I've had to top up my car's radiator fluid twice in the last month after discovering that the expansion tank (plastic container where one usually tops up) had completely drained out (roughly a gallon). The first time, I noticed a puddle of coolant under my car in the morning. I live in a hot climate and make heavy use of the A/C.

What possible reasons could cause radiator fluid to overflow? It looks like there is too much pressure inside the cooling system. Is this symptomatic of a clogged line or something else? If so, what action could be taken to minimize/eliminate loss of coolant?

My car has an LS1 engine, in case that matters.

4 Answers 4


I doubt the fluid is overflowing. You most likely have a leak somewhere.

Common locations:

  • The plastic container that you top off has a leak. (Check underneath it for puddles, also make sure the hose between it and the radiator is not leaking).
  • Radiator (Look on the bottom for drips, it is typically behind the grill in front of the engine)
  • Water Pump (This one can be harder to spot leaks with, normally is powered by a belt and has at least one large hose if not two between it and the radiator)
  • One of the cooling hoses (You can give them a little squeeze to see if they are hardening up and to see if fluid leaks out anywhere. You should be replacing the upper/lower radiator hoses occasionally as it is, so definitely give them a check if you haven't replaced them in the last few years.)
  • Head Gasket or other engine gasket (harder to spot, check the other places first and look for oil in the coolant)
  • Radiator cap

The key really is to find the leak, so if you can see where fluid lands on the ground, try to follow it up to the source.

  • It was a small leak after all (which manifested itself when the car was parked on an incline)
    – Zaid
    Commented Dec 31, 2011 at 22:25

The overflow is a possibility if you are overheating or do not have the proper mixture of antifreeze and water causing the fluid to boil creating excessive pressure and evacuating the system.

Water alone has a boiling point of 100C/212F. The addition of antifreeze to the mixture alters this boiling point (and lowers the freezing point, hence the name antifreeze, but the "antiboil" nature is just as important).

Are you adding straight water to the system? Pre-mixed 50/50 coolant? Or home mixed coolant?

When it was last flushed and filled, what was it filled with? (same choices as above)

You can purchase a coolant freezing/boiling point tester from most auto parts stores and even in the auto department of places such as WalMart for a few dollars. You suck up some of the coolant into the tester and it tells you what temperatures it can handle.

  • 1
    This may be the reason why... I have no idea what my coolant is at the moment (car's a second-hand purchase). The first top-up was with tap water... the second was premix.
    – Zaid
    Commented Jun 12, 2011 at 13:01

Various possibilities come to mind based on issues I've had with my cars over the years:

  • Leaky radiator cap
  • Cracked expansion tank
  • Cracked radiator
  • Cracked hose/hose with pinhole in it
  • Loose hoseclamp
  • Improperly seated hose (at an angle, not straight on)

I had a head gasket leak from the cylinder into the coolant that was pushing coolant out the top of the overflow tank. Wash off the overflow tank after filling it, so it's easy to see if it's coming out the top or from somewhere else.

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