If it is leaking at the base of the thermostat housing, that means you have to replace your intake manifold. I have seen these leaks occurring more and more often. Almost everytime I check out these forums, I find that someone else has the same leak. I replaced the intake manifold on my son's 2003 GT Mustang 4.6 engine for the same reason. My personal feelings are that there is a design flaw with this intake in that area just below the thermostat housing. However, the Ford Motor Company will never admit it. I have posted one picture here to show you what the O ring on my son's intake looked like after I removed the intake manifold. This is a view of the bottom of the intake manifold just below the thermostat housing. I'm willing to bet that this is the same area where your coolant leak is. I can tell you that I believe what is contributing to this problem is a defective thermostat. When the thermostat freezes up, the coolant and pressure will look for the weakest point to escape, which just happens to be at the base of the thermostat housing. Notice that the O ring is not seated properly. I'm talking about a factory installed intake manifold which was suppose to correct previous coolant leak problems. If you need more info just ask. Hope this helps. Good luck, Tommy.
The best thing to do is to remove the intake manifold to check the extent of the damage. If the port on the intake has not melted downward like the one shown in the picture below, then perhaps a new O ring with silicone may resolve your problem, not sure? The aluminum cross over channel cannot be removed to replace the O ring with the intake installed. No matter what you do, the intake manifold must be removed, if the leak is at the base of the thermostat housing. Notice the damage to the O ring in two locations. Again, notice that the port on the intake has melted downward. This condition will create a larger gap that will almost definately cause coolant to leak from that seal area. I got this picture from another 2003 GT Mustang 4.6 owner. He chose to replace the intake for a new one. Wise choice! With that said, there is still a question of what caused this to happen. As mentioned in my previous thread, I believe the culprit is a defective frozen closed thermostat. This is just my opinion. In any case, think about it. What else could it be? Hope this helps. Good luck, Tommy.
silicone works perfect for that!! my intake is less then a year old, and started leaking there, i pulled it apart, cleaned it, and used silicone sealer, and have no problems anymore.