warned that it will not stay exactly where it was because the spring will push it back up. Try to make sure the o-ring is seated correctly also for a proper seal.

Step 10
Now get your torque wrench. Set it to 18 ft/lbs or 25 Nm. You may want to start the bolts with a ratchet but make sure you switch over to your torque wrench before the bolts are even snug. When you hit 18 ft/lbs on both bolts, the housing is done. Sorry for the blurry picture here (I may take another one later).

Step 11
Reattach your alternator bracket. There weren’t any torque specs that I could find for these bolts, so just snug them on with your ratchet. Don’t over tighten them, remember you’re dealing with aluminum and you don’t want to strip these bolt holes. I don’t have a picture of this, it just goes on like it came off.

Step 12
Now close your radiator drain plug.

Step 13
Refill your coolant. You can use pre-mix, or concentrated as long as you dilute it with distilled water. If you wanted to do a coolant flush, this is where you would deviate from these instructions. Because I have done a coolant flush in the last month, I only needed to fill.

Step 14
When you have refilled the coolant reservoir completely or while you are starting this process, you need to unhook the positive battery terminal wire. This will require an 8 mm socket.

Step 15
After waiting several minutes, to reset your ECU, reattach your battery terminal. After this, you can restart your car and drive it around to thoroughly mix the coolant and to work out the air bubbles and pockets. You will most likely have a fair amount of air that works its way out so remember to check your coolant frequently during this time. You are now done.

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