I am not familiar with the 2008 EXPLORER, I am assuming it has a traditional cooling system. I have a 97'.
Temperature spikes are a symptom of air pockets in the cooling system.
Fill coolant in the rad. to the top, start engine and let the engine warm up with the radiator cap OFF, it will go down slightly when first started, as the engine warms up and the thermostat open the water will go down, this is the time to add more coolant, keep doing this. Some engines have a bleeder valve on the highest point of the cooling system to let air out, if you have one, crack it open and keep an eye on it, when you have a steady stream of coolant flowing out the bleeder valve, close the bleeder valve. As the engine warms up, keep adding coolant in the radiator. As the thermostat opens, you should see water flowing in the radiator looking though the cap location (maybe if visible). There will come a point where no more water can be added, this is the time to install the radiator cap and top off the overflow jug. Go for a test drive and get the engine to operating temperature and watch you gauge (if you have one). After the first overnight cool down check you over flow jug, if might have gone down, add coolant to the "FULL COLD" line. Keep and eye on the coolant jug for the next few days, it might go down again. this just means more air is being expanded from the system, just top off the over flow jug, after a few heat up and cooling cycles it should find the "sweet pot".
