As I understand it, one end of the matrix gets clogged. It just happens to be the drivers side. You will still get water to flow through and because the stuff that clogs the matrix doesn't budge, you won't see anything come out. My money is still on the matrix.

 

+1 heater core

 

It seems to me that Brutal's answer / description makes perfect sense. I will be pursuing a new heater core RNR. Thanks everyone.

Cheers

 

Brutal is absolutely right! You can at times flush the core from the feed lines the the firewall...I have seen the matrix door shut, and also even temop blend go bad...But I would try the flush anyway ...

 

Okay, so here they are ... images of the infamous X350 heater core. This is a used unit that arrived today from a salvage operator in GA.

Where I was most confused and rejecting of the clogged heater core theory was my assumption that this was a flow-thru core. It clearly is not. When I mean is that fluid through this core is such that instead of being encouraged to flow from one end to another, coolant will always find the shortest path of least resistance. This could go a long way toward understanding how, and why, these have a reputation for clogging.

As I've not seen the way this is oriented in the vehicle, I can only wonder what might happen if coolant flow were reversed (top to bottom vs. bottom to top?) Would sediment push itself through more effectively, or would it just swirl and settle? Hmmm.

I'm not yet committed to the swap as we've not yet had a day warm enough to do a chemical flush. I really am hopeful that I don't need to go under the dash.

 

I believe the flow is top to bottom, there is a plastic restrictor in the outlet ,bottom, to give the coolant a little more residence time in the matrix.

 

Hey guys
Listen to Brutal. He knows and is right.
Replace the heater core and change ccoolant every couple years.