I got s4 water pump housing/thermostat with the rad cap setup, *I put it on my car w/new s4 water pump since I couldnt source s5 parts at the time*, and S5 radiator with the rad cap on the long neck.


So dual cap setup with 2 overfill nipples. I have tee'd off the line going to the overfill tank. A radiator cap rated for .9 at the thermostat, and a 1.3 at the radiator. I seem to be filling up the overfill tank after a few weeks , and getting the low coolant buzzer. I was thinking the one at the thermostat would be the higher point in the system and would use the lower rated cap to do the expansion. *I could also have a slight coolant seal problem if im filling up every few weeks, but ill have to diagnose that later, it could just be that im dumping the coolant into the overfill and not pulling it back in, my running temps are great, 180 all day *.

My guess is that T, or the rad caps are not functioning the way I would assume, hence it could be releasing coolant but not pulling it back from the overfill tank. What do you guys think? I know I cant be the only one rocking this setup, its basically like a s4 w/ an s5 rad.

Maybe I need to switch out to a S4 thermostat cap housing that doesnt have the overfill nipple?

 

remove the line from the cap with 1.3 pressure rating and run a single line from the .9 cap to the tank. it should work fine in that configuration.

if you replace your plastic waterneck cap piece with one that doesn't use a releif cap it would solve the issue completely. (pretty sure you can use the one from an FD)

 

...or you could use a stock S4 rad cap (non venting type) on the radiator and cap off the nipple on the rad.

 

One of those caps should be just a cap, not a pressure relief cap. Get a non pressure release cap for one of those spots.

Both of those caps have a vacuum release valve in the center to allow the cooling engine to suck the coolant back from the overflow. Check to see if they are free moving and unclogged. If they are, then move on to the cooling system.

One problem you are going to have is that when you pressure test on the rad opening, the coolant neck cap will pop, if you test on the coolant neck, the rad cap will pop.

If your caps vacuum valves are free and clear you have some type of leak that is permitting air to enter the cooling system instead of coolant returning from the tank. The first job is to pressure test the cooling system. 'Rent' a cooling system pressure tester from autozone or oriellies. Pump it up to 25-30 psi. You will likely find the leak at a hose clamp on the radiator or the heater.

If you don't find the leak on the coolant system, then the leak is probably in the return line from the expansion tank at the tee or there is a pin hole somewhere in the 20 year old line. Since you have now replaced one of the caps with a non pressure release cap, you can run new line with no 'tee'. The new line will be tight and leak free.

Problem should be solved!

Good Luck and don't let anyone tell you that you need a rebuild!

 

so the non-relief cap looks like this?

Seems like it would still allow access to the relief nipple since it doesnt plug the hole. Perhaps I would just than need to block off the nipple at the rad?

On mazdatrix site they got this which would probably do the job but im thinking id want the highest side to have the relief cap and thats the thermostat side.

 

i have dual caps as well. i have the plastic piece under the cap on the thermostat housing without an overflow nipple. stock cap on the radiator, 1.3bar mazdaspeed cap on the thermostat housing.

 

I bought that aluminum piece when my plastic one cracked...didn't realize there was no nipple on it. Since I didn't have an S5 rad I just drilled/tapped it and installed a barb fitting. It would probably be easier if you just got a non-venting cap and bought that piece and used the rad nipple. However, I agree that I would want to relieve pressure from the highest part of the cooling system.

87 t-66: If there is no nipple on your thermostat housing, and a non-venting cap on your rad....where does the pressure in your cooling system go?

 

there is a nipple on the radiator, its an s4 koyo.

 

I just made a block off plate, will see how it works out.

 

I've got a Fluidyne rad on my s4. I've got a .9 bar cap on the rad and non venting cap on the thermostat housing.

 

I know that this is an old thread, but basically you have the same setup that I have currently.
I ended up with a s4 front cover on an engine, so I am using my s5 radiator with a s4 waterpump, housing, neck..
Did the block off plate work?
I'm trying to figure out what to do with the situation that I've got right now.
I figured since the t-stat housing is the highest point you'd want a "radiator cap" there and then a normal cap on the radiator.
And then run the overflow line from the t-stat housing as well.
If your setup worked then I will probably copy what you've got going on....

 

yeah it worked til i blew the engine in december from another issue. I will be using it again. Eventually thru running the car , it should knock out all the air bubbles from that area being the highest point.

 

Hey guys, I seem to have the exact opposite problem as hypertek: s5 with s4 rad. And, I've just realized there doesn't appear to be a rad cap anywhere, and so my cooling system keeps overpressurizing and blowing weak points in the system. I'm new to cars/rx7s and I was wondering if anyone could provide any insight on how to go about fixing this?

 

You need an s5 rad. You cant fit an s4 filler neck to an s5 front cover. S5 rad, no way around it.

 

That is exactly the route I went since I kept having issues. Then last summer I got a machine shop it put a freeze plug in is what it looks like. I put some jb weld over it so its sealed and I have no issues. My dads T2 I got that aluminum piece that you see at mazdatrix. I have heard that because its the highest point in the system that you should keep if but I gave up on my n/a and I have a koyo n flow so I said screw it. Looks like you are doing a lot of work from the threads I have seen.