Our family has 3 vehicles with Northstar 4.6L engines build on the same line as Cadillac DTSs. Two are 2006s and one is a 2007. They have from 100,000 miles to 140,000 miles on them. They heat up quickly from a cold start, taking about 5 minutes for the temperature to indicate exactly mid scale and they usually faithfully hold that reading. However, all 3 have demonstrated a mysterious overheat episode. The 2007 misbehaved about 14 months ago. After a 2 hour 50mph highway trip, I must confess I didn't notice the temperature gauge when I left the car, nor did I notice the cooling fans running. I came back in about 10 minutes to get something and the electric cooling fans were going full blast and the temperature gauge was almost in the red. An hour later the fans were off. I slowly released the coolant cap with no fuss and noted the coolant level was normal. I added about a quart of 50/50 coolant anyway with the car idling and the temperature quickly indicated mid range. The car didn't give me any more trouble until today, 14 months later. After 10 minutes driving, the overheat light came on and the gauge was in the red. I pulled off the road and let it idle. The dash told me it was overheating, then told me it was turning off the AC and then it was going into idle mode. So I shut it off and the fans kept running. After only 5 minutes I was able to slowly release the coolant cap with only a slight hiss and a perhaps a hint of a gurgle. The coolant level was normal but I added about a quart anyway. When I started the car, the temperature quickly reached normal and stayed there for my 3 hours of driving today. My sons reported a similar one off experience with the other two vehicles. We all did the same thing - release the pressure, add a bit of coolant and things immediately went back to normal. We have replaced all the cooling hoses and thermostats and used a vacuum tool to get rid of trapped air. The only thing I can think of is a stuck thermostat or an air lock. Any ideas? I just hope it isn't head gaskets. We had another bad scare this summer when all three engines suddenly had their bottom halves covered in oil. Oh no, not oil pan seals! But how could all three possibly do it at once? I got underneath and with the engine idling used an air gun to clear the oil away from the joints and see where it came back. I started at the oil filter, lucky me. That's where the oil leak was. It had been the first time we used white NAPA oil filters and had tightened them the full turn as labelled on the filter. When we replaced them with Frams, we noticed the gasket was a lot stiffer and took a lot more torque for its full turn. No more oil leaks on all 3 cars. I would hate to think of people getting $4,000 gasket jobs because of lousy oil filters.