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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02 E320 Black (198K)
85 300D Blue (147K)
83 300D Blue (299K)
Plus 2 300D Parts Cars
05 ML350 Silver (165K)
02 ML320 White (207K)

My GUESS is it won’t cause a leak and if it does it would have leaked in the near future anyway. Can you flush it backwards with hot water? Hot would probably work better than cold water. I would avoid using a pressure unless it was regulated to below 10 PSI or so.

Also if it leaks you could bypass the core.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

i'm in the same spot with the heater on my '83 300SD. high heat on low fan speed, decreases as fan speed ramps up. once the weather warms up to be playing with plain water, i'm going to do a citric acid flush. if pinholes are revealed, they were there to start with and time was running out. besides, mebbe a stopleak would seal it back up if grunge was all that was sealing the core.

Yep ;

It's time for a careful citric acid flush .

My very first 300CD's cooling system was terrible, it rusted out welch plugs and thermostat housings, when I flushed it the heater began to work but also dripped coolant ever so slightly .

Since the car never ran hot, even in Death Valley with the A/C in 'refrigerate' I simply used a no pressure radiator cap .

I was unwilling to take that damn dashboard out .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better