Radiator overflow cap pops off by itself?

1996 Tacoma V6 4X4 auto. Recently the radiator ran damn near dry. I found the radiator overflow tank's cap wasn't on. Is it possible it popped off by itself due to a problem or did a human take it off? The cap fits like it should. Also, why would the cooling system go dry from the overflow tank's cap being off? My truck has never had any drops or puddles of coolant under it, what's going on with my truck's cooling system? Thank you.

 

No leak, and low on coolant?

Fill the overflow tank to the appropriate line, monitor after some driving and report back.

If you're like me and drive through some bad roads, it could've popped off. My gas tank door pops off the hinge every now and then.

 

No rough roads at all.

 

Drive it 'til the wheels fall off then!

Really though, if it's only your overfill tank, then just top it off.

It also costs nothing to check your oil, though I'm sure if you were burning coolant, you would've seen the white smoke coming out of your exhaust.

 

check you radiator level, should be to bottom of cap when cold, if your over flow is dry, you’re using coolant somewhere.
check the hoses behind the engine on fire wall at heater control valve, mine were brittle and cracked from the heat of the engine and exhaust and 20 years of age.
keep tabs on the coolant level on a regular basis, if the engine gets low and over heats, you’re buying heads

 

I had to add 1 1/3 gallons to my radiator and its overflow tank. No water in oil, no steam or smoke coming out the exhaust, and it runs great. I'm just confused about why it went dry, how it went dry, and why having no cap on the overflow would make the radiator go near dry.

 

Every single hose was replaced about 2 years ago when I had to replace the engine due to lack of coolant.

On the last engine I had the cylinder head gaskets replaced, heads rebuilt and checked for cracks, and when it was all said and done the engine had the same problem due to a cracked block ($1,800 down the drain).

I'm definitely going to be monitoring coolant levels a lot more often.

 

it’s going somewhere
are you smelling coolant when the vehicle is hot and parked ?

mine has a faint smell of coolant, i’m fairly certain it’s a seal for a heater tube under intake, i just haven’t had time to do anything about it. but it’s been like that for a few years and i check things regularly. ( once a week minimum, wife drives the hell out of it, so i gotta keep it moving)

there is and oil cooler on the driver side of engine block has coolant hoses going to that.

smell the exhaust for coolant on start up, steam is normal on cold start sweet smelling steam is not

 

Replace the radiator cap for starters

 

I've never smelled a coolant smell from this truck. I agree the coolant is going somewhere, but I don't have a clue where it's going.

 

It's funny you would mention the radiator cap. When the engine, all the hoses, and the radiator were all replaced the radiator cap has never been replaced. I'm not sure how that applies to the problem I'm experiencing.

 

The springs get weak over time

 

Spring fees weak and the rubber seal deteriorates
The cap on my runner wasn’t sealing when I first bought it, once I replaced no real issues til a head gasket went a few years later