Hey Littlebookworm,
Great call about the trapped air in the system! I was having a very similar problem this weekend after replacing my heater hoses. My engine seemed to be running hot and, with the heater fan turned on, hot air would blow out for only a few minutes and then, turn stone cold. At this point I wondering if the thermostat, water pump, and/or heater core had suddenly packed it in? In short I was pretty frustrated, as I wasn't looking forward to spending another weekend working on this.
After reading your post, I checked my Haynes manual and sure enough, there's a small section that describes the need to "bleed" air from the system when filling the rad with coolant. For V6's, there's an actual bleeder valve located on the neck where the upper rad hose attaches to.
Car runs great now (IMG:
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