Okay so I recently got into the rotary game and I've had a bunch of issues already. This issue being that the car will not pull coolant back into the system when the car cools down. I've vacuum filled the system to be sure there are no bubbles and I've ran it with a coolant funnel attached to bleed it. First thing I thought was rad cap or coolant seals so I waited a day and did the cold crank watch for bubbles thing with good results. Cold started the car and saw 0 bubbles. Tested the rad cap, it failed so I bought another one and tested it and it failed! Only to grab my tester and find the other one I was using was broken. So I have 2 good rad caps now. Then I tested the tube from the filler neck to the reservoir, its fine. My hoses aren't collapsed in the morning (or ever), and its not loosing coolant, it takes the same amount back into the system to top it off, never more. I pressurized the system to the spec on the rad cap (I forget,13 psi or something) and came back 18 hours later and it had only dropped 0.5 PSI! What would cause the car to push coolant into the bottle but not bring it back? Good rad cap, Coolant seals seem fine, no visible leaks, no loss of coolant as far as I can tell. Ive been using a 4L jug that I empty the reservoir into and then top the system off with and its still full. Car has 70K Km on the rebuilt motor. Possibly the original water pump, would that cause my issue somehow?

Sorry if this post is jumbled and shitty or if I'm leaving out some really obvious information, its my first post

Car is an 87 with s5 rotors and housing, s4 front cover and everything else I believe. rtek 1.5, 720cc secondaries, slightly bigger turbo (Trying to figure out exactly what it is), koyo rad, blitz blow off valve, wideband AFR, omp delete running 1:100 premixed fuel, N/A racing beat exhaust, no cats, and suspension mods.

 

Hold on, your not overheating? Or losing coolant? Have heat in the car?

Why are we so worried? Also, the cooling system doesn't pull from the overflow unless it's under pressure, and low on coolant IIRC

 

Actually it does when it cools down. Coolant expands when heated, so the extra volume gets pushed into the overflow. When cooled, a sealed system creates vacuum and pulls some back in.
There is supposed to be some liquid in the overflow in order for it to work properly. It's not a catch can.

OP: Check that the hole in the filler neck isn't clogged. The inside opening is quite small. Other than that, is there coolant in the bottle to begin with? Is the straw in the overflow tank submerged?
There's only a couple of parts to the system and none are particularly expensive.

 

Filler neck isn't clogged, neither is the tube to the bottle or the bottle cap itself and the tube inside. Air is getting into the system but it doesn't seem to be losing coolant it just ends up in the overfllow bottle. I always leave it at the full mark with the tube submerged an inch or so. If I don't empty it back into the system manually it will eventually cause the buzzer to come on and the overflow will be completely full and spilling. System holds pressure fine. Rad cap is new from Mazda and I tested it before I put it on.

 

Not overheating, not loosing coolant, have heat. Im worried because when it cools it doesnt pull back into the system from the overflow or collapse the hoses it just gets air from somewhere. Its not exhaust either, cooloant is clean and the overflow bottle isnt getting dirty.

 

Okay so I figured I should follow up on this. It was the coolant seals. Didn' think it was but after a while of it running with a coolant funnel attached at the rad cap there were very small champagne like bubbles coming up into the funnel. Very small. Even if I pressurized the system and fired the car up it wouldn't smoke or drop pressure so I thought they were okay. Had enough of it, pulled the motor for a rebuild and sure enough they had failed. So if your coolant isn' getting pulled from the resivour suspect coolant seals if you can't find any thing else wrong. I thought cap or a small puncture or blockage in the line to the resivour.

 

How and where in the housing the coolant seal failed can hide the issue sometimes. The only other thing I would have suggested trying would be to replace the overflow hose. They’re thin-walled and I could see them staying open with pressure but collapsing under even the mild vacuum seen during cool down.