You didn't say which coil pack- I'm assuming #4 since this is the one that fails all the time. Have fun it is on the right side at the back. Also double check the heater hose connections to the core b/c that's why most of them fail- coolant drips on it. I work at a Ford dealer and see it very often
Has it been #4 each time? I don't think a dealer will warranty it b/c of the mileage. You can put a dry rag over #4 and let it idle a while to see if the heater hoses are leaking. If so, then replace the o-rings in the coupler.