could i have a problem with a new thermostat

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on my 96 oldsmobile ciera 3.1 v6 i bought a new napa # 222 thermostat and installed it now it seems when i start it cold i get no heat sometimes and my temp gauge swings up to 3/4 semi fast then after some time it comes back down and then a few minutes later i get heat then sometimes the heat turns cold and the gauge starts going up again after running for over 25 minutes everything is good. the system is bled correctly could the brand new napa thermostat be bad? I changed the thermostat because the original was stuck open, i also replaced the water pump because it was starting to leak and there is a new radiator cap also.
 
I suppose anything is possible, but what you describe are classic symptoms of the system having air in it.. How did you verify the system has been bled correctly?

These 3.1s can be a real booger to get bled.. Believe me. Do a search on the web there are several recommendations on bleeding these, and mods that can help your cause.. Off the top of my head one that stuck out to me was drilling a small hole in the lip of the thermostat to allow easier bleeding.
 
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sounds like you have air in the system. I would drain the fluid remove bleeder and then with car off pour fluid back in and when it pours out of the bleeder you are done, reinstall bleeder and top off let run till warm with heater on and make sure its full and its al done.
 
Back the car in a ditch or jack up front of the car, gets the front of the engine tilted upwards to help bleed out the air. There should be two air bleeds. Don't worry about making a mess when bleeding system, doesn't hurt a thing.
 
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If this trouble continues, drill a tiny hole in the flange of the thermostat. If you get an air bubble behind the pellet, it never sees heat and never opens.

Also, that hole will help generate a slight water current behind the stat, which will help tickle the pellet and keep it up to date with what's going on, temp wise. If, heaven forbid, you get a tiny amount of exhaust in your coolant due to a HG leak this hole will let that gas find its way out, too, and not cause future problems the next cold start. (Not saying you have this)
 
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