My car had the same problem. I now have some cheap "hood pins" on my car(hood is trash anyway, so i didn't care about a sloppy job). That isn't what i suggest you do though, the reason i thought my hood would always pop up was because of my accident, but what i found was that the spring that pulls the latch closed wasn't strong enough anymore to pull the "hood pull handle" and cable all the way back into the closed position. Now, i just need to make sure that i push the handle all the way back. So, i don't even need the pins on there, but i still have them anyway. So first thing i'd do, is just make sure you're pushing the hood "pop-open" handle all the way back in(not that you don't, but that's what my stupid problem was), and then if that doesn't do the trick, you might try hitting up your nearest hardware store for a new spring that you will find underneath the latching mechanism behind the grill. And then hopefully your problem will dissapear. Good luck!
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Kyle - attending Ore. State, while my lil 68 142 (74 b20, broken m40) sleeps... (check out my website, www.cardomain.com/id/kneedsacar