Ok, Ive had cooling problems, and I'm sure I can help you out there.
First things to check is to make sure your fan is working and coming on.
It cant hurt to goto the stock water pump pulley, and its going to cost you maybe 2-4hp at the most.
If you have the stock radiator I'm sure that is the problem. Get a good 3core brass unit. I run one and it works great. I think it was about $150? Maybe less, I dont remember it was a while ago.
I run that 3 core with 80/20 water/coolant and redline water wetter with a permacool 16" electric fan and the stock water pump, with a 180 thermo and the car stays dead cool under ANY conditions. 105+ degrees or more.
Changing the thermo if your car isnt cooling already will do NOTHING for you. Get the radiator and your problems will be solved.
Skyler
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-1989 Saleen Mustang #406- TFS Heads, E-303, edelbrock intake,70MM TB, 73mm MAF, off road H, headers and 3chamber flows.
12.55@107mph
Going for 11's on 87 octane with A/C and 20+MPG!!!