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AC works, heater doesnt


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Although I never owned a big-block Cadillac, I do have a lot of experience in owner-maintenance of GM cars of that general era, including 1969 and 1973 big-block Chevrolets, and I once did a tune-up on a friend's 1973 Eldorado that included replacing most of the vacuum hoses.

I'm assuming that you know that your thermostat is OK and the right temperature range. Be sure and use the recommended temperature range (195 F; lots of people use 180 F but 195 F is better for your engine). I'll not belabor you the details of reverse-flushing or replacing your heater core if it turns out to be clogged.

The first thing I would do is make sure that the new heater valve is getting vacuum. pull off the vacuum hose, start the engine, turn on the heat if the vacuum opens the valve or turn off the heat if the vacuum closes the valve, and see if you have vacuum. If so, push it back on the valve and see if the vacuum is sufficient to control the valve. If not, you know to find and replace any cracked or deteriorated vacuum hoses. On my friend's 1973 Eldorado, I found that just about all the vacuum hoses had rotted out on the bottom where they rested on the engine, but they all looked fine from the top. You should just have to do this once because newer hoses don't rot out with exposure to oil like 1970-era hoses.

If the valve is operating properly, the next thing to do is to see if hot water is flowing through it. If you can safely reach the valve with the engine hot, try that; if not, find a heater hose that you can feel, use an IR temperature-measuring gun -- or get some temperature-indicating tape from your auto parts store that is used with thermostat housings to determine whether thermostats are opening at the right temperature.

If the heater core is getting hot water, you have to go under the dash and fix the temperature control doors. These are vacuum operated and thermostat controlled, and this is a likely trouble area for no-heat problems for any year car. Without further information, all I can tell you from here is to make sure that all the vacuum hoses are OK and pushed onto the right connectors.

-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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