When you get air into your fuel line, you get a whiteish/blueish (light) smoke. Replacing the return lines to the tank won't do a thing for you in this instance.

Tell me more about this voltage check of the glow plugs. Are you talking about checking to see if they glow before you start the car? That will give you a rough idle for a while. After the engine heats up, the idle should smooth out. This is a diesel we're dealing with here. Once the glow plugs heat the engine so the compression ignition can start, I don't believe they are a factor. You can remove the battery, the GP relay, wire harness, you name it, and the thing will continue running. They do not "spark" like a gasser engine does.

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1982 300CD Petrol/Black Leather