Basically you need to disconnect the two rubber hoses to the heater core and using a hose on the heater core OUTLET flush water through it. Hence back flushing as the water is flowing backwards through the heater core.

You'll need to do it steps.

First, locate the in and out hoses of the heater core. The in comes off the head, and the out goes to the water pump housing. Remove the two lines. Now, take your garden hose and flush the heater core backwards, putting the garden hose onto the output hose and letting the water flush the crud out through the input hose. Don't use too much pressure since the heater core is probably fragile at this point. Flush it until the water runs nice and clear. Then, blow the water out of the heater core with an air compressor and low pressure, or with your mouth (carefully, to avoid ingesting coolant). Hook the lines back up to where they came from. Top off the coolant. Then get the car situated so that the front of the car is up higher than the rear. You can jack up the front, put the car on ramps, park on the end of your driveway if it's a steep incline, etc. Start the engine and run it at ~2000 rpm with the heat on full hot and low fan speed. Once the engine comes up to operating temp (the temp you see every day driving) make sure you have nice hot heat, check to see if the coolant level has dropped, and check the upper rad hose to see if it's solid or full of squishy air. Put the car back on level ground, let it cool, and top off the coolant.

Done.

Better yet, watch this
https://www.youtube.com/watch?v=baAAYM5Cl00
(and we can skip all the comments about the guy that made it, regardless, its a good video)

Dave has great vids, but I would skip using any chemicals until you try plain water first. These heater cores are old and burst just from normal use. I wouldn't put anything stronger than vinegar in one of them. That said, I'd need to flush with water a couple times before I even thought about using even vinegar. Heater core R&R is such a PITA that it should be avoided at all cost. I did it once on my 84 190D. The only way I'm taking the dash out again is for a crack free 87+ replacement.

Haven't backflushed the core yet because it's been freezing. Raining now so i might try today... my coolant looks clean and new though... if the core was full of crud wouldn't my coolant look nasty too?

In other news, opened up the monovalve and the diaphram is ripped. It pops out when I turn the CCU off, and pops back in with any of the air buttons. I'm assuming it pops out to seal off the hot coolant, and the ripped diaphram lets hot coolant into the system... so this isn't my problem.

I could feel the aux pump running with the key in the on position. I tried with the monovalve open but didn't notice any coolant flow...

No, not necessarily. The heater cores are normally made from Copper or aluminum, and as a consequence, tend to corrode differently than the steel of the block. You made not see much in the rest of the cooling system. Also, water flow is truncated away from it when it is off, therefore it is more sedimentary in the core vs the rest of the system.

I know this sounds strange but the next time you drive it at highway speeds do this when its safe of course- apply your brakes hard and see if temp increases coming out of the vents. I've had 2 MBs that would only produce heat at idle or during rapid deceleration.

...and what the hell is the explanation for that

good luck

The heater core collects all the junk that circulates in the cooling system. It isn't a high flow components and it's out of the loop a bit. Back flushing loosens all the crap and blows it back out the input.

how do you know the monovalve is working? its the weakest link in the system, i'd thoroughly rule it out before doing complicated flushing of this and that. they break easily. if yours is still original to the car it could very well be shot. they can fail in multiple ways such that you get full heat all the time, or no heat, or weak heat, etc. even if you're getting voltage to it the valve itself could be faulty

I would repair/replace the mono valve before proceeding.

It could mean the rapid deceleration may be:

1. Causing a blend door to shift to max heat position temporarily.

2. Causing the monovalve to temporarily shift into a position to let max coolant flow.

3. Causing sediment blockage in the heater core, to/ from the heater core to temporarily shift to allow more coolant flow through the heater core.

Any more guesses?

Agree with replacing the torn diaphragm monovalve with a known good unit before doing anything else.

One more thing: I would never do a citrus acid flush unless I am prepared to replace the heater core- a big job from what I've read. Radiator may also need replacing after the citrus flush.

Awaiting the monovalve diaphragm part... i'll pop that in and see if it helps.

@ivandrocco - THIS IS THE EXACT SAME PROBLEM I AM HAVING NOW

I am having clicking from the CCU, lukewarm heat, and very low power (3.5v) to the monovalve on defrost position.

The clicking is driving me mad...

The clicking completely stops ONLY when I disconnect fuse 8 at the fuse block (heater blower motor/AC compressor)

I put in a good working CCU and the same clicking sound occurred, so the CCU is not the problem.

Can't figure out what is wrong, but this is what I've found so far.