I recently replaced my radiator and thermostat due to a small leak in the radiator and highly fluctuating temperatures during driving. After I replaced my thermostat and radiator, I noticed that the running temperature no longer rest's at the 210 mark but instead rest's inbetween 210 and 0. After about 2 weeks, this temperature started to fluctuate from the 1/4 mark to the 1/2 (210) mark sometimes reaching even 260 briefly. My coolant seems to be running low but I can't notice any apparent leaks. Even when I completely re-fill the coolant though the temperature still fluctuates and whenever the temp gets too high, my heat stops working. My radiator fan is working completely fine i checked that.

Anybody know what could be causing these problems?

 

When the cooling system starts getting low on coolant, the first thing that happens is you lose heat intermittently. As it continues getting lower, you totally lose heat. As the coolant level continues to drop, the engine starts to overheat.

Pressure test the cooling system and look for leaks. Lower intake manifold gasket failures are very common if it still has the OEM style gaskets. Check the underside of the oil filler cap: if it looks like a chocolate milkshake, coolant is getting into the oil. If you continue to drive it like this, it will destroy the bearings in the engine.

 

I checked my oil yesterday specifically for that milky white substance and no, the oil was normal colored. Sorry, I forgot to mention that.

Will a typical Pressure test tell me that there is a leak or will it tell me where the leak is also? Im guessing it's basically if you hear any "leaking air sounds"?
Also, if its the Lower intake mani, that should show up on the pressure test also I assume right?

One more thing. Could it be my water pump?

 

No coolant in the oil means the intake gasket is not leaking internally, (into the engine), and that's a good thing!

Most auto parts stores rent/loan testers for free, and they also sell them. The tester pressurizes the entire cooling system, except the overflow reservoir and the radiator cap, (they are tested seperatly). It's up to you to find the leak. Very common for lower intakes to leak where the intake meets the cylinder heads. Check each of the 4 corners. Probably will need a small mirror and a flashlight for that. Quite difficult to spot if it's behind/under the AC compressor or the alternator. Water pump can also leak, so can the radiator, core plugs, heater hoses and heater core. The parts stores have a florescent dye that can be put in the radiator. Leaks can easily be spotted using a black light. Look for the dye, shows up florescent green, can't miss it.