Originally Posted By: sciphi
Thermostats do not need to be OEM. They do need to be a high-quality part. The cheap thermostat for my car is a piece of trash. The one for $2 more is a much nicer part, and is resold by performance companies for much more than AutoZone.
Been there done that, and its been hit or miss for me since the mid 80's with my Ford Thermostats. I've tried the Stant (top of the line), and a few of the cheaper brands and soon after installing several of them I ended up replacing them with the Ford part. In a 4.9L Ford and a 3.0L Ford engine here is what happened more than once. Install the thermostat, the engine temp climbs into the red, then quickly drops and stays in the normal zone, as if the Thermostat suddenly popped open. This happened everytime you started a stone cold engine, not good. The Ford part climbs to the Normal Zone and stays there, it never goes to red then drops down. Another issue I had with an aftermarket T-Stat was it would pulse up to just under the red zone, and down, then up to near the red zone, and down, constantly. Not worth the $3 savings. As soon as the OEM Ford part went in problem solved.
Now when I need a Thermostat or a PCV valve I go OEM. YMMV, I learned the hard way.