Help why does coolant spill out when I take off the radiator cap on cold engine



I got a 2002 mustang v6 and I hadn’t flushed the coolant since 5 years ago so I decided to take off the radiator cap off too check the status of the coolant. As I took the cap off, coolant started to slowly spill out of it so I quickly put the cap right on. What causing this? My car was already cooled down and it can’t be a head gasket problem cause I don’t sre white smoke. And I don’t see any leaks from hoses and water pump . I park my car outside could it be the sun heating it up and making the coolant flowing out when I take the cap off? What causing this? Please help btw coolant is still green


Is the heater core higher than the radiator cap? If so the coolant will drain down to the radiator and out the open cap. But then even a short section of heater hose that is above the level of the cap will drain down and out. Water will seek its level. As will coolant.


I just got back from LA. It’s been reaching over 100F there this past week. Yes, your theory is plausible.
Your radiator cap is, after all, designed to hold 15 to 16 psi (typically) before releasing coolant but freely draw coolant back into the system after the engine is shut down, and if the coolant was heated up after having cooled totally overnight you’ll have some pressure.


Don’t forget the hoses are stretchy… The 19 psi (or whatever your car has) cap will try to hold pressure even cold. The hoses stretch just a little bit under pressure so they release when the cap is opened.

What you describe is pretty much my experience with most cars I’ve owned. They piddle a little coolant when the cap is opened. It should just be a little though.


Thanks for your help guys


I don’t think this is something to be concerned about. It only overflows for a few seconds, then stops, right? As the engine cools while parking the car overnight, the coolant in the overflow tank gets sucked back into the top of the radiator. And as this happens it fills the radiator right to the brim. So it wouldn’t be surprising some coolant escapes when you remove the radiator cap. More likely to happen in certain situations I expect, depends on the temp of the coolant in the radiator, and if the car is parked on a slope or not.

If still concerned, you could do a quick check for a head gasket problem. Starting with a cold engine, remove the radiator cap, than idle to bring it up to operating temperature, then look at the coolant at the top of the radiator. See any bubbles coming up? Rev the engine to 2500 or so, check for bubbles again. No bubbles, unlikely to have a head gasket problem.


Thanks for the help man I’ll sure do the head gasket test when I get my car back from my brother since he’s borrowing it for a week


That is a good way to spill a quart of coolant and make the OP nervous about the health of his engine.

@Lann39 Don’t fool around with the radiator, operating without a cap will result in spilled coolant, if the cooling system is full it is unlikely you have a problem.


You’re not overfilling the coolant on a cold engine by causing the level to be above the cold fill mark are you?


Nope, when the engine is cold, the coolant level is on the cold mark right now, so it isnt over filled


So I’m guessing I shouldn’t be checking for bubbles? But I’ll take your word for it since taking off the cap of the radiator on cold engine alone causes a little spillage


I do I do think this is pretty normal and I would leave it alone. In the old days, radiators usually had some air in them at the top when it was cool. Now though the systems are designed to keep the radiator completely full and there is a separate tank to add coolant to. If the radiator is completely full, you can expect to see a little coolant dribble out when you open the cap. In fact a lot of radiators now don’t even have a cap anymore.


Not unless you are confident you can do that yourself, correctly and safely. If not, and you want it done, ask your mechanic to do it. That’s what they do.