1987 300d. Am I leaning in the right direction the heater core is clogged? I have no heat even on defrost. The monovalve itself is hot and the hose going to it from the engine is slightly warm, but the hose on the back of the monovalve going to the cabin is completely cool. No warmth to touch.

If it is the beater core. What hoses do I use to flush it and How? Sorry aytocrect keeps calling it a beater core.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k