I am having heater issues with my 83 300D. Thought it was monovalve but have replaced with a Klimakit and still having the same issue. For this test I have the valve zip tied open so I'm sure I have good water flow. When I have the fan on low, the heat is reasonable (100 degree) but when I turn it medium or high the temp drops to 85. So I tried a heater core flush with only water (no chemical, no air pressure). A LOT of rust came out in both directions. Flushed both directions (forward flow and reverse). Since heat is limited, I am guessing that the core is still clogged. Have gotten some counsel that doing a chemical flush might result in "exposing" holes in the core and then having to deal with a core leak (which would likely be a core replacement) .

Q: Do you believe there is a significant risk of exposing a leak after a flush on a core as old as mine? I would rather live with limited heat than HAVE to replace a core.

What I may do is have a few more garden hose flushes and see if they will loosen up the gunk.

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