Weird temperature issue on a 1996 Ranger 4x4 4.0 v6

All you cooling system gurus --


1996 Ford Ranger 4x4 4.0 v6 108,000k miles.


When I have the heater on, the dash temperature gage will cycle between 80% hot to 10% cool. The temp needle will go up and then rapidly go down (like the thermostat is opening). This occurs on about a 6 min interval regardless of the heater temperate setting and driving conditions (freeway or city).


Here is the kicker...


If I turn the heater off, the temp needle rests about 50% and rarely fluctuates – this is normal.


I have replaced the radiator cap, thermostat (twice!!), and verified the coolant level. There is no coolant loss and the water pump seems to be functioning.


My next plan is to change out the vacuum operated heater coolant valve that routes coolant to the heater core. Maybe this valve is cycling.


Anybody experienced this type of problem?


Chris -
Seattle, WA

I would venture to say it probably is the vacuum operated bypass valve or whatever is controlling it. Why change it, just disconnect and plug the vacuum hose. See if that makes a difference. You can use a hand vacuum pump to see if the diaphram is any good.
Clark