I have a 2016 Nissan Frontier 2.5L. A couple weeks ago, somehow, the oil cap came off and it slung oil on everything. That in turn led to it throwing the belt. I drove it home without the belt, but was careful to stop before it would get too hot and add more water. OK... so about 3 miles from my home that day, I filled the radiator to the top with water then drove the 3 miles home.

The next day we put a belt on. Since then I have drove it ~150 miles without issue.

Today on the way home from school....it overheated. When I opened the radiator to add water.....it was down 1.5 gallons. I don't know where that water went! After filling it.....I drove about 7 miles home and it never offered to get hot again.

Once home...I checked the water and it was full. I then removed the plastic skid plates under the radiator and engine, parked the truck on top of cardboard, and let it run for half an hour in an attempt to find the leak. There was NO water on the cardboard and the truck never got hot.

I checked the oil....it looks clean and clear and it's not overly full. I don't think it's a head gasket.

Any idea what might be going on here?

Also....thoughts about Bar's Stop Leak? Think that might help here?

Don't use any stop leak hocus pocus unless its an emergency.

I'd drive it more, and keep a close eye on the coolant level. You could have run it low, and got air trapped, then it finally burped the bubble out.

I am FAR, far from a mechanic but you may have had an air bubble in it when you filled it the last time that finally burped itself out. I would just keep an eye on it but it may not give you anymore problems for a while.

What 160 said , I think it was air locked and came out of it , keep an eye on the coolant level. Hopefully someone that knows for sure will chime in.

Some vehicles require special procedures to burp the coolant system. Back in the day, just filled everything and only put cap on half way and it would take care of itself. Many newer vehicles need to be jacked up in the front, or some cases, jacked up only one side in the front. No idea why your oil cap would have popped off for no reason...

Are you running actual water or coolant?

You could have purged some remaining air from the system, or if your using just water it could be slowly evaporating.

You can buy a pressure tester fairly cheap to check. Or you can check the coolant periodically.

Always add coolant into the ovah flow tank...in the morning...

When the engine is cold.. Then keep an eye on it...but never open it when hot.

KSnow nailed it ...air pocket ...it needed to burp

Well I had coolant in it up until the day I threw the belt. Since then I have been refilling it with straight water (and I had to do so 4 times on the beltless trip home) so imagine it is mostly all water now.

You best get some antifreeze in there .

Drain it first....then add the 50/50 mix

I would have the system flushed if it has not been done recently and have it refilled with the 50/50 stuff
Pretty sure that antifreeze doubles as a coolant (better cooling than straight water)
And also if you put anything but distilled water in it you will have the stuff from the tap water causing issues inside the cooling system

Another thing I would do is see if I could find a "weep hole" on the water pump. Search the www for your vehicle and see if it has one you can see.
That is how you could tell if the old school water pumps were going out.
Pretty sure there is not one on my Tacoma and if there is you would have to take the belts and covers off far enough that you might as well change it

you refilled it 4 times in how many miles? Was it boiling off?

You might have gotten it too hot and warped the head.

First mistake was buying a nissan frontier.
Sorry YT somebody had to say it.

I use 50/50 on a drain and refill but not a flush.

On cars that don't need a flush I do a single drain & refil with 50/50. Its fast and easy. It only drains about half the coolant, but I do it early and with high quality coolant so that's fine.

My last few purchases of old things (03 dump truck, 80's dozer), I drain and refill several times with tap water. Then drain and refill 2-3 times with distilled water. At this point it is almost entirely clean distilled water. Then drain and refill once with straight coolant concentrate (NOT the 50/50). Since usually draining just gets around half the system capacity, getting the system to straight clean water, then doing a drain/refil with concentrate gets it to just slightly stronger than the desired 50/50 mix.

Yeah I don't believe that is it though as I said in my original post. About 20 miles. Whenever the heat gauge would get over 3/4 I would pull over and refill it...... but most times it only took 1/2 gal or less...

ah, that's encouraging.

I would drain and fill with coolant , it works better at not boiling off than water.

here is the issue you may have

the water pump is belt driven , so as soon as the belt is off ,you get very little time to drive.

in the future safe money is stop and get a tow

because the biggest danger in overheating is the hot steam pushing through the weak point which is usually the head gasket.

filling really does very little because you are not circulating coolant through the motor if the pump isn't turning

also the coolant temp sensor may not be where it gets hot first , when the system is circulating it measures the circulated temp, not circulating you get steam pockets

anyway fill it watch your level closely daily and see how much you lose , if you lose no more then you won the lotto , if you are losing some every day/week you need a block test done it is a die that tells you if have a head gasket leak.

Could be head gasket and not putting water in the oil. Put antifreeze in it and check exhaust. If it is steaming and smells sweet get your checkbook ready. You mentioned not leaking on driveway. Not in oil
Good luck

How did throwing the belt off cause you to lose coolant?

Crack your oil plug and drain some oil out and see what color it is. if its milky white thats where your water went, which will be at the bottom of the pan near the plug not at the top for the dipstick. Oil floats. Id say thats the most likely. Even though you were trying to not let that happen where else did the water go?

Id probably change the thermostat and maybe the radiator cap. Those are cheap things to try. Maybe the radiator cap wasn't on all the way?

You don't have any known confirmed visible leak it doesn't sound like. Im not sure a stop leak would help.

Cracking the oil plug for 100% confirmation and the thermostat and radiator cap is where I'd start while I watched for leaks and tried to figure out where it was going if its not in the oil and theres not oil in the coolant.

Flushing the heater core and radiator would be next after the easy stuff all while trying to find the leak if its still going missing. The air bubble is possible.