There should be a heat shield under each injector, they are also a one time use item as they form a seal between the cylinder head and injector nozzle by getting crushed. I have good luck with reforming and reusing, but that's another topic.
Sometimes when someone removes an injector or injectors they reinstall injectors without replacing the heat shields, when this happens carbon will build up all over the injector body and also covering the heat shield. One way to find out is to slide a small maganent down the injector bore to see if it sticks to the base of the injector bore where the injector heat shield should live. If it sticks there is a heat shield there, if it doesn't.. Well there isn't one.
If there is heatsheilds down there, spray some penetrating oil down there to help loosen them up. Sometimes you can blow the shields out by cranking the engine over.
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