I have a 2000 Pontiac Montana, I have antifreeze mixing with my oil, what could be causing this and how do i fix it?

Sounds like the water pump is bad.

Could it be the water pump

if the thermostat is good you could have the system flushed as it may be clogging up, especially if the dexcool antifreeze is old.
make sure you use only the color the vehicle has in it, do not switch from the orange to green or vice versa, be sure that dexcool is mixed properly, 50/50 ONLY otherwise it becomes corrosive.

Get the radiator and heater core, hoses and thermostat replaced. Also replace the mixing valve if needed. A radiator shop could give you an estimate and diagnose the problem better than I can online. I hope sincerly that this helps.

They decay because off antifreeze breakdown. If you don't change your antifreeze yearly, there is a precipitate build up that eventually eats away the gasket & causes leakage. It is very important that this is done especially now that aluminum heads are the mainstay. In the old days every engine was 100% steel & we had a lot less problems with this issue. Aluminum & steel don't mix well with antifreeze. Good luck!

YOU HAVE INTAKE GASKET LEAK.ANTIFREEZE LEAK ON VALVE LIFTERS DILUTING OIL CAUSING LIFTER NOISE.

It could be the water pump. They will leak and sling the coolant so it doesn't get to the ground. You can't see the leak because it's hidden under the pulley. It's called the weap hole in the pump.

There has been a problem with the Dex cool orange antifreeze deteriorating the upper and lower intake manifold gaskets, causing coolant to leak in to engine.If there is antifreeze in your oil, it will ruin your engine. If you find antifreeze in the oil it will cause the oil to become thick and frothy and will not lubricate the bearings and other moving parts, causing an engine failure. If there is evidence of coolant in the oil, do not drive the vehicle. Have it towed to a repair shop and have the gaskets replaced, oil and filter changed, cooling system flushed and refilled with GREEN antifreeze that is compatible with aluminum radiators and engine parts.

remember new head bolts also, torque to yeild bolts. i would recommend to get a service manual for torque specs and procidures for removal of heads. also head gaskets are leaking for a reason have heads checked for straightness and for cracks at local machine shop, keep all pushrods and rockers numbered for reassembly, pushpods are different and will cause major engine damage if mixed up.

That's normal, that's on the high side but should drop soon as it get't that high, You can have the thermostat replaced with a lower rating but your going to have the colder running engine in the winter, Just keep an eye on the radiator fluid level and make sure the antifreeze is at a 50/50 mix (antifreeze/water). The higher the concentration of antifreeze the harder for the heat to transfer from the engine. Water is a great heat transfer but it boils over, that is why antifreeze is mixed. 100% antifreeze will gel when heated, causing poor circulation making the engine run hot, you can buy a 50/50 pre mixed antifreeze if your not in to mixing the two together and you can also buy a antifreeze meter to know what your mixture is at. Good luck and hope this helps. The other thing to is to make sure the radiator is clean and not full of dirt from the winter driving. Good luck and hope this helps. Keep me posted, be glad to answer any question you may have.