Although my 4.3 was running at about 195 degrees (that's with a stock spec thermostat) and seemed pretty stable, I decided to flush the cooling system because the coolant looked rusty. After the flush, I refilled it with a50/50 mix of antifreeze and water. Now it runs hotter on the highway. The upper radiator hose is the highest point in the system, that's why I'm curious about the direction of the flow. I wonder if the coolant is flowing toward the engine from the top tank. I had always thought it went from the bottom into the engine and the hottest water exited the top and flowed into the top tank. The system is not the best design. 4 row radiator, AC condenser and a pusher fan. But it worked fine until I fixed it. Any ideas? I'm thinking of adding a coolant recovery bottle and a bleed valve at the highest point since it probably won't hurt to do so. Then I could at least be sure it's not air bound Any suggestions?