I replaced my radiator cap with a new, OEM cap and now my coolant system has sprung a leak. From what I can tell, it is dripping from under the alternator. I already replaced the thermostat with OEM and used a upgraded metal gasket. The mating surface was clear of the old paper gasket but still had sticky residue (RTV from previous owner?) I couldn't scrape any further off with a razor and a brass brush couldn't take any further off... I'm not sure where to go from here. She's my daily and it only leaks while running after like 30 seconds of start up... Any ideas or where to go? I can't tell where the leak is originating from... 06 Revup

 

If you have a friend who is a mechanic, see if they have a coolant system pressure tester you can borrow. That way you can hopefully hear / see the leak when the car is off.

 

OP doesn't mention whether he's still running the OEM radiator or gone aftermarket, but aside from running a pressure test, a new radiator cap causing a coolant leak sounds like it could be the radiator itself. The OEM radiator uses composite end caps that can separate from the aluminum core. Rather than guessing and fooling around, I'd suggest pulling the radiator itself to look for the leak.