After seeing a couple under hood pictures of Rx7's I am getting the impression that the cooling system on my "new" RX isn't functioning as it should. Mine appears to have a moroso pressure cap spliced into the upper radiator hose that returns excess into the overflow tank. Since I only have experience dealing with european stuff that has pressurized overflow tanks, I am wondering where things like factory overflow valve is supposed to be. Also is the overflow tank supposed to somehow tie back into the system?

I was also having a hard time bleeding the system. It seems that there was an air bubble at (what I am guessing is) the water pump outlet and upper hose that was preventing water from flowing through the radiator. What is the proper procedure in bleeding this?

 

Have a picture of your engine bay? There are a few variations among FCs (your profile says '86), and all of them have a line running from under a cap to the overflow tank. Coolant returns to the engine through this same line. There will also be a short line open to atmosphere on the cap of the overflow tank.

Here's a picture of my engine bay out of my album. I have an aftermarket radiator, but everything is set up just like it was when stock. The radiator cap is on the thermostat cover, the hose to the overflow tank runs along the upper radiator hose, and there is a black bleeder bolt/screw on the connection to the radiator.

 

Ah! Now it makes sense about how the overflow works! I don't have pictures of my bay; I'll get some tomorrow if I can't describe it well enough. So my motor does not have a cap in the t-stat housing. It is a turbo motor from a S5, so I'm assuming S5's had the cap elsewhere. Either way, I have an aftermarket "splice-in" fitting with a cap on it about half way down that upper hose, which most certainly does not put it at the highest point of of the cooling system. I'll see if I can dig up some pics of an S5 motor.

 

Found one of an S5. This is the water neck I have. Anyone know if an S4 thermostat housing would bolt to an S5 block?

 

S4 and S5 water pump housings are not interchangeable. Differences with the front covers make it so they won't fit correctly. The thermostat covers also can't be swapped since the bolt pattern is different. What you need is a radiator with a cap on it. Bleeding the system with a cap halfway down the radiator hose is going to be just about impossible.

 

for the record, this is what it looks like:

 

looks like something someone rigged up cause they weren't using the right radiator.