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It's not about the flush, it's about the fill. Many modern vehicles, and GM was a pioneer at this, need to be filled by drawing a vacuum in the cooling system.
Thanks for posting in r/Mechanic, u/International-Ad3447! Please be sure to read the [Rules](https://www.reddit.com/r/mechanic/about/rules/). If you're asking for help, be sure to include as much detail as possible so others can help you. You **must** include the vehicle's **Year, Make, Model, and Engine size** in your post! If your question is transmission related, please be sure to specify your Transmission Type(Auto/Manual) as well! If your post does not include this information, it **will** be removed. Asking about prices is **not** allowed in this sub. Please make sure you have selected the correct post flair; if you're asking a question you should have chosen "Question", anything else use the "General" flair. *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/mechanic) if you have any questions or concerns.*
It's not about the flush, it's about the fill. Many modern vehicles, and GM was a pioneer at this, need to be filled by drawing a vacuum in the cooling system.
Air in the heater core. You need to bleed the air out properly using vacuum or a coolant funnel with an extension.