My car started overheating. I check the coolent level in the plastic and it is at full. When the car cools off it is still at full and my radiator is about 3/4 of a gallon low. WTF?! Isn’t the full reservoir supposed to do SOMETHING to keep your car from blowing up? I don’t think I’ve ever had a coolant reservoir work on a consistant basis.
On a properly performing system, the radiator remains full regardless of the coolant temperature. As the coolant heats up, it expands, and the excess gets pushed into the reservoir. As it cools, it contracts and pulls a slight vacuum, and sucks the coolant from the reservoir back into the radiator. But in order to do this properly, the radiator cap must be functioning correctly.
Try replacing your radiator cap.
You also need to verify that the hose between the radiator and reservoir isn’t kinked or blocked. Start with a system properly filled and purged of air, and then see how it goes.
And make sure the hoses are good. I had a very old Volvo that started overheating and the reason was that the hoses from the radiator to the reservoir and to wherever it else it was going were cracked and losing coolant. I just ran the heater–in the middle of summer, unfortunately–until I could get replacement hoses (not easy with this car.)
No. You’re totally misunderstanding its function.
In the old days, cold radiators were filled to an inch or two from the top to allow expansion room as the coolant heated up. But even properly filled they would occasionally lose some fluid, and needed to be checked and topped up regularly.
Coolant overflow jars allow the radiator to always be totally full. Expanding coolant goes into the jar, and with contraction when the engine cools down it gets sucked back in. The problem of coolant loss out the overflow hose is eliminated.
What overflow jars don’t do is give a reliable indicator of coolant level in the system, as you have discovered to your consternation. Now, sometimes they’re helpful for this - with some coolant loss situations, the level goes down in the jar. But there are leaks where, instead of sucking coolant back from the jar, the system will suck air in through the leak. The level in the jar gives no clue.
So the proper way to check engine coolant level on overflow jar systems is to remove the radiator cap - COLD ENGINE ONLY - and check the level there. The task of doing something to keep your car from blowing up falls to you, or the service facilities working on your car.
Don’t confuse a surge tank - which has the pressure cap or “radiator cap” on it - with an overflow jar. Overflow jars are not pressurized, and have dust caps, not pressure caps. Just to make your life harder, 98% of automotive parts and service people don’t use the term surge tank, and call both surge tanks and overflow jars “overflow jars.”
Make sure your radiator is completely cool when you remove the cap. The radiator cap is under pressure and if you remove it on a hot radiator you may got skalded by steam.
My suspicion is the radiator cap. There was coolant leaking out from under the cap and NOT the blowoff valve and this was after i had filled it completely. I’m taking the car in anyways since the sunroof gets stuck and it’s still under warranty and they know to check the cooling system.
On a real warm/hot day you might notice the level up slightly but it should be neglible.
It is entirely possible even probable that if there is an internal coolant leak and being burned in the cylinders along with the fuel (no green stuff on the garage floor) that the reservoir will remain full.
Look for a puff of white, acrid smoke on startup, or even semi-constantly, usually the engine will likely run badly and overheat. Oftentimes the cause is a leaking engine head gasket, which costs about $5, the labor may run into four significant digits. Plugs will look a bit green if I remember. If significant coolant is leaking into the oil sump, then oil on the dipstick will be milky. Best to just drop a remanufactured engine in a lot of cases, as the cost is vanishingly close to repairing the original.
The rersevour was origiinaly added to reduce polution, but it’s well beyond that now. The resevours hold more than expansion coolant, they hold some extra. One that is always full when the radiator is missing water, after cooling down, indicates the radiator cap is in need of replacing. Replace the radiator cap and everything is cool.
Is the only reason for this one of injury to the person and not the car, or will opening a hot radiator potentially damage your engine and/or cooling system as well?
Maybe, but not necessarily. Any leak that allows air to be sucked into the system can cause the system to get low while the reservoir level maintains. There are many possibilities besides a faulty radiator cap.
$5? Not for many decades. Probably at least ten times that much. And a head gasket set, which is usually needed, is quite a bit more.
So if the coolant reservoir is just for catching the overflow, why did the guy at Jiffy Lube sound so concerned when he pointed out during my last oil change that it was empty?
Because likely you had a leak where the system was sucking the coolant out of the jar (rather than sucking air as in the OP’s case).
An empty overflow jar is almost always an indication of a leak, but a full overflow jar is not assurance of no leakage. So if the jar’s empty, a leak test is called for. If the jar’s full, you may be okay, you may not. Inspect the level in the radiator (COLD ENGINE ONLY) - if it’s full, you’re fine. If it’s low, test for a leak.
Plus the non-reusable head bolts. Those beasties can be close to $5 on their own.
And it’s pretty unlikely that swapping out a full engine will be cheaper than replacing the head gaskets by the time you button up all the hours of labor needed to move over all of the this-and-that parts like the alternator and distributor that are attached to, but not part of, the engine block.
The tank holds the water until the radiator cools causing low pressure in the cooling system which sucks the coolant back into the radiator so it stays full. An empty reservoir means eiather it’s cracked, the connecting hose is bad, or the radiator system leaks and could lose enough coolent that the engine overheats.
Removing the radiator cap when hot and pressurized, can result in explosive decompression. It can even lose much of the coolant after you open it as it continues to boil out when the engine is extremely hot. Do not add water until after the engine cools or you can crack the head or other parts of the engine. I was taught to use a rag opening a radiator cap, and to be sure to only turn it initialy to the pressure relief point. You can turn it the rest of the way after you know it isn’t pressurized. Radiator caps didn’t always have this pressure relief feature and they would just launch into the air if you opened them under pressure. Now you have to push down to get past the safety feature.
Replace the cap and monitor the radiator. Take it in if the radiator keeps going low.