I have an S4 n/a with a koyo. Since I got the car a few months ago I've had issues with air in the radiator. I would bleed it, only for the coolant sensor to start going off a few days or a week later. It only goes off occasionally, whatever bubble is in it is small enough to get circulated through the system away from the sensor pretty quick. When it goes off it's usually at idle and if I rev it goes away, but sometimes it happens at half throttle or wot only to go away a second later. I thought at first it was just a persistent bubble (it still could be) and have bled it probably 3 times, but I'm starting to think air is being introduced somehow. The radiator has no visible/significant leaks, it doesn't seem to lose coolant. The overflow is above full so maybe some is coming out and not being recirculated. Also the coolant is clean and the engines a very fresh rebuild from a reputable shop, so I don't think its the coolant seals giving out already. I've been told to check the radiator caps to make sure they still seal, I think they're fine but I'll look again. What else could introduce air into the system?

 

welcome to the board.

if i had to guess, you're probably not getting new air in. you probably just never fully got rid of whatever was already in there when you first filled the system.

at any rate, why don't you walk us through your bleeding process.

definitely check (or replace) the radiator cap.

 

if there is a leak somewhere, even a tiny one will act in just the way the OP describes.
so maybe a pressure test is in order, if the engine is new, and the radiator is new, its probably a loose hose clamp or something
also i've seen junk on the sealing surfaces of the radiator caps, its all basic stuff

 

To bleed I remove the top and bottom radiator caps (top located nearest to engine and bottom attached to radiator near battery). Using a bleeding funnel I fill it from the top until the bottom part is full and overflows just a tiny bit (I put rags around the bottom cap so coolant doesn't get everywhere). Then I turn the car on and crank the heat on full blast. I squeeze the coolant hoses to release any big bubbles, rev the car, shake it at the strut towers, and watch for bubbles. I ran it for about 30 min last time and then sealed it up and went on a drive.. no low coolant buzzer on a few drives until a few days later. I will say maybe I should have topped it up cause I did read some people let it cool down and then pop it back open and add a little more coolant, and I didn't.

 

I will check the caps and seals, how would I check if the hose clamps/fittings/etc are an issue apart from things that will be obvious when visually inspected?

 

you look for signs of leakage. if you used worm drive/P clamps you can check those for being tight too

 

If you still have the cold start themowax, you need to crack that water hose that goes up to it, I had so much air in mine and was the source before I got rid of all of that when I went to standalone.

Also, with the top cap off, blow into the air vent on the overflow tank, to force water back up the tube, or when the engine cools off, it sucks in the air in the line first. That tube likes to drain because of gravity if you crack the top water fill cap too.