Why would a 99 Malibu dump coolant on the ground after the engine's been turned off?

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Three times now, my '99 Chevy Malibu (70,000 miles) has lost a good amount of coolant (nearly a gallon) AFTER the engine's been shut off. I only notice the puddle after ...show more
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First a quick overview of the coolant system on a vehicle - the vehicle's coolant system builds pressure when it is heated (heat causes expansion, which in turns causes pressure). A vehicle's coolant system is made to do this to help keep the coolant from boiling in the engine as the temperatures of an engine do get above the boiling point of water and this pressure in turn raises the boiling point. Once the temperature gets up to operating condition (depending on the thermostat - which is usually around 195 degrees) - the thermostat opens and allows the cooler dexcool/water mixture into the engine and releases the hot mixture out and into the radiator so it can be cooled.

If the pressure gets to high in the coolant system, the radiator cap will allow the pressurized system to vent into the overflow container. (The pressure allowed is determined by the radiator cap - usually around the 13 -15 psi mark). This usually happens if there is some problem in the cooling system - and your vehicle is usually overheating (possibly thermostat sticking or blown head gasket which allows combustion pressures into the coolant system).

Once the vehicle is turned off, the temperature decreases and so does the pressure - and if any had gone into the overflow tank, it will suck it back in when there is negative pressure on the system through the radiator cap (this is what the little springy thing on the bottom of a radiator cap is for).

If there is no cap directly on the radiator, (I am not sure about your Malibu) - then the overflow container is also called an expansion tank and allows the coolant to expand into it. There is a fill line around 1/2 way down that tank so that it will allow for the expansion of the coolant (from the heat).

Usually when the water pump fails - there is a little hole on the bottom which allows coolant out of to signify that the bearing is worn and the seal can no longer seal it - so this determines when the water pump needs replacing.

For your vehicle to leak when the the vehicle after the vehicle is turned off and depressurizing I would think that you will have a small leak that is possibly leaking onto something hot and the coolant is evaporating while running (since the engine would he hot enough). Once the engine is turned off and cools down, the system retains pressure longer than the it takes to cool off and at this time it doesn't evaporate and leaks onto the ground.

This would mean that it may be a pin hole in a rad hose above an exhaust manifold, an intake that is seeping slowly onto the engine, possible heater hose, etc.

A person would need more information for a further diagnosis - for example - where is the leak located - front or back of the engine, and if you open the hood after it has leaked - can you see a puddle anywhere above the leak on the engine.


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Great info on how the system works for a lay person (such as myself). It helps me deal w/the dealership greatly. Thanks!
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  • rocket7126 answered 8 years ago
    Why replace the water pump? You should be able to check that by taking the cap off (when cold) and seeing if the coolant enters the engine block when the tstat. opens. Either way if it was an underperforming waterpump, you would notice the problem when driving since you would overheat.

    Gaskets? Why? You dont leak gallons through gaskets? Do you know where it is leaking from? To dump a gallon when stopped there are only a few places I can think. Petcock valve, Radiator overflow, and cap.

    Recommendations: Check cap pressure, replace (cheap, and worth a shot), I am not familiar with this engine, but if it has an electric fan, check if that is functioning. Leak in radiator/hose is also a possibility. It also sounds like the system is not holding pressure (lowering booiling temp).

    There is also the possibility the engine cooling system is clogged or restricted. Did you try a reverse flush?

    Hopefully this will give you some ideas. Without exact specifics, it is a tough one to diagnose. I am also making an assumption here that you are having a boilover condition. If it is just a leak, chances are it is leaking all the time. In this scenrio I will check for leaks and if I see any coolant, make a note of it and the steam clean the engine (when it has not been running). Then if I can duplicate the scenrio, I will get a fluid trail or a leak point.

    I just had this same type of scenrio on a 95 ford explorer, I had fluid on the block and knew it was leaking from the manifold gasket, just couldnt see where. I now have pinpointed the leak and will replace the gasket.

    Feel free to contact me at mikekrejci@comcast.net.
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  • KC answered 8 years ago
    What Else does it do ,any mechanical problems like overheating,or something. when was the last time you had the thermostat replaced ..... just an idea. .maybe the check in and see if their is any other problems first. ... my truck used to do that and it was the thermost
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  • sethle99 answered 8 years ago
    How is it leaking all over the ground? The thing has an overflow tank. If you're having it leak on the ground, make sure that the tube is still hooked up between there and the radiator, or that the tank isn't overfilled or cracked.
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  • rhajha answered 8 years ago
    overflow.....overfilled it.....was the water and coolant added while the motor was running?...If it's the overflow...it will quit and you will still have a full radiater.If not...check your hoses,one could have a pinhole in it,so it would leak worse while under pressure,otherwise it will show only beaded leak.They could have a dryrot crack on the bottom side,where,it isn't as noticeable from the topside.Or a pinhole in the radiater.
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  • BabyDaddy-U Gone Smell my cologn answered 8 years ago
    I used to have a 98 Malibu, same problem. My co-worker had a 2000 Malibu, same problem. Do you have a warranty? ALL MALIBU'S HAVE THE SAME PROBLEM, WE WILL NEVER KNOW THE ANSWER.
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