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 .'

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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

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

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 157K (Nancy)
1983 500SL 125K (SLoL)

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

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

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

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

Dose the SD have the protruding tubes on the fire wall that are part of the system that feeds the heater cores. If it does can you fell if all of the tubes are hot?

On W123 on the suction side between the Monovalve and the Engine is the Aux Water Pump. If the suction side of the setup is clogged up like in the Aux Water Pump it would of course keep coolant from circulating. You could also feel that return line to see if it is getting hot to determine if there is circulation.

If you are getting heat to all of the tubes on fire wall of the heater core and heat returning to the engine is it I would guess your the little heater radiator/s are plugged up with something like a mouse or rats nest in that area that is keeping air from passing through it or there is something else keeping air from passing through them.
How about if there an issue with the ducts and flaps that control where the heat would go?

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84 300D, 82 Volvo 244Gl Diesel

Those heater hoses to and from the monovalve are indeed hot. As stated, heater core has been flushed in each direction.
I think my problem is the blend door covering the heater core is stuck closed. Does anyone know the location of the heater core and/or blend door?

__________________
'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