Heater cores on early 90s Cadillacs are a weak point. Let's all share our experiences on what we have tried, sucessfully and unsuccessfully to resolve issues and get the good heat back. Perhaps this thread might make a good "Sticky".
I have a 1993 DeVille and a 1992 Seville and I've been into the heater cores a couple times. These are my experiences.
For starters, we have owned our 1993 DeVille since 1998. From day one it never threw heat of any measure. Living in Wisconsin and traveling to Minnesota regularly, it is not a good thing to have a weak heater. I can remember driving from St Paul and after 3 hours of driving (with blankets on our legs) it would still be chilly (not freezing, but annoyingly uncomfortable) in the car. On top of that neither of our cars have heated seats!
Finally, 2 years ago I removed the heater core in the DeVille and attempted a fix. I did the drain-cleaner trick which I have used successfully on other heater cores in the past. That is, ya use drain cleaner to clean out the deposits in the heater core. Afterwards, it was better but not significantly. I ended up buying a new one from Cadillac for about $90. The original heater core is an aluminum unit with plastic end-caps. Aftermarket copper replacements are available for less but aluminum has a better heat transfer cooefficient than copper. The car has thrown a good amount of heat ever since. It's a shame I didn't work on it right away after we bought it.
Our Seville (1992) never threw good heat either. We have only owned it a few years, now and since we do not typically drive it in the winter, or very seldom, the lack of a strong heater was tollerable. Until now. Recently, I have been driving it in the winter as I am in the process of repainting our 1993 DeVille. The temp has been hovering around 0 or into the low teens for the last two weeks. It has been cold and I had enough. Last week I removed the heater core (fortunately they're easy to remove on these cars) and gave it the drian-cleaner flush treatment. This time I performed several applications using drain cleaner (crystals) and was more careful than I was on the DeVille project to swish the drain cleaner around inside the heater core to get the ends and portion between the inlet and outlet. A lot of black colored and chunky deposits were flushed out. It would get so hot, in fact, that I could barely hold onto it. I was using rubber (PVC actually) gloves. Afterwards it looked like brand spanking new inside. But then again, so did the DeVille heater core when I finished cleaning that one.
The proof is in the pudding (whatever that means) and the fact is, the additional time flushing it out with drain cleaner was a successfull fix. It works great now, throws a good deal of heat! Before there was scant heat in cold temperatures even when set at 90 degrees. Now, I get wonderful heat and it keeps the interior toasty warm and comfortable. I keep it set at 73 degrees. This time it was a success.
Heater Core Removal Steps:
Removing the heater core from this year of vehicle is easy. Remove the glove box. Before going on any further, turn the ignition on and move your heat/cool setting from 60 degrees to 90 degrees and see that the actuator moves the damper door on the heater box. If it moves, then you know that's not the problem and the heater core probably is. Proceed by removing the computer and pull it away for access.
[It would probably be a very good idea to remove the air bag fuse and separate the yellow connector under the steering column before digging into the electrical components like the book says.]
Continue by removing the AC/Heat mixing damper rod (the little metal rod) by CAREFULLY lifting up the threaded end out of the plastic clip. Then remove the heater box cover (2 screws) and move it to the side to gain access to the heater core. Move to the engine compartment and remove the hose clamps that secure the hoses to the heater core. They're against the firewall and you probably won't even be able to reach them. Use a long screwdriver. On our DeVille, there are spring clips instead of threaded hose clamps and I had to make a special tool in order to remove these spring clips. Again, it's tight in there and its unlikely you will be able to get your hand in there. I will try to post a picture of the tool for others to see on my website. Then, take a long, strout screwdriver or prybar and carefully pry the hose off the heater core inlet and outlet ports. Oh yeah, put a drain pan below the car to catch the antifreeze and also remove the radiator cap.
Go back to the inside of the car and remove the two bolts that secure the heater core to the heater box. You should then be able to remove the heater core pretty easily and pull it from the car. Do your repair or replace the heater core with a new one.
The installation is the reverse of removal. The only thing of note is once you get everything back in place (except the glove box), you need to be sure the little metal hot/cold damper actuating rod is adjusted properly. To do this, turn the ignition on (don't start the car) and turn the heat to 90 degrees. Wait a minute to allow the actuator to move all the way to the end of it's stoke. When it's there, it should be pointing all the way towards the PCM computer on the Rt side of the car. Pull the rod over to the connector and put a little tension on the rod and then snap it into the plastic connector on the actuator arm. Use EXTREME care and support the plastic actuator arm and don't break it! That would be a costly mistake. When I say apply some tension to the rod, I mean you want to be sure that when the door is fully closed there is some tension on the arm to hold the door in it's closed position.
Once you have done this, change your climate control setting to it's lowest or coldest setting 60 degrees and watch the arm stroke the opposite direction. If it doesn't move, check your wiring. Once it reaches the other side check for a slight amount of tension holding the mixing damper door shut. If this is right, you have it adjusted properly and you can put the glove box back in, fill the radiator and try out the heat. You're done!
What are your experiences with your car's heater?