So i noticed my car temperature getting hot and the heater blows only cold air. First thing i did was change the thermostat. That didn't fix it, so i changed the radiator next, that didn't fix it. I Notice that at first the heater does stat getting warm but after i drive it a little bit it will start getting cold again and the temperature gauge will start going up. I've tried bleeding air out by jacking it up and running with the radiator cap off but doesn't seem to do much. One question i have is someone told me that when you install the thermostat the little hole on it has to be in the 12 oclock position which i didn't do. Could that be the reason it still overheats or am i looking at maybe the water pump? Also the big hose at the bottom of the radiator doesn't get hot like the other hoses.

 

That maybe a clogged heater core! Bypass the heater core and test it out! Try to ran water through the heater core via a garden hose or compressed air!

 

TM: If the hose from the bottom of the radiator doesn't get warm before the engine gets hot, it could be air or the pump. Both the VG30 and VE30 can be an SOB to get air out of. Yes, you need get the thermostat right. Check it while it's out to make sure it still moves freely. Jacking helps- it's the recommended procedure. You should also remove the bleeder screw near the throttle body while you initially fill with coolant (then put it back), and maybe after you finally think you're close to filled and have put the cap back on the radiator and the system is under a little pressure, just loosen it a bit (not completely) until it bleeds coolant. It will take some patience. If the engine gets hot while trying to fill it and spits back out the radiator stop and let cool down: if there is air trapped behind the thermostat, it won't open until it gets really hot or hot coolant finally gets there. (audible pop/thunk) Even after all that, it may take a few days to get every bit out- check radiator after it's cooled overnight, keep an eye on the over flow and keep it fed. Service manuals for later vehicles like an Xterra with a VG33 actually specify using a rig to pull a vacuum on the cooling system and then have it draw the coolant in to replace the vacuum. Yeah. Gives ya a clue about the potential difficulty level.

Having said all that, it wouldn't surprise me in your case that the water pump is NFG. I pulled the water pump off a VG33 while I was getting deeper into the engine. The cooling system had obviously been seriously neglected. I don't think it was the original pump- looked like it was an inferior cheap design- but regardless the fins were so far gone from corrosion that they broke off in my fingers. If your cooling system has been neglected- or shows signs that it was previously, this could be the issue. Don't worry about the heat just yet, but a bad water pump would cause a no-heat and an overheat- though a clogged heater core from neglect could be a problem on its own. If you get the system full and you're sure the air is out and it's not taking any more coolant over a few days (hot/cold cycles) and the engine is still getting hot- suspect the water pump.

You didn't say which engine you have. If you have a VG: if it's time or close to time to replace the timing belt, replace the water pump. If you go to replace the water pump and the timing belt isn't fresh, replace it while your there Don't bother trying to get the last 20 or 30 thousand miles out of it. If you have a VE, let me know- I have some advice for you for replacing the pump.