QuestionI have a 1979 caddy coupe deville and the heat does not work. I removed the normally open heater valve and it is fine. I then blew compressed air thru the coil and that was fine. I think the vacuum system has several problems because the blower only runs if the temperature control bar is all the way to the right. I have a 425 engine and any information would be greatly appreciated.
Thank you.
AnswerHello,
Before anything is done, all the vacuum issues must be solved because that it not only what controls heated air, but also controls the direction the air flows from the vents.
These cars also had issues in the rotary valves in the heater control where if leaking would not direct air flow from floor to center vents to defroster.
The best thing you can do since you say you have known vacuum issues is to purchase a hand controlled vacuum pump. This way, you can apply vacuum to all areas to see if doors are opening and closing as they are supposed to.
Now, if hot water/coolant is going into one side of the heater at the hose, but is not hot on the return heater hose coming out of the heater core, you have a plugged core. If it does not flush out, the only thing you can do is replace the core.
I do not know what you blew compressed air through you refer to as the coil, but blowing air through the heater core would not have a cleansing or diagnostic effect and can potentially cause the core to leak. Back flushing with water/coolant is the only way to check or diagnose.
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