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I have an 08 impala with the 3.5L engine...

So the other night i was idling the car to to run the block heater and notice there was no hot air coming out, despite the engine being at normal temperature. So I checked the coolant reservoir and it was empty(and I can't remember the last time I checked it). I do hear a running water sound going threw the block heater(even when its turned off). for the last 4-5 months...

as it is...
1. The engine is not overheating what so ever, and is running at normal temperature
2. I can't see any viable signs of leaks..
3. I took the rad cap off, and turned the engine on, I could see coolant running threw the system and it was coming up to the top of the cap.(there were a lot of air bubbles in it, but then went away after a few seconds, this probably the reason why i'm hearing running water)
4. I have checked the oil for signs of anti-freeze, but can't see any
5.There is white smoke coming out of the exhaust, but only during a cold start, goes away after the engine warms up.
6. this is the strange part... I topped up the coolant reservoir, and then checked it after I have driven to work(45 minute drive). and it only dropped a little bit below the full-cold line. I then checked it 3 hours later when the engine had cool, and it was empty... When I checked the level after I have gotten home, there was coolant again in the reservoir and when I just checked it(an hour after I got home) its empty again...

What I think it is...
-A bad radiator cap
-A Crack in the head gasket(I’m going to check the spark plugs on the weekend)
-A Blockage in the system somwhere(I Noticed a few specs of dirt in the reservoir)
-Something wrong with the block heater, I did not run the block heater today while driving, but I did run it while idling to see if hot air would come out(which it seems to work fine now)
-Water pump(maybe, but very unlikely)
 

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The coolant tank is NOT a reservoir. It's only an overflow tank... Always fill your coolant at the cap (when its cold!).

My 02 is leaking coolant somewhere as well. Not in the oil, but I'm thinking maybe intake... I don't think the radiator cap would cut the heat off completely. I would check the blockage (however, unlikely if the system hasn't been damaged/tampered with).

Water pump is a good place to start, as it's cheap and an easy fix, however, I would agree with you, that its unlikely to be the cause. Spark plugs would be a good place to check as well.
 

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Interestingly, I don't even remember seeing a radiator cap on the 2012/2013 Impalas! As far as I know, you can only add coolant through the "surge tank" (what they call it in the owners manual). The cap on the "surge tank" is called a "coolant surge tank pressure cap" - it's almost like they integrated the radiator cap right into the "surge tank"...

I'll have to really check for a radiator cap the next time I pop the hood - like I said, I don't remember seeing one at all...

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The only common coolant leaks on the 3.5L is a water pump, if can see signs of a coolant leak wet or dry on the a/c compressor, and coolant cross-over pipe gaskets, may see signs on the back of the engine belt side near the head gasket. When cool make sure the radiator and surge tank are full and pressure test the system, hopefully you'll find the leak.
The 2012 and up impala's with the 3.6L engine have a coolant reservoir with an integrated radiator cap. These cars do not have a surge tank since the cooling system is completely enclosed system.
 

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The only common coolant leaks on the 3.5L is a water pump, if can see signs of a coolant leak wet or dry on the a/c compressor, and coolant cross-over pipe gaskets, may see signs on the back of the engine belt side near the head gasket. When cool make sure the radiator and surge tank are full and pressure test the system, hopefully you'll find the leak.
The 2012 and up impala's with the 3.6L engine have a coolant reservoir with an integrated radiator cap. These cars do not have a surge tank since the cooling system is completely enclosed system.

Question Bart77, I have a 2012 Impala with the 3.6! My water pump is leaking and I'm replacing it within the next week or so. I will be repairing it myself. You said the cooling system is completely enclosed. So do I just refill the coolant through the reservoir and leave it at that? Do I still need to "burp" the system?
 

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The cooling system still need to be "burped" or purged of air otherwise the engine could overheat. The water pump is great at pumping water but sucks at pumping air.

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I had same problem. Turns out the overflow tank had a crack in it. Caused excess coolant to overflow onto the ground and air to get sucked into the rad when re-starting car. Puzzled me and a dealer and auto repair shop for months, but that is the problem. No cracked head, no head gasket in need of repair, just a new coolant recovery tank. Never heard of that happening. 2007 Chevy Impala LT 3.5 litre purchased new.
 
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