Just wanted to post for those searching (since specific information was a little thin)... been battling with coolant dumping out of my overflow (and still am), and rented a *very* useful tool I thought - never having used one before.

Coolant Pressure Tester ~$75
http://www.autozone.com/autozone/accessories/CST-Pressure-Tester-Adapters/_/N-2674?counter=1&itemIdentifier=84330_0_0_&target=shelf

^^^ Used to test Radiator Cap ONLY (use smallest adapter, and be sure to wet the rubber for a good seal or it will not hold pressure.

Import Radiator Adapter ~$45
http://www.autozone.com/autozone/accessories/OEM-Import-Radiator-Adaptor/_/N-2674?counter=3&itemIdentifier=311465_0_0_&target=shelf

^^^ Used in conjunction with Tester to test the entire cooling system (attach to fill neck, and be sure not to exceed 30lbs per manual or things WILL break - I went to 20lbs).

So for ~$120 worth of rental (all refunded), you can very accurately test your radiator cap and (more importantly) the entire cooling system. No more 'just buying' another radiator cap and 'hoping it works'. I found that my upper radiator hose (near hot turbo) was seeping air into the system (you can hear the air bubbles going through, very obvious). It still didn't fix my overflow problem, but I would have never noticed the hose otherwise (one step closer - hopefully).

Just be sure to rent BOTH tools,
- J

For those interested, what I've ALREADY done:

1.) Replaced headgasket (two weeks ago, second since rebuild)
2.) Replaced thermostat w/ 160' (VERY nice)
3.) Replaced radiator cap w/ Mishimoto 19lb (tested good above)
4.) Replaced upper radiator hose (seeping air)
5.) Replaced silicone tubbing to & inside overflow tank
6.) Removed Weather Stripping from firewall/hood/windshield
7.) DEI Heat Wrap on Downpipe, Upper Radiator Hose, Big Metal line between Lower Radiator and Water Pump
8.) DEI Heat Sleeve/Hose on Heater input & return metal lines
9.) Flushed & filled the entire coolant system, several times
10.) Cut out stupid black ribbing on front bumper AND crash/support/foam behind it (HUGE help here)
11.) Tested and confirmed the entire coolant system will hold 20lbs of pressure, very steady with no drop over ANY length of time.

What I've got left to do:

1.) Washers on hood, to raise up 1/4" - high pressure cowl effect
2.) Replace Water Pump (Yes, I KNOW I should've checked w/ Headgasket, skipped right passed it... d0h!)
3.) Replace Turbo->Downpipe gasket (again), leaking some exhaust under hood (raising underhood temps).
4.) Airbox/Duct from Radiator->Intercooler->Bumper (huge loss of airflow here for sure, I may be starving the radiator w/ my big a** Starion intercooler)
5.) Brand new VENTED ViS Carbon Fiber hood, that I want to paint to match >>> Can you put Redline Tuning QuickLift Elite hood struts on Carbon Fiber hoods?!?

X.) I had disconnected my heater for quite a while by running a line directly from the lower radiator to thermostat housing and eliminating the metal piping (although it was still in the car). In attempt at increasing the coolant capacity (more fluid in system), I've reattached the heater. The heater has NEVER worked, not since DaveFL had the car, and I have absolutely no idea why. I really don't care if it works as I'm in South FL and it only gets cold down here two days a year.

Dave's initial post in 2002:
http://forums.2gnt.com/dcboard.php?az=show_topic&forum=5&topic_id=38124

DSM Registry:
http://www.dsmregistry.com/137

- J