Heat isn’t hot over 65

Why would my heater stop blowing hot air when I get up to 65 or 70mph? And how could I fix it?

Thanks!

[QUOTE=70vette]Sounds like a thermostat. If it is stuck open the engine will cool down at highway speed because of all the air going in the rad.[/QUOTE]

That's not it.

The thermostat was just replaced.

Well if the thermostat is new and working ok, what does the temp gauge say? If it goes down, then maby the viscous (spell?) is bad. Check to see if it spins freely when the engine is cool. If it does, then perhaps the vacuum controls for the heater door are allowing the system to go to the vent, the vacuum signal for the heater hot water control closing? My .02

or the new thermostat could be bad.. 

it does happen and the syptoms really point to it sticking open

[QUOTE=70vette]Sounds like a thermostat. If it is stuck open the engine will cool down at highway speed because of all the air going in the rad.[/QUOTE]

That's not it.

The thermostat was just replaced.

[/QUOTE]

It is true that sometimes you might have a new thermostat that is bad. Also the vacuum could be a posibility. I had a 69 Scout that had vacuum wipers. If I was accelerating they would not work. Maybe at 65 you are at a high enough RPM that not enough vacuum is made to keep it open. To check this all you have to do is turn on the light. If there is not enough vacuum to open the heater vent then there will be not enough to open the headlights. That would be a bad oneway valve then.

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