I would guess that it is most likely the cap or it was not turned on tight. On the cap you have two tabs that must be aligned to the slots on the top of the housing, pushed down and under the ridge. Then the cap must be turned until the two tabs come against the stop. Sometimes a small amount of silicon grease (but don't use std grease) on the housing ridge helps turn the cap more easily and seat it.
BTW, what does your temperature gauge read? If the coolant is getting too hot and building too much pressure you will see it spilling out quickly from the radiator cap to the coolant overflow bottle.
Forgot to ask, are you using the GTP or Fiero thermostat. If you are using the Fiero thermostat make sure that it is pushed in a bottomed out/seated correctly or the cap will not close properly.
------------------ " THE BLACK PARALYZER"
-87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Powerlog manifold, Northstar TB, LS1 MAF, 3" Flotech Afterburner Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite. "THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H " ON THE LOOSE WITHOUT THE JUICE "
[This message has been edited by Dennis LaGrua (edited 12-01-2014).]