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Vacuum football


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AFIK no vac to the heater valve is full on, not off. In other words, low vac to the valve could put the coolant full thru the core instead of blocking it off. If it's a big leak, ALL the heat would default to the defrost. If it's a small leak, it could only default part of the heat upwards. Also the heat/vent issue would change when blasting uphiil or decellerating downhill, or even decellerating on the flat.

 

If in fact no vac to the valve is full closed, it would stand to reason that you're not getting heat out the lower vents, or at all.

 

Either way, you should trace all the vac lines on the passenger side, all the way from the black line coming out of the firewall down under the battery tray (common source of corrsion and vac line failure), AND check the tan/pink line to the heater valve. I like to blow some smoke down the line and see if it comes out where you don't expect it ( I call it the Stogie test).

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That's good advice from ParadiseMJ. If however all is well, pull the dash bezel then the screws holding the HVAC control module. It will come out about and inch or so. Operate the slide lever, make sure the HVAC rotary valve is rotating at each position, and that all linkage, electrical, and vacuum connections are in place. Once had a problem on an XJ the the vacuum rotary connection was loose on the rear of the switch and caused intermittent HVAC door problems.

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