My wife turned on the heater in her '93 FB the other morning, but nothing but cold air came out. She set it all the way to 90 degrees and still cold air. Everything seems to work OK otherwise. The vents change over, defrost to floor, the other way around, air comes from dash vents when it is supposed to. I checked the hose coming from under the water pump and it gets plenty hot about 2/3 way up. The water is not going through the core for some reason. I have a '91 Olds Custom Cruiser with a very similar HVAC system and I checked the service manual I have for it and it said the gray plastic line, located under the glove box, beneath the dash, controlled the hot water valve. I hooked a Mity-Vac to this line and attempted to open it, but still no warm air. There is plenty of vacuum coming from the throttle body, and as I say everything changes over, so I don't think there is a vacuum problem. However, the colors of the lines might not be the same for Cadillacs and other GM cars. Does anyone have a factory service manual that they would look at and see for sure what color line goes to the water valve? Also, any ideas or suggestions as to what the problem might be? Thanks for the help!!

My 94 LT1 did that shortly after I got it, I popped loose a couple heater hoses and flushed it with the hose, then blew out with my mouth as much water as would go, and then filled it up with CLR with both hoses held up high, and then let it sit for 10-15 min, then flushed alternately back and forth till it was clear. Now I have heat that would rival the sun...

I would imagine a 93 would be similar to my 94 (likely the heater core is the same).

Check the thermostat first, make sure it is ok, then go after core....

I will give it a try tomorrow. This will be the second Winter that we have had this car. She said that last year the heater did OK, but nothing like you should expect. I put a new thermostat in last year and the hoses are getting plenty hot, so I feel confident that it is reaching operating temp. Maybe the heater core is blocked. I will pick up some CLR tomorrow and give it a try. I really don't look forward to tearing out the dash to get to all the switches and actuators. I don't know much about the Caddy HVAC system, anyhow. It is a little too high-tech for me. Thanks for the suggestion. I will let you know how it turns out.

IIRC one hose should be hot (supply, top line) and other should be cooler, as it has given off some of its heat.

If the core is plugged, next is the programmer. Check for codes to ensure that it isn't a problem there.

Core on Cadillac's is very very common, more than any car I have ever owned.

The top hose is hot about 2/3 way from the engine to the core inlet. There is no water flowing through the inlet hose. Where is the programmer? How do I check for codes? Thanks again!

How about flushing the core? If the hose is only hot 2/3 way in then pull the entire hose and see if there is blockage in the hose itslef. Usually the core will get clogged and not the hose -- but stranger things have happened.

Just hook up a garden hose and flush into either end of the heater hoses. In one hose and there should be sufficient flow out of the other hose. Just pull them loose at the water pump and let her rip!

Unfortunately with such an old core, they can be clogged. Another downside is that monkeying around with it can provoke it to spring a leak. You might be able to get away with it so in your shoes I would try flushing first.
Just keep your eyes peeled for a leak down the road!

The FWB heats like Hell! I pulled both hoses loose and jammed the garden hose in the long one that goes to the front of the engine. I opened it up and water flowed from the other top hose that goes to the rear of the intake. The water was pretty and clear with no signs of rust or scale that might have restricted the flow. I looked at my Olds that has the same setup, except my water valve is under the hood. By the looks of it, there is a bypass, meaning the water valve must be opened to let water flow through the core. Anyhow, I put a Mity-Vac on the gray line under the dash, (the one my Olds service manual says opens the valve) and pumped it up hoping the valve would open. I pushed more water and compressed air through it and not feeling too confident, put it all back together. A couple minutes after I fired it up, the lines were getting warm, as was the air coming from the heater. After about 5 or 6 minutes, the air was hot enough to bake biscuits! Well, maybe not that hot, but plenty warm. Anyhow, looks like Mama will stay warm on her daily 4:30 AM commute this Winter. Thanks for all the help and suggestions.

I alternated back and forth to make sure I broke up as much as I could. I will probably do it again soon just to do it. If it plugged up last year, I am sure it is building up now. It would flow clear, then when I reveresed it, it would go cloudy again, then clear and then reverse it, cloudy again, etc. It took a while.

To reduce it, always use distilled water to fill up the cooling system.

Also, do not remove the restriction you see in one of the hoses, it is needed to prevent premature heater core wear and to keep cooling system noise down (some people complain about hearing the water move and gurgle). When it is removed, the cooling system will bypass the heater core, with it 100 plugged, max cooling system performance. Different geographic areas may change the restriction (Alaska vs Florida).

I guess I neglected to say in my other post, I also alternated the flow with the hose. I started with the long hose from the front and then to the one in the rear behind the throttle body. I flushed in both directions. I did not see any restrictors. Could be someone has already removed it, as it looks like the hose assembly has been replaced. The clamp on the back hose is a screw type instead if the spring type. Where would the restrictor be? The car was originally purchased in Texas, if that would have anything to do with the type of restrictor or whether or not it had one.