Coolant cap cant get coolant reservoir cap off

So the level keeps dropping in the reservoir? If you have to keep adding coolant to the reservoir, the system is leaking or losing coolant somewhere. Have the system pressure tested to find the leak. Is the reservoir itself leaking? When the level gets low enough, first the reservoir will go dry, then the radiator level will start to drop.
Here's how the reservoir works: coolant is under pressure from the radiator cap seal. As coolant heats up from hot engine, it expands, and pressure forces the radiator cap seal up, coolant is forced into the reservoir-remember the hot mark on the reservoir? When car is shut off, coolant contracts, creating a vacuum in top of radiator. This vacuum works to suction coolant back from reservoir into the radiator, keeping the radiator topped up, and coolant should now be at the lower cold mark on the reservoir. It is a closed, sealed system. The only loss of coolant will be a very small amount of evaporation from the reservoir. If coolant level keeps dropping, there is a leak somewhere.

Has coolant level been going down in return reservoir at all, as in have you had to add coolant recently to bring it up to top level? Have you tried running engine from cold, with heater on and let it run up to normal temperature with cap off return reservoir? If not do so, and watch level in reservoir and if it goes down, add as needed until engine running at normal, and not overheating. Allow rad fan to kick on & off a few times before re-installing cap on reservoir. Pay close attention to engine temp while doing this. If it goes high, shut it off and let cool. Be careful, my son's Passat just had overheat problem and turned out to be impeller on water pump ( internally) was spinning on shaft of pump. Thermostat is certainly a consideration if coolant is full, and all air bled out. Keep me posted on what happens when you run with cap off and keep reservoir full. Don't forget, heater on while doing this.

Many Newer Vehicles do not have a "Radiator cap" any more. all coolant checks and filling goes through the coolant reservoir. which is usually located under the hood left or right side of engine near inner fender.

WAIT UNTIL YOUR ENGINE IS COLD,YOUR RADIATOR PRESSURE CAP IS LOCATED ON THE COOLANT OVER FLOW RESERVOIR, THATS WHERE YOU ADD COOLANT WHEN COOLANT LOW,YOU POUR COOLANT IN THE OVER FLOW RESERVOIR UNTIL COOLANT LEVEL REMAIN AT FULL COLD MARK DONT OVER FILL OVER FLOW RESERVOIR, WHEN COOLANT LEVEL STOP DROPPING WHEN ADDING COOLANT AND COOLANT LEVEL STAY AT COLD MARK ON THE RESERVOIR YOU RADIATOR HAS THE CORRECT COOLANT LEVEL IF COOLANT LEVEL KEEP DROPPING CHECK FOR COOLANT LEAKS AROUND RADIATOR AND THE TOP AND BOTTOM RADIATOR HOSES, LOOK FOR COOLANT LEAK AT WATER PUMP GASKET OR SEAL OR LEAK AT WATER PUMP WEEP HOLE IF YES REPLACE WATER PUMP.

radiator caps went out a long time ago, coolant is added via the fill cap on the overflow reservoir.

You probably have a slight leak from the coolant filler cap and you have lost coolant by evaporation over the four months. Check the seal on the cap and replace it or the cap.

If there is no cap on the top of the radiator, you add fluid to the coolant reserve to fill the radiator.

just fill from the coolant reservoir; the radiator itself doesn't actually have a cap, so the pressure release valve is on the cap to the coolant reservoir. Make sure to tighten it well, though, because the cap is plastic and known to leak a little fluid if not tightened all the way.

Twice in two years now my Saab 9-5 has had a pin-hole in the oil cooler, which has caused oil to flow into the reservoir. A way to tell is if coolant/oil is coming out of the reservoir cap. The reservoir cap is rated for the pressure of the coolant system, which is significantly lower than the oil pressure. When there is a hole in the oil cooler, oil will be forced out at relatively high pressures, causing the reservoir to fill and then over pressurize and spill out of the cap.

this was caused by the cap being gone, there is zero doubt of this, I hope u have not damaged the cylinder head gaskets, time will tell.