Heater cores on 89-93 Cadillac Deville's are a very common problem, usually clogging on the lower half and causing the core to have a half hot and half cold situation. On 91 & up Deville's they have a split level heat duct, which means that the top half of the heater core is divided from the lower half of the core. When a heater core gets clogged it usually causes the lower half or return half of the core to get cold, which will result in a loss of hot air to the driver's side of 91 & up Deville's. This can be a real chore to troubleshoot. The first thing people usually go for is the heater core, but they only warm up the car and feel the feed and supply hoses of the core in the engine compartment, and often times they will feel hot. That doesnt necessarly mean the core doesnt have a restriction in it. You have to check the core itself. Now, the 89 & 90 Devilles have a hollow heater box, which means no split level duct system. So, when you get a clogged core, you just lose heat. These problems are common on these Deville, I own a 90 & 93 Deville, and I just went through all of this, and it wasnt easy finding the problem, but fixing it was. Thats why I developed this site, to help others with the job and save them from having to go through what I did just to find the problem.
So, now we have to figure out how to confirm if the heater core is good or bad. To get to the heater core on 89-93 Devilles is about the same. The first thing to do is, get those bottom panels out from under the dashboard, next would be the glove box, after that you will look inside the dash and see the heater box and about a million wires. On top of the heater box, next to the programmer, you will see a little box with a peice of linkage connected to it going toward the driver's side, pop that peice of linkage off on the heater box side. Next, disconnect the wires that go into the top of the little box (mix door motor), then if you look to the right of the motor, you will see another bigger box screwed to the side of the heater box, its about 10 inches tall and about 2 inches thick, thats your programmer for your climate control. There will be 3 screws holding it on the heater box, one is on the top of the programmer towards the back, then one on the bottom and one in the middle, the screw on the top is a little difficult to access but I used a 1/4" nut driver with a 3 inch extension and a 7mm socket, and got it off without a problem. Ok, now you have the vacuum lines on the bottom of the programmer, in the middle is a little nut, I used a pair of needle nose pliars to unscrew it cause its a pain to get a socket in there, it will be a little difficult to pull of once you get the nut off but it will go, just be careful not to break the lines. Next is going to be the wire harness you see next to the vacuum lines, ok now the programer is disconnected, just pull it out through the glove box opening and its out. Now if you look next to where the programmer mounts you will see a seam, thats the access panel for the heater core, there will be 2 screws, one on the top corner and one on the bottom corner, take them out and seperate the panel from the box by pulling it to the right and then straight out towards you, and remove through the glovebox opening. Ok, now all thats done, make sure you have all the hardware in a bag and put it to the side. Now take your programmer and slide the vacuum lines back on it and re connect the harness (as per photo at bottom of page). Start the car and use the climate control panel to turn on the heat, set it at 90 degrees. Now, wait for the car to warm up and carefully, feel the heater core for cold spots. Now if you have a section of the core that is cold, or it never gets cold, and your feed hose is getting hot, you have a clogged heater core, and it needs to be replaced. Don't bother trying to unclog it because I have never heard of anyone having luck with that, and since the car is not new anymore the core will probably start to leak soon anyway, and new one is 36 bucks at autozone, do it right and replace it while you have it all apart.
Now that you have all that stuff apart and you know the core is bad, you need to remove the bad heater core. Now here is where the 89 & 90 and 91-93 Caddys are different, On the 91-93 Deville, access to the clamps and heater hoses are ridiculous. GM has more stuff tucked into that area than anything you could imagine, depending on what position the squeeze type clamp is in, the job could get ugly here. It took me about 30 or 45 minutes to get the clamps and hoses off, and a lot of cursing but it can be done, just have patience. Basically you remove the two hoses making sure to have a drip pan under the car, and then pull the heater core out through the dash, making sure to hold it up-right and put down a towel on the carpet to keep coolant from staining. Now if you are doing this same job on a 89-90 deville, you are in luck! It took me about 30 minutes to do the whole job, there is a lot less stuff tucked away by the core hoses than on the 91-93. The installation is reverse of the removal. Hopefully that helps some people solve there heating problem on there Caddy, I know mine are both nice and toasty now. If anyone has any questions about this procedure feel free to e mail me at firstname.lastname@example.org Happy Motoring!!