*HELP* losing coolant fluid by having no leaks visible or no

[Pr0t3g3]

So for a while now I've been losing coolant on my car constantly and every week or so i would have to top up my coolant just so my car wouldn't overheat. My guesses were the head gasket at first burning off the coolant because my car is known for bad head gaskets. I have a 2002 Nissan Altima 2.5S. So just today i'm like hell if i want to know what it is and why i'm losing coolant let's do a pressure test on my cooling system and also a compression test. Also i did change the headers today from my pre-cat to a straight racing pipe because of the infamous cat failure which shoots the particles up into your engine basically causing the engine to fail. Besides that i got the results from my mechanic and on the pressure test there are no leaks whatsoever and it help the pressure for over an hour. As for the compression test it went great on my car all systems are great cylinder #1 got 180p si #2 got 175 psi #3 got 185 psi and #4 got 185 psi. So now i have no idea what to do to find the problem of losing coolant if anybody could help me out it would be great. I was thinking the overflow coolant tank but that's just a guess. Also my heater has stopped working in idle which is weird and works when i'm driving and pressing the gas pedal down. I did a coolant flush and refill just recently before the headers change, pressure test and compression test.

[tealseal26]

it's the head gasket. if you pressure test the system and leave the pressure tester on the car overnight you will probably see coolant on the piston in cylinder 3. just went through the same thing with mine and replaced the head gasket.