Opened Filler Neck Cap, Heard Flushing Sound, No Coolant inside
 

I'm a new FD Owner and while getting know my car I opened the Filler Neck Cap and heard a flushing sound and I don't see any coolant inside, its empty.
Shortly after I put the car in the ON position the add coolant buzzer lights and makes the sound as well. I'm afraid to turn on the car.
I've checked the coolant level in the Coolant Overflow tank and everything is good there.
Is there supposed to be coolant inside filled in the Filler Neck? and if there is, any reason why the coolant 'flushed away' when I opened the Filler Neck Cap?
Any help is greatly appreciated!

You don't check your coolant level through the overflow. You check it at the filler neck and yes it's supposed to be at the top. The black sensor on the front of it is the low level sensor. If it's dry then the buzzer goes off. If you took the cap off and heard a flushing sound then it's 99% likely you did it when the car was still warm and the system was slightly pressurized. What you heard was normal. It was the system equalizing it's pressures with the atmosphere. You need to fill and properly burp your system. Buy a lisle funnel and do it. Don't not buy it and don't do it. Don't not buy it and do it some other way. Buy a lisle funnel and bleed your system

When cold, an intact and sealed cooling system should actually be under vacuum. The flushing sound may have been as a result...filling an air pocket.
I agree the Lisle funnel makes it easier, but in a pinch....fill it at the neck with a 50/50 mix, replace the cap and squeeze the upper radiator hose 5 or 6 times. Open the cap refill and repeat until it's full. Check the over-flow tank's dip-stick and make sure at the full mark...not above or below.
Continue to monitor the levels both at the filler neck and at the overflow after running it up to temp a few times and letting it completely cool.. If the overflow becomes 'overfilled' when cold, then check for any leaks and replace both coolant caps, making note of where the spring cap and non-spring cap go. Continue to monitor. If you continue to loose coolant or the overflow continues to be overfilled, have the system pressure-tested.

Really? I never knew that. I don't recall ever removing a cold cap and having any sort of effect on any car. Or maybe it has and i never noticed. I'll be more mindful next time

Yep. If your system is nice and tight, try pulling your spring cap off when it’s completely cooled. The vacuum formed naturally will remain. It’s not huge, but I can easily feel it on my car. I have to lightly pull up the cap with my finger tips to break it. If I watch closely I can usually see the fluid level fall a couple of mm too.

My car definitely doesn't do that. It just comes off lol. I know I don't have any leaks though, internal or external on any of my cars. It's not something to do with elevation? I'm in florida. I feel like this is something I definitely would have noticed at some point in my life.....

Mine definitely “sucks” when i pull the filler cap cold. And the fluid level drops a bit. I thought mine was the weird one.

:dunno: The vacuum that’s formed by the coolant cooling and contracting pulls coolant from the overflow back into the system. On my car that vacuum definitely remains even after going completely cold. In fact just yesterday I pulled my filler cap (AST deleted) and it was still there after several months sitting over the Midwest winter.
You might check to make sure your overflow is full and not running dry. Or your caps might be getting a little old and not sealing quite as tight?

You forgot to write warming up the engine with the open cirkulation of the heater system to get out the air.

Thanks for the replies guys. I'll definitely buy a Lisle funnel, fill it at the filler neck and bleed the system. I double checked the overflow tank and It is overfilled and way above the Full mark. Is that something I should be concerned about before I fill the filler neck, and bleed the system?

It could be. Check the thin-walled hose going to the overflow tank to make sure it’s not kinked or plugged. Then have the system pressure tested. It sounds like the coolant isn’t being pulled back into the system during cool-down.

I finally went ahead and bought the Lisle funnel and a new radiator cap(AST deleted) and started the burping process. weird thing is the air bubbles never stopped even after a while. is it possible air is coming in through a leak somewhere and making endless air bubbles?
I also did some more testing. Coolant is being drawn into the system during cool down. Unfortunately, I still hear the flushing sound when I open the filler neck cap when the car is completely cold. Another thing I found interesting was when I top off coolant to replace the coolant lost from the flushing sound and the next day I turn the key to the ON position, the add coolant buzzer doesn't go off/sound. Shortly after when I open the filler neck cap, it makes the flushing sound with no coolant in sight and the add coolant buzzer goes off when I turn the key to the ON position again. Not sure what that means..
I'm thinking air is coming in through a leak somewhere but any other suggestions to what is happening?

The constant bubbles isn't a good sign. Your next move is a coolant pressure test. Pressurize it to 18-20psi. It should hold it. I always pressurize a little higher than operating just to be sure that nothing is JUST barely holding.

I was able to do a coolant pressure test. I opened the filler neck cap, flushing sound, no coolant as usual so I topped it off. I put on the coolant pressure tester on the filler neck and pressurized it to 18 psi and it did not hold. It went down to 16 psi in about 15 minutes. It took about an hour for the needle to go from 16 psi to 15 psi. I didn't see any visible coolant leaks but I'm new to fd ownership so I don't know where to look.
When I took the coolant pressure tester off the coolant was gone as usual.

Its going somewhere. If its not on the ground, it could be pooling on top of the engine (likely you would smell it burning on a hot engine, though), its filling up your overflow tank, or the engine is burning it.

look in the 3rd gen faq section for the big cooling thread. Have a look at all of your hoses and connections.

It could also be leaking from the tester itself. Don't immediately go to the worst thing. Verify that it's a proper seal at the neck. When you attach the tool and pressurize it, spray it with soapy water and make sure it's a good seal.

I think you're right cr-rex, I might have not done a proper seal at the neck the first time I tried. I did another test today for an hour and 30 minutes. I pressurized it to 18 psi and it did hold, needle did not move at all. After I took off the tester the coolant was actually there where it's supposed to be unlike yesterday where the coolant was barely visible.

Update: checked coolant today in the morning and for the first time there was no flushing sound, and coolant was actually present about halfway full in the filler neck. I didn't add any coolant and just left it as is. I checked later again in the afternoon, and coolant was full to the top where it should be.