How do you repair a heater coil on 1990 Cadillac?

You generally don't repair a heater core in a 1990 Cadillac (or any car, for that matter). Heater cores are complex, finned structures made of thin, fragile tubing. Any attempt at repair is almost always unsuccessful and usually leads to more problems. The cost of labor to replace it often outweighs the cost of a new core, and a poorly repaired core can lead to leaks in your vehicle's interior, causing mold and mildew and potentially damaging other components.

Instead of repairing, you should replace the heater core. This is a significant job that typically requires:

* Significant mechanical aptitude: It's not a simple job. You'll need to be comfortable working with automotive systems, including potentially coolant, and using specialized tools.

* Access to a repair manual: A factory service manual for your specific 1990 Cadillac model is essential. It will show you the exact location of the heater core, the steps involved in removal and installation, and torque specifications for fasteners.

* Time: Expect this repair to take several hours, possibly a full day or more, depending on your experience and the specific model of your Cadillac.

* Tools: You'll need a variety of tools, including wrenches, screwdrivers, pliers, possibly a coolant flush kit, and potentially specialized tools depending on your vehicle's design.

* A new heater core: Purchase a new heater core specifically designed for your 1990 Cadillac model. Using the wrong part will lead to further problems.

* Coolant: You will need the correct type and amount of coolant for your Cadillac's cooling system.

In short: If your 1990 Cadillac heater core is leaking or malfunctioning, your best course of action is to have it replaced by a qualified mechanic. Attempting a repair yourself is strongly discouraged. The labor cost of replacement will likely be cheaper than the cost of parts, potential damage caused by a botched repair, or the headache and time involved.