1997 Ford Ranger XLT 4x4 3.0L so ive replaced my thermostat thinking that would fix the coolant vanishing problem, turns out it did not. make tank became dry once again and my temp gauge keeps going back and forth from cold to hot. plz help

 

Is your rad cap bad,,,could be slowly venting off coolant to air so you see no leak,,,,just a wild guess

 

im going to check that when i get off work in a few hours. hopefully thats what it is. thx

 

My 72 Ford E-200 did the same and once I replaced the cap, I no longer had to add
coolant.

 

Welcome to FTE.

Perform a cooling system pressure test, or run it by your favorite auto parts store & have them do it. They can also pressure test the radiator cap.

Did the problem come about slowly over time, or suddenly after some event?

What kind of condition is the coolant in now & has it ever been badly neglected in the past?

If the cooling system doesn't pass the pressure test, are there any signs of coolant in the engine oil/chocolate milkshake like deposits on the dip stick, or moisture signs on it?
Any signs of a spark plug being way cleaner than the others from the steam cleaning by coolant leaking past the head gasket into the combustion chamber?
When it's low on coolant & you remove the radiator cap for a cold start, do you smell exhaust fumes coming from the radiator, or see exhaust bubbles in the coolant?

Any smell of coolant in the cabin, if so look under hood, firewall area at the rubber A/C condensate drain hose, to see if coolant is dripping out from a heater core leak.

More thoughts for consideration, let us know what you find.