Dragged my 85 300SD out of an 8 year hibernation. I did some tune up stuff and took it for a shake down run. I promptly cracked the radiator so I installed a new one and flushed the system at the same time.

I now have no heat. I rebuilt the monovalve. no change.
I unplugged the monovalve. no change.

I do no suspect the CCU as I hit the buttons you can hear it working and I can adjust which vents to blow from.

I'm thinking I need to flush the heater core.
What is the best way to do this?

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

This is what I did to regain heater funtionality for 'Stormcloud'.
‘Stormcloud’ - Coolling System Remediation–Part 2

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Ok. So I can flush the heater core from the two lines in the engine bay?

One enters the rear of the head on drivers side.
The other runs from firewall to water pump on passnegers

__________________
'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

Yes, just disconnect the heater hoses from the heater core nipples on the firewall.
Repeatedly backflush/flush with a garden hose, until the water runs clear.
Normal coolant flow through the heater core is from left to right.

There can be a lot of packed sediment trapped in the heater core that won't dislodge with just a single flush in each direction.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

If you unplug the monovalve it stays wide open , enabling better flushing .

The heater matrix sits below the level of the inlet & outlet pipes, this causes sediment to settle in and be tricky to remove .

I undo the hoses and use a low pressure garden hose to flush it back and forth, each time you reverse the flow expect the first bit of water to come out rusty....

It takes a while then button it up and use Citric Acid mix (one pound Citric Acid to three gallons of distilled water) to fill it up then DRIVE it for an hour or more ~ my '82 20D required me to drive it 35 minutes each way to work and home again for about a month before I got all the rust and sediment out of it....

Sitting is murder on water jackets and heater matrix's .

You can do this and will do a far better job than any shop because you won't rush the job .

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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

Update.

Heater core flushed. Put some more treatment/cleaner in the system and I have been driving it around the neighborhood. Still no heat.
I can feel the hoses to/from the monovalve behind the false firewall and they are hot. I can push the desired setting at the CCU and get defrost/side/bottom vents to all blow but its cold.

Is my Blend door stuck? What is the best way to access it on a 85 300SD?

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

I have been contemplating gushing my cooling system but I don't want the strong acid inside of the Heater Core.

So I have been looking on other sites. One guy on another site suggested cheap Vinegar white is usually %5 acid.

Harbor Freight has a hand fluid transfer pump which I have used in the past for Fuel, Water or Coolant. I figure I can use that to circulate the Vinegar through only the heater core and the Vinegar is not likely to eat up the Heater Core.
I think using the hand pump is also safer then using a garden hose. It gives circulation without a lot of pressure.

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84 300D, 82 Volvo 244Gl Diesel

Vinegar is far more acidic than the citric acid .

Do it correctly and you don't have to worry, I hear your concern about damaging the water core but if any leaks appear, it's because that part was already bad and only the rust crud or solder bloom was keeping the coolant in .

As far as no hear, push the defroster button, this will by pass the entire system and give you max heat to the windshield .

If you get a blast of cool defroster air I'm betting your heater core is plugged, that's very, *VERY* common .'

__________________
-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

Please explain how 5% Vinegar is more acidic then the recommended then the FSMs 10% Citric Acid.

I can tell you for a fact that Vinegar does not clean my Distiller as well as Citric Acid does.

On top of that since you are flushing only the Heartier Core you are not getting in hot and heat increases the reaction when it is connected to the Engine.

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84 300D, 82 Volvo 244Gl Diesel

Here is a picture of the:
From Harbor Freight
PITTSBURGH AUTOMOTIVE
Multi-Use Transfer Pump 3257 $6.99

Not for hot liquids.

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84 300D, 82 Volvo 244Gl Diesel

Vinegar typically has a much lower PH.
Sure, this is affected by concentration, but 5% vs 10% isn't a meaningful distinction without an accurate PH reading.

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1981 240D 4sp manual. Ivory White.

Citric acid is also far more effective at dissolving lime and calcium buildup than vinegar without attacking the metal.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

People drink in particular apple cider vinegar. I tried it 1/4 cup; it did nothing harmful.

I don't think I would do that with 10% citric acid. Even though some beverages have citric acid in them.

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84 300D, 82 Volvo 244Gl Diesel

I have been getting setup to do a acid flush and filled the block with water to check for leaks. What I found is I am getting bubbling even after the air should have been gone. I think either the head gasket has issues or the head itself.
Note that in the attached pic the temp is not even up to 40C.

I also manage to badly crack my Windshield so this Month had not been a good one for me.

I am going to be even more controversial. The FSM said you can use Oxalic Acid. And as it happens I have more Oxalic Acid powder then I have Citric Acid Powder on hand. So I am going to use that on only the Block.

From the other sites I have looked on as the Oxalic Acid reacts it turns green on to a dark green. Also on one of the sites a Chemist said that the Oxalic Acid chemically works better on rust then Citric Acid.
If I picked the right site this guy tests various metals in various acids and gives results.

Since the Radiator is Aluminum I assume the Heater Cores are Aluminum but I don't know as I have never seen them. In the test the Vinegar did not react much with Aluminum: "vinegar-also caused bubbling on the lime deposits, though less vigorous, and very few bubbles appeared on the aluminum coupons."

https://www.practical-sailor.com/boat-maintenance/descaling-solutions-for-boats

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84 300D, 82 Volvo 244Gl Diesel

Heater core was flushed with garden hose from lines entering engine bay. I spent an hour flushing each direction until it came out clear. Still NO HEAT even on defrost setting.

__________________
'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R