Ok, i have an S4 NA, and it has been loosing coolant, enough to set off the buzzer about once a week. It doesnt seem to be burning it (i cant smell it out the exhaust at all... it smokes a tad on startup but i have a leaky injector or two that i think are the culprit there), but the overflow bottle is completely full. Even when I remove some, it promptly fills it back up, and I think it overflows, which would be where the coolant goes. Would an incomplete or improper seal of one of the radiator caps (specifically the one on the metal filler neck coming off the engine) cause this? Much of the time when i open it up after the car has had time to cool down, there is a vacuum inside the coolant sys. My first step is gonna be to replace both caps and see what happens, I just wanted to ask whether you think this step would be futile or could possibly be the cause.

Cheap enough to change the caps.



With a bad coolant seal, you will see like soda bubbles floating through the coolant at the filler neck.

yea, thats what i figured, and i havent noticed the bubbles, so i dont think its the coolant seals (and certainly hope its not)

caps are #1, could be a thermostat too



and next in line, it might have a pinhole leak somewhere else, that will keep it from pulling a vacuum and being able to suck the water back out of the overflow.

well, just picked two new ones up, so we'll see how that goes. i havent replaced all the coolant hoses, but they all seem to be in good condition

Theres also an o ring on the piece that bolts onto the filler neck. S4

I think it had been the 2nd rad cap, since i noticed it would start to randomly hiss at times (when the car was off and cooling), and if i cracked it open to relieve some pressure before topping it off after the coolant buzzer went off, id close it and it would keep venting. Plus, the seal and inside of that one looked pretty bad, so Im hoping thats the problem

Well, replaced the rad caps and its still doing it... What I found to be odd was that the overflow tank filled up, and then when i released the pressure (got one of those caps with the handle that lets you do that before actually opening the cap), it promptly spewed another pint or pint & a half out of the overflow tube. So remaining suggestions on the board are thermostat, small leak in one of the hoses, or the o-ring on the filler neck. Anything else?

I had the same kind of problem and it turned out to be a broken water seal in the engine.

wonderful... thats what i was hoping that it wouldnt be. Whats normal operating pressure for the cooling sys?