Balance with the overflow bottle

JHZR2

Staff member
Joined
Dec 14, 2002
Messages
47,845
Location
New Jersey
I am not sure what should be expected, since all the cars I’ve owned have either been new with pristine cooling systems, and/or used a pressurized tank with no overflow.

For vehicles with overflow, like my 96 Ram CTD, is it expected that coolant should balance and “pull back” from that bottle to maintain level?

I know that the truck seeps slightly overboard from the front part of the HG. Very common. Ill fix it one day but for now it’s not a big deal given other priorities at home.

But one day after a long trip, few thousand miles, I found the radiator level low, like maybe an inch or a bit more when I looked inside. The overflow bottle was at the full line. So obviously as the system heats up, pressurizes, then cools down, it does not pull any coolant back from the bottle. I thought it was supposed to develop a bit of a vacuum and suck back from that bottle, because the inner seal is not as tight as the outer seal.

Maybe I’m wrong. Maybe I don’t understand. Or maybe it points to some other issue (?).

Is it expected that the level is maintained by the overflow? If so, what does it mean when the bottle stays full and the system is not?

Thanks!
 
Joined
Sep 27, 2015
Messages
3,985
Location
USA
In the unpressurized tank system, the unpressurized tank is more properly called the expansion "vessel". When the system is working properly, the radiator and engine will stay completely full at all times. As the coolant expands from heating up it moves into the bottle, and it returns as it cools down.

The return depends on a slight vacuum forming, and a leak in the pressurized side will prevent that.
 
Joined
Jan 24, 2013
Messages
1,000
Location
Upstate NY
In the unpressurized tank system, the unpressurized tank is more properly called the expansion "vessel". When the system is working properly, the radiator and engine will stay completely full at all times. As the coolant expands from heating up it moves into the bottle, and it returns as it cools down.

The return depends on a slight vacuum forming, and a leak in the pressurized side will prevent that.
Exactly what he said. Get your system pressure tested for the fastest answer.. Maybe Vatozone rents one? Hmm.
 
Joined
Jun 3, 2002
Messages
8,617
Location
MI
Thanks guys. I have a workplace standby generator with Ford engine doing the same thing.
 
Joined
Dec 13, 2006
Messages
1,127
Location
midwest
If your head gasket leaks there will be no seal to pull the vacuum against. Water will push into the overflow container but will not return. I have the same issue with my civic. I’ve changed the cap and overflow hose to no avail. I move coolant from the overflow container back to the radiator every week or two. Once there is air in the system it will continue to expand on a long drive pushing out enough coolant to the point it will overheat.
 
Joined
Jun 5, 2003
Messages
24,268
Location
Apple Valley, California
It can't pull the coolant back if there is a leak. Also a bad cap can keep it from sucking it back. Caps fail more often than people realize. Any cap that's say 5 years old or more needs to be replaced.
 
Joined
Jun 22, 2015
Messages
1,353
Location
montreal ,canada
If your head gasket leaks there will be no seal to pull the vacuum against. Water will push into the overflow container but will not return. I have the same issue with my civic. I’ve changed the cap and overflow hose to no avail. I move coolant from the overflow container back to the radiator every week or two. Once there is air in the system it will continue to expand on a long drive pushing out enough coolant to the point it will overheat.
Sounds totally logical but OP's didn't overheat even after a trip of a few thousand miles.
 

JHZR2

Staff member
Thread starter
Joined
Dec 14, 2002
Messages
47,845
Location
New Jersey
If your head gasket leaks there will be no seal to pull the vacuum against. Water will push into the overflow container but will not return. I have the same issue with my civic. I’ve changed the cap and overflow hose to no avail. I move coolant from the overflow container back to the radiator every week or two. Once there is air in the system it will continue to expand on a long drive pushing out enough coolant to the point it will overheat.
Well my HG doesn’t actually leak. Theory goes that the front few head bolts don’t clamp quite enough, so under high boost, load, and rpm, the head lifts a bit, allowing some seepage.

There is a lot of excess capacity in this system of course, and it works very well. Ive seen no sign of coolant in many weeks, including highway driving, because I havent been doing hundreds of miles at 75+, which is when it happens…
 
Joined
Dec 13, 2006
Messages
1,127
Location
midwest
It could be that your cap or overflow hose or container is preventing coolant from returning to the radiator. Or your engine is putting air into your coolant system from the combustion chamber which expands with heat and pushes coolant into the overflow. Maybe some seal pulls air in as the engine cools off and doesn’t pull coolant back in from the reservoir. Or there is an air pocket in the cooling system that can’t self bleed it’s way out. If your overflow is filling up and the radiator is getting low it will overheat if you don’t check it. I’ve had a similar problem for several years but it is now getting worse and I have to pump coolant from the reservoir and pour it back in the radiator every couple hundred miles. To answer your original question the level in the reservoir should rise as the engine heats up and return to its original level as it cools. When the engine is cold the radiator should always be full.
 

JHZR2

Staff member
Thread starter
Joined
Dec 14, 2002
Messages
47,845
Location
New Jersey
It could be that your cap or overflow hose or container is preventing coolant from returning to the radiator. Or your engine is putting air into your coolant system from the combustion chamber which expands with heat and pushes coolant into the overflow. Maybe some seal pulls air in as the engine cools off and doesn’t pull coolant back in from the reservoir. Or there is an air pocket in the cooling system that can’t self bleed it’s way out. If your overflow is filling up and the radiator is getting low it will overheat if you don’t check it. I’ve had a similar problem for several years but it is now getting worse and I have to pump coolant from the reservoir and pour it back in the radiator every couple hundred miles. To answer your original question the level in the reservoir should rise as the engine heats up and return to its original level as it cools. When the engine is cold the radiator should always be full.
Overflow hose is clear. If anything, the cap seems a little bit soft, not as firm as I’d expect for a 16lb spring. But that being the case would make it easier to push and pull fluid to the overflow, wouldn’t it?
 
Joined
Sep 27, 2015
Messages
3,985
Location
USA
The radiator cap has two rubber seals. The one on the spring loaded piece keeps pressure in the radiator. The other seal, in the top of the cap, is often overlooked. It seals to the top of the radiator neck and is important to hold the vacuum that returns coolant from the bottle.
 
Top