1G Coolant pressure test: Is ANY leakdown a fail?

[GSLENK]

Short version:
When performing a coolant pressure test, is about/less than .25 PSI (a quarter of one PSI) leakdown over the course of 2 minutes acceptable? No visible leaks and a block test returned a satisfactory result.

Long version:
I was trouble shooting a coolant leak that I could not detect. First I did a block test (suction thing with blue fluid, I did that for a long time slowly on the T-stat housing till the car warmed up, careful not to pull coolant into the tester. No color change. Verified this against sucking exhaust air out of the tail pipe, fluid turned yellow pretty fast. So most likely not a HG/cracked block leak right?

then I rented a brand new Coolant system pressure kit. Hooked it up to a warm car. Pressurized the system and it bled down slowly, but I could see the needle move, and dripped coolant. Found the leak! some pinholes in the OEM radiator. Bought a mishimoto radiator and installed it.

Now I hooked the pressure tester up to a warm car (perhaps a bad idea? see below) and it holds pressure, but leaks VERY slowly, no signs of leaks. When I say slowly, I mean after 2+ minutes the needle is barely on the side of the 15psi line (started dead center covering it). Cannot tell the needle is moving if you look at it the whole time.

Is this acceptable? Or should I still be chasing a leak? Could this be caused by the car cooling over 2 minutes and loosing a little bit of pressure? There is 1 psi Mark to mark, so I guess .25 psi leak down after ~2 minutes would be a generous (slightly over) estimate of how much pressure was lost in that time period.

Anyone else done this test on a "perfect" system? Similar results? Or has my quest not ended yet?

 

[curt-s]

There is also a large assumption that this rented tool is operating correctly as well. A cooling engine will also cause a drop in static coolant pressure. There is likely a big formula to represent this..

 

[Boost-Junkie]

In my experience wrenching I've seen pressures drop Almost 1psi and still be a sealed coolant system. You have to take into account the quality of the pressure tester. According to your hot test I see no issue with .25 drop. Try a cold test and verify results if you want to eliminate all doubt.

 

[GSLENK]

Actually that equation is small... Ideal gas law: PV/(NRT)=P'V'/(N'R'T'). Assuming Volume, N(number moles), and R(constant) remain constant (no leaks...) they cancel out. Then Pressure/Temp=Pressure'/Temp'.

After converting units to SI and back... at 160 degrees F, 1 degree F change is worth 0.0242213 PSI. So 10 degrees is worth .24 PSI. I don't have a thermometer to confirm. Its 40*F out and the pressurized air tested is relatively exposed. I wonder if it did lose 10*F over the course of the test. I guess some of that leakdown can be attributed to temperature change.

Pretty sure I am ok. I returned the tool and got my $211 deposit back. Still, thanks for the input.