My '87na coupe is running well. Problem is that it's too damn hot where I live (SoCal desert) and the a/c is marginal.
It's been converted to R-134 and I put an oe electric fan on the condenser core; still running the mechanical fan and all the shrouds. I have a new blower motor to install when I get to it (anyone done that? Must be hard to access in the passenger footwell...).
When I installed a/c in my '66 Mustang, I put a mechanical shut-off valve in the heater hose that supplied hot water to the core. Many manufacturers use those, with a linkage to open the valve when you select heat at the dash. In my case, I just opened it by hand. Winter came, I opened it up. It worked well.
I think I should install one on my RX. Looking at the coolant flow, the driver's side hose is the supply hose from the water pump, the passenger side returns to the radiator. I'd rather put it on the passenger side hose due to accessability, but I'm concerned about too much pressure in the heater core causing a rupture. I could just run a hose from the block outlet to the radiator inlet, but I'd be concerned about rusting out the core.
Anyone out there done this to their car? Any experience with a shut-off valve (a $10 item at the parts store)? I need a cooler cab!

 

The blower motor isnt to hard to get to. It drops right outa the dash kinda nicely. Not to sure about the shut-off valve idea. Drive faster maybe?

 

Perfect car for that..

 

Its what I do, I didn't like the AC.

 

The black line is the old way it was hooked up. Undo the hose at the firewall and swing it around so it is horizontal instead of vertical. Get some more hose and run it up and into where the water comes out of the block and into the firewall (like the red line). Basically, the coolant will bypass the heater core and run back into the radiator. I had a parts car that was like that and I had no issues with overheating or leaks. Ran fine. Just make extra sure that the hose clears the clutch fan and all those other belts.

 

The PO of my 87 NA put a valve in just like you talked about because the logicon didn't work and he was too old to know what the internet was. It was his way of controlling heat or outside air coming through the vents. He put it in the engine bay on the drivers side against the fire real close the oil filter. I attached a pic. He closed it when he wanted outside air, and opened it when he wanted heat. Never had any pressure problems or overheating. I'm not saying its smart, but he didn't have any problems.

 

That's exactly what I'm talking about. I was hoping to cut the passenger side hose instead, because it's cheaper and easier to get to, and I did the driver's side hose and the pedestal o-rings a couple of years ago, but I think I probably should do it the way it was done on your car. That keeps the pressure off the heater core, but allows it to stay full of water so it won't go bad. And I'm sure it will drop the a/c temp 5 or 10 degrees, which will be great around here. Thanks for the pic!