thermostat and rad. cap

Going to a cooler thermostat with your OBD II system might not be the best thing without a slight reprogramming. I definitely wouldn't go to a lower pressure cap. Water in a pan boils at 212degrees, but the more pressure its under, the higher temp it takes to make the water boil. The area where the coolant contacts the cylinder can be like dumping water into an empty pan on the stove that has been on a full up burner for 5 minutes...If the coolant cant contact the metal, it cant cool it.

you gain aprox 2.8 degrees for every pound of pressure, so a 15-16 pound cap will raise the boiling point of the coolant 45 degrees . The computer system expects to see a certain operating temperature. If the vehicle does not reach a high enough temp the computer assumes the engine is cold, and the fuel management system runs on a preset set of parameters called open loop, i.e. it doesn't read from knock sensor, it retards the timing, it ignores other sensors. Its basically like driving with the choke on on a carb. In the early days of replaceable prom chips, there were chips designed to work at 180-190 degrees instead of 212. Fuel mileage still suffered. The whole idea of running at higher temps is to get more efficient burn of fuel. The engineers would love to see 250, but pistons, heads, and gaskets aren't there yet. As stated in another post, there are surfaces internal to the engine that exceed the temp you see on the gauge. The boiling point for the coolant must exceed these temps, or we have problems, remember early 400 small blocks before they added steam ports to cylinder block decks and the heads? (400 blocks the cylinders were siamesed, ie there was no clearance between the cylinders in the cooling system)

Whitelightning

I had a feeling you were gonna chime in with the right degree increase per PSI...Thanx

Another issue with a lower thermostat is the cooler water temp might 'fool' the computer into thinking the engine is not up to temp and run richer as if the engine was still in warm-up mode.