Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Leaking thermostat and boiling reservoir

I have a 98 mustang. It has been having best issues for a while and a boiling reservoir. The thermostat is brand new as I replaced it last night. Car ran fine this morning but it overheated when I got off work and was driving home. The reservoir has a hole in the top corner of it and may have something to do with it, and the thermostat has leaked some liquid. I couldn’t see the color so I’m not sure what liquid it could be. What could be causing this? Also has a new radiator and I’m not sure if the fan even turns on. I haven’t checked that.

If the cooling system has a leak, it can’t hold pressure. And the engine can overheat.


Tester has provided the answer for the OP.

Im not sure where the leak could be though. I replaced the thermostat last night and its now leaking. It hasn’t leaked much, but it didn’t leak with the old one but does with the new one.

That’s where the coolant leak is.


Do you have the V6 (unpressurized reservoir):

Or the V8 (pressurized reservoir)?:

I have the v6

“Boiling” in an unpressurized reservoir is many times caused by a blown head gasket. Try a new radiator cap…and pray…


I presume you mean the thermostat housing is leaking. When you buy a replacement thermostat, you also have to buy a new gasket that fits the housing. The gasket doesn’t come with the thermostat automatically. Did you do that?