'05 Sentra won't start. Coolant disappearing from

Blown head gasket if oil and water are mixed

Head or intake gasket leaking. Is the coolant milky? Or do you just see a little oil in the colant? If it's not milky i would not worry. If it's milky ,To buy you some time add some bars leak into the radiator. Then change the oil and antifreeze.

probably either the intake manifold gasket, or a head gasket has failed allowing cylinder compression to escape into the cooling system and boiling the coolant. also look for milky oil(water in the crankcase), or for oil in the coolant system(rainbow sheen on top of coolant in the radiator, or overflow reservoir).

Well ima pretende that you are talking about the coolant system on you car that's going somewhere and you don't have any leaks? Well the best thing ill suggest to do first is check your engine oil stick and make sure you don't see any coolant mix with oil wich it will be like milky and second I want to ask you if you usisng you heater if you do you may probability have a heater core leak and next thing to do is performed a pressure test on you coolant leak if you don't have any tools to do this take it some where and let then do the pressure test you may have a minor leak somewhere try that and let me know.

If you are adding coolant every week, and you don't see any external leaks, the coolant is probably being burned thru the engine.
You can burn a couple of quarts a week and not see any smoke.
At some point in time you could pull the spark plugs and find evidence from the burning coolant.
Most common cause is a leaking head gasket or intake gasket.

This is NOT a good thing...Better check you engine oil. When engine coolant disappears without any signs of it leaking on the ground, it is usually because it is leaking into the crankcase via a blown head gasket, blown intake manifold gasket, or cracked cylinder head. If your engine oil looks milky, like a chocolate milk shake, it is time for major engine repair or replacement.

milky oil would suggest water has mixed in with it. this added to a slow, unfound water leak points toward a head gasket problem with the coolant escaping from the cooling system, into the oil.
this needs to be sorted out fairly quickly. apart from the obvious problems of a water leak and the risk of overheating, oil with water in it will not lubricate the engine properly and there is a serious risk of engine seizure.
a garage should be able to do a simple test on the oil to discover if it is coolant contaminating the oil.
a pressure test on the cooling system would also confirm where the coolant is being lost.

How about a crack in the cylinder head or a gasket leaking into either the inlet manifold and being sucked into the engine, or being blown into the exhaust system and being boiled away. Ask a radiator repair shop to carry out an O2 test on the coolant. They may detect presence of the gas in the water confirming a leak into the exhaust system.

you need to take the oil dipstick and see what color the oil is if it is milky color your piston rings are leaking or maybe main seal look on back side of motor side if real dirty oily wet also check your thermostat maybe leaking slowly on motor burning it off hope this helps where i would start

there's a chance you have a blown head gasket. If that's the case, your coolant isn't disappearing, but leaking into the cylinders. Check your oil. If it looks kind of milky, it means it's mixing with your coolant, which indicates a blown head gasket. Any white smoke coming out the tail pipe is also a tip off.