Permanent oil cooler vs thermostat

[Deuce Cam]

From what you just described it sounds like the valve is sticking so it might be dirty or clogged. I had the same problem - it took a really long drive for the idle to drop completely.

I removed the valve and found there was some some type of loose debris (hard black pastic or something) causing an obstruction inside the valve assembly where the water passes through. Flow was really restricted when blowing air through the water tube on the valve. I cleaned it really good, got most of the debris out to open up the restriction, and now the valve works correctly.