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Can someone explain how the coolant resevoir works?

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Old 31 Aug 2019, 07:16 pm
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Default Can someone explain how the coolant resevoir works?

I apologize if this is a dumb question, but I'm leaking coolant somewhere and saw a few videos on Youtube on how to refill engine coolant on a PT Cruiser.

Some of them poured the coolant by taking off the radiator cap, but others poured coolant into the reservoir.

But, there is a hose that goes to the reservoir, but none that come out of it.

So I don't understand how that is supposed to work. If coolant gets sent to the reservoir how does it get fed back into the radiator?
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Old 31 Aug 2019, 07:52 pm
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Default Re: Can someone explain how the coolant resevoir works?

In more detail: as the coolant is heated by the engine, it expands. As it expands, some flows out of the overflow fitting on the radiator's filler neck, and into the reservoir.
Once the coolant in the radiator cools down (i.e. you park the car for a while), it contracts, sucking the coolant from the reservoir back into the radiator. Note that the overflow hose runs into the bottom of the reservoir- this way the coolant gets siphoned back into the radiator as the coolant cools down.
Illustrated in less than two minutes:

Last edited by SpaceMouse; 31 Aug 2019 at 07:56 pm.
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Old 31 Aug 2019, 08:03 pm
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Default Re: Can someone explain how the coolant resevoir works?

In the old days, the radiator flowed top to bottom. The top of the radiator was also the reserve "tank". You filled the radiator so it was a half inch or so below the pressure cap when hot. When cold, the level would drop, however the core of the radiator always had coolant in it.

Sometimes around the '70s the side flow radiator became popular. The coolant flows from one side to the other. Obviously, with this type of radiator using one of the ends as a reserve "tank" is not practical. The radiator core would not be 100% filled with coolant.

About this time the expansion tank (or reservoir) came into use. It allows the side flow radiator to be 100% filled with coolant, all the time, hot or cold.

A few words about pressure caps. In essence, it's a two part cap.
- The upper part of the cap contains a seal to keep the coolant from overflowing on the radiator. This makes sure the coolant flows to and from the reservoir.
- The lower part of the cap is in essence a pressure relief valve. It holds the pressure in the cooling system, usually to about 15 psi, so the coolant gets hotter and the engine runs more efficiently. There's also a one-way valve in this portion of the cap. When the system in not pressurized, the one way valve opens up. Remember that one-way valve.

In a properly serviced cooling system the system is filled with coolant, there is no air in the system.

When the system is cold, the coolant is shrunk in volume and the coolant level in the reservoir is at the "cold" level. As the system heats up the coolant expands. The expanding coolant flows into the reservoir (expansion tank) and rises to the "hot" level. With the system hot the cooling system is under pressure, maintained by the pressure cap, and regulated by the thermostat and fan. When the engine is shut off, the pressure drops, and the coolant shrinks in volume. Once the system pressure drops, the one-way valve in the radiator cap opens and allows the cooling system to draw the coolant back from the reservoir. The cycle repeats every time the car is used.

A few words about leaks.
- A fair sized leaks can flow both ways, they can let coolant out of the system and let air into the system. Leaks can foul up the operation of the reservoir (expansion tank). With a leak in the system, instead of the system drawing coolant back from the reservoir, the system draws air through the leak. If a leak is let go long enough, the coolant leaks out, the air leaks in, again and again, there's not enough coolant to cool the engine, then you have a very sad person when they learn their head is warped from overheating.
- A very minor leak may allow the reservoir to work, but you find in needs fairly regular refilling. Early in the PTs life, I found myself having to top up the reservoir every two or three weeks. That was a bad head gasket, which was replaced in a timely fashion. Topping up a properly filled system should only very rarely be needed, it's not like the old days. But do keep an eye on coolant level, it can tip you off to a problem.

I had a '65 Dodge Dart where I stripped the black paint off the top of the radiator and polished the top of the brass tank. I thought it looked rather sharp, at least for a while.

Last edited by RickinPA; 31 Aug 2019 at 08:14 pm.
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Old 31 Aug 2019, 08:19 pm
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Default Re: Can someone explain how the coolant resevoir works?

Thanks for the explanation guys, I'm trying to pinpoint an intermittent overheating issue.

I'll definitely have to replace the plastic spout the radiator cap and thermostat are connected to, since it's missing a large piece of it.

When I tried to refill it through the radiator cap it spilled out and then I noticed the missing piece. I may as well replace the thermostat as well.

That's probably why I saw a load of steam coming out of my car and coolant leaking onto the ground.

But, the car starts and runs perfectly fine and also no coolant in the oil. So I don't think the head gasket failed.

So to start I'll replace the spout and thermostat, then go from there.
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Old 01 Sep 2019, 12:31 pm
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Default Re: Can someone explain how the coolant resevoir works?

There will be either a small hose running down the side, or a short spout for the "overflow".

What I have experienced with 6 PT Cruiser is...
They don't like the reservoir "topped off"!
Every time I have done that, they end up spouting off on the driveway until they get to about the half-full level of the reservoir.

2003 PT Cruiser GT Panel Van Conversion as new day driver.

Have now owned 6 PT Cruisers = Me PT Crazy!
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