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

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

I don't know about the best, but here's how I did it on a W123 300TDT:

Removed all 3 hoses from the heater core. Flushed each hose with a garden hose, making a simple seal with my hand. Repeated several times. Reconnected the hoses. Drove car until it's at operating temp. Shut it down, let it cool and changed the coolant. The last two are because hard water was introduced into the system.

I went from no heat to face melting heat.

The three hoses you disconnected were at the auxillary water pump?

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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

On mine when I did it I disconnected the hose at the Monovalve (hose from the Monovalve to the fire wall) and the Hose at the fire wall that come off the Cylinder head (the one that is next to the oil filter).

The other Hose on the Monvalve goes from the Monovalve to the Aux Pump to the Engine. It is part of the heating system but not the Heater Core. That is on the suction side of the circuit (returning coolant).

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84 300D, 82 Volvo 244Gl Diesel