No heat on the highway

[ewbennett]

I have a 04 X 4.4 with 107K on the clock. My wife complained last week that there was no heat in the X. Each time I went out to check it in the driveway it worked perfectly. I told her that she must have not been setting it correctly and began to become frustrated with her inability to learn. She has a tough time putting her sneakers on some days but that is a story for another time.

After driving it myself this weekend I discovered that I have no cabin heat when on the highway only. It works fine in the city and on back roads at slower speeds. It seems that when the HVAC thermostat is satisfied and the valve closes it will not reopen on the highway. If it is heating when I enter the highway it will heat until it satisfies and then no more heat till I get off the highway. Then it works as normal. I found that if while I am on the highway if I pop the transmission into neutral for a 5 seconds and the RPM's drop back to idle, the heat will immediately come back and the cycle will repeat.

In attempts to determine the root cause I have disconnected the water valve wiring and jumped back on the highway... This resulted in run away heat with dash vent temps of about 170 degrees no mater what you set the HVAC system for.

I have read the posts on HVAC issues but there is nothing quite like this issue that I could find. It is getting cold in New Hampshire. Any ideas on this besides the need to eat crow with the wife?

Thanks,

Eric