I have pressure tested the engine. No seal leaks.
I have traced down all the weak clamps and replaced them all.

I am now finding that ALL the 14 PSI caps I find seep pressure down to about 11.8 PSI.

Should I return to using the original 16 PSI caps and see if if those seep down to 14 psi?

ARGH!

 

Are you testing the caps themselves? OEM or aftermarket? If you're testing the system and its still seeping down maybe you have a leak you haven't found (hopefully not a coolant seal).

Matt

 

Yes I tested the caps

Now testing the whole system.
I distinctly hear bubbling from the overflow tank

This means the cap there is seeping.

Why can I not find a way to keep the pressure at the 14 psi as prescribed?

 

Are you using an OEM cap? Or an aftermarket one? OEM or aftermarket AST? Deleted AST?

 

The car is stock

Using aftermarket caps
All new
All act the same

 

time lapse of about 40 minutes of time.


eliminating the caps as the cause, i have blocked all overflow holes in the necks and there is no air escaping to the overflow tank.

This leaves a completely closed system and yet the pressure slowly goes from my testing pressure (18 PSI) to 0 over the course of an hour or so,

I cannot locate the leak.

Is this slow depressurization normal or should it hold.

I recall that it should hold until released.

 

It is possible it is your tester itself? All possible vent sources blocked off, along with no visible external leaks, usually indicates an internal leak.
Do the test several times in a row letting it leak down each time without running the engine. Pull the plugs and rotate engine by hand, check for coolant expulsion. If none in combustion chamber, drain oil and check for coolant in sump. Theoretically possible for a outer coolant oring to leak into oil pan.

 

Andre...

I did run the engine with the tester still at 16 PSI. It didn't budge when I started the car.
It only started to indicate a pressure increase once the car started to warm up.

Regarding your advice...

The advice you gave me is really good and not at all improving my mood.
You know you could try harder at being more upbeat.

I'll do this now I suppose - it seems the only way to know for sure.

 

My engine failed due to coolant seal failure. The combustion gases would not pressurize the system, but failed the "champagne" test, and consumed small amount of coolant from week to week.
Mine would also leak down very slowly and only get to 5 psi over night. 16 to 10 psi in a couple hours.
Sorry to lead you in a negative direction but I know you are trying hard to solve the mystery. I removed and tore down the engine after I heard the coolant buzzer for the first time.

 

Definitely not trying to jinx you or anything....