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Coolant reservoir is non vented at cap how does this work?

My Buick’s 03 lesabre overflow reservoir cap is screwed on and has a gasket in it so it cant possibly leak air Hmmm?, I’m still wondering how does air or antifreeze get displaced in a overflow reservoir container that has no outlet to breath unless its forced through only one hose at the bottom in- and there is always a little pressure in the container to push the antifreeze back into the radiator when it cools off at the radiator metal cap?

The reservoir is not an overflow container it is a sealed expansion chamber. The COLD level allows for enough air to remain so that the coolant can expand when hot compressing the trapped air.

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My Insight also doesn’t have a vent or overflow for its non-pressurized reservoir. My guess is that the crude plastic cap won’t hold a seal. It’s easy enough to test; just open the reservoir cap with the engine hot (max coolant level) and see if air wooshes out under some pressure.

please do NOT open a pressurized coolant tank if you value your skin and eyes! the steam/hot fluid burns will be awful…

The pressure cap will have this warning on it.


Take a closer look at the inside of your cap. It actually is a pressure relief cap. It relieves pressure from the underside of the cap thru internal passageways. If you look closely you can even see the pressure relief spring thru the slots.

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Dorman HELP! - Coolant Recovery Tank Cap
Line: MTM | Part # 82594
1 Year Limited Warranty
Coolant Cap
Without Vented Cap -------------- says non vented cap
Type: Coolant Cap
Quantity Per Vehicle:

this sounds most logical because the radiators cap is a normal cap with a relief valve in side to allow coolant to come back into the radiator when it cools down. I just never thought that under a heated condition with engine running at normal temperature that the expansion tank would have any pressure in it although the way you explain it it would have some built up pressure from expansion . Thank you

Check your radiator itself for a second cap. It has to ventilate somewhere or you would blow a hose or, worse yet, the radiator. Some systems use two caps.

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There is usually a standard pressure relief cap on the radiator on cars with expansion tanks. And if the expansion tank is over filled there will be no room for expansion whitch will result in the pressure blowing off at the radiator and being dumped. Many cars, especially FWDs, have the radiator mounted at or below the level of the thermostat so an expansion tank above the thermostat is needed with a fill cap. It is impossible to fill a cooling system from the bottom or even an inch below the top.

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It’s a good question. I don’t see how an auto cooling system would work very well if it didn’t have a spring-loaded gadget of some kind, somewhere, to maintain a constant pressure as the coolant heated to normal operating temperature. But maybe the engineers at Buick have figured out a way. Maybe they discovered if there’s a big enough air space it buffers the pressure enough as the engine heats, that’s all that’s needed.

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I believe your right on that ,I even looked around the plastic reservoir to see if there was a outlet somewhere near the top because of its closed non vented cap and there isn’t any so its like you say a buffer the airspace in the top of the tank.thanks

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There MUST be a pressure relief somewhere in the system…otherwise, during an overheat condition caused by whatever, pressure would build until the weakest part in the system failed…

OP already said it has a normal radiator cap, right? Here’s a pic showing both caps:

yes you have a nice clean engine like mine. see what happens if you overheat the coolant will rise all the way in the expansion tank then the rest of the coolant will blow out the cap the metal radiator cap that’s why its rated for 15 lb’s, I see the rear rubber hood seal is hanging over the alternator ,mine came down also so I glued it back in place .

You guys need a history lesson. In days of yore, vehicles didn’t have overflow reservoirs. They only had the radiator with a pressure relief cap on top. In those days, you did not fill the radiator all the way to the top. You left about 2" of air space below the cap. If you filled it all the way, the radiator cap would let some of the water out and eventually, the water would seek the correct level.

The disadvantage to this was that as water flowed back into the top tank of the radiator, it would be turbulent and pick up some air. The air reduced the effectiveness of the water and overheated cars were a common sight along the highways.

Then along came the overflow tank (reservoir) which would keep the water from becoming aerated. Also ethylene glycol antifreeze was gaining acceptance and between the two, cooling system problems were greatly reduced.

On some GM vehicles, they went back to the old design. They removed the radiator cap from the radiator and put it on the overflow reservoir, turning the overflow reservoir into an expansion chamber. Its basically its a new style, old school cooling system, but with the expansion tank remotely located away from the turbulence of the radiator. For FWD, it has a bleed line built in so it does fill the engine completely from the bottom, or anywhere below the top.


yes your right about the old school methods they were sorta crude however they worked .

I recall some late 50s and early 60s Fords with large V-8s using an expansion tank installed between the radiator and thermostat. The ‘radiator’ cap was on the tank and the cold water level was near the bottom of the tank.

I hope this works

cool the old T bird had that set up

@lesabre1, did you try removing the reservoir cap after a drive? I did immediately after a 70 mile drive in my Insight and there wasn’t a hint of pressure build-up in the reservoir.

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