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Last July, my '88 740 non-turbo wagon overheated one day, and when I opened the hood I found that one of the nipples connecting the heater water valve to a heater hose had broken off resulting in a massive coolant loss. Rather than mess with it much more at the time, I just spliced the heater hose together, bypassing the valve. I just left the broken valve where it was, still hooked up to the vacuum line, and laid it on top of the intake manifold so it would be out of the way. So far so good, except that in the past month or so, we've finally been getting some cold weather here in the deep South. Not having the heater water valve meant that the coolant was allowed to flow through the heater core all the time, and the engine took a very long time to warm up. In fact, on the really cold days (by Southern standards: 20 degrees F), the engine would never really reach "normal" operating temperatures at all. So I went out and bought another valve at CarQuest, brought it home, spliced it into the heater hose, hooked up the vacuum line, threw away the old broken valve, and voila! Nothing really changed. The engine still takes forever to heat up, which means it's a long wait for any hot air out of the vents, and when I do crank up the heat, the engine cools off again. So what's going on? How can I tell if the new heater water valve is doing its job? It looks okay, but I don't really know. Maybe it's not getting the proper vacuum signal, but there's no obvious signs of a vacuum leak, like a broken line or a hissing sound. The heater worked just fine as recently as this past April, and I can't see what harm could have been done to the vacuum system by sitting connected to a broken valve for a few months. Maybe there's something wrong with other cooling system components, like the thermostat or the fan clutch. How can I tell if they're failing in a way that would lead to overcooling?
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