1. Preparation is Key:
* Consult a Repair Manual: A factory service manual for your specific year and model GMC Sierra is invaluable. It will show you exactly where the fitting is located, what type it is, and the proper procedure for removal.
* Safety First: Wear safety glasses to protect your eyes from coolant spray. The coolant is toxic, so wear gloves and be prepared to clean up any spills. Disconnect the negative battery terminal to prevent accidental shorts.
* Coolant Drainage: Allow the engine to cool completely before starting work. Drain the coolant system to minimize the mess. Locate the drain plug(s) at the radiator and engine block (check your repair manual for locations).
* Gather Tools: You'll likely need a set of wrenches (metric), pliers (possibly line wrenches for stubborn fittings), a drain pan, rags, and potentially penetrating oil (like PB Blaster).
2. Identifying the Fitting Type:
* Compression Fitting: These are usually held in place by a clamp or crimp. You'll likely need to loosen or remove the clamp (if present) before attempting to remove the tube. Sometimes a small pick or screwdriver can help to separate the tube from the fitting.
* Flare Fitting: These are secured by a nut. Use the correct size wrench to loosen and remove the nut. Be careful not to round off the nut; use a wrench that fits perfectly. Penetrating oil may help if it's stuck.
* Other Types: There could be other types of fittings, including push-on types. The repair manual will specify the type and the removal method.
3. Removal Process (General):
1. Loosen the Fitting: Carefully attempt to loosen the fitting. If it's resistant, apply penetrating oil and let it soak for a while. Try again. Use the appropriate wrench or tool, and avoid applying excessive force, which could damage the fitting or the tubes.
2. Remove the Fitting: Once loosened, carefully remove the fitting. If it's a compression fitting, you may need to gently wiggle or pry it off. If it's a flare fitting, unscrew the nut completely.
3. Inspect: Inspect the fitting and the tube for damage. If the tube is damaged, it will need to be replaced.
4. Important Considerations:
* Rust and Corrosion: In older vehicles, rust and corrosion can make removal very difficult. Penetrating oil is your friend here. Be patient and work slowly.
* Fragile Fittings: These fittings are often brittle and can break easily. Use caution and avoid excessive force.
* Professional Help: If you're uncomfortable working on your vehicle or encounter significant resistance, it's best to take it to a qualified mechanic. Damaging the heater core tubes can lead to significant repair costs.
Remember, this is a general guide. Always consult your repair manual for specific instructions for your 1999 GMC Sierra. The location and type of fitting may vary, and the manual will provide the most accurate and safest method for removal.
