Coolant level in reservoir

Please settle an argument I've had recently. It was about whether the coolant level in the reservoir is also an indicator of the coolant level in the radiator. Not at any given time, of course, but -- say -- in
mornings, before you start the car first time for the day. My position was that dropping coolant level in the reservoir under such conditions usually means the loss of coolant in the system requiring also checking the level under the radiator cap. However, with stable level in the reservoir I can open the radiator cap less often to check the coolant level there, too. My opponent said that I should check under the radiator cap just as often, even when the reservoir level is stable. What say you?
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My Opinion--When at ambient temperature --that is, in the morning, the total volume of coolant in the system is the amount in the radiator/engine plus what is in the reservoir. The total volume of coolant changes with its temperature--since the radiator/engine volume is constant the change in coolant volume is reflected in the reservoir. That's why you have a reservoir--to accept the the change in coolant volume as the coolant expands due to its temperature change. As the coolant cools it contracts, creating a vacuum in the radiator/engine which in turn opens a port in the cap sucking the reservoir coolant back into the radiator As long as there is coolant in the reservoir and the radiator cap is working properly the radiator/engine will remain filled--no need to check it. If there is a noticeable change (loss) of coolant level in the reservoir (at ambient temperature) then it indicates a loss of coolant is taking place. When there in no--Zero- coolant in the reservoir then it's time to check the radiator. MLD
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On 4/25/2012 12:34 PM, MLD wrote:

Thank you. That's exactly what my understanding is. Last time I found a rapid disappearance of coolant in the reservoir was when the heater core developed a leak. Then, when I checked the coolant level under the radiator cap, I could see the lower level there, too. I still open the cap occasionally even if the coolant reservoir level looks the same just to verify that the cap's rubber seal and spring look OK.
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Don't ever take the radiator cap off when the coolant is hot--you'll get an ugly surprise. If you take the cap off without allowing the pressure to bleed off--remember, the radiator/engine volume is under at least 15 psig and the water temperature is approx 235 F---there will be a flash of steam as the coolant (exposed to ambient pressure) quickly boils (water boils at 212 F) and you're going to get burnt. MLD
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On 4/25/2012 4:26 PM, MLD wrote:

Oh, come on! I'm not that stupid. As I mentioned earlier, I do my checking in the morning, before I get out of the garage.
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<snip for brevity>

I'd add a bit of a clarifier to your statements: The reservoir level can be used as a proxy for the rad level so long as you keep an eye on the coolant when hot and when cold, and are aware of what the level should be under each circumstance.
It is possible for the rad level to decline while the reservoir level remains the same, but you need to know the information I provided in my last paragraph to be able to tell that. One possible cause of such a situation is an air-leak in the tube to the reservoir.
The upshot is: Check your coolant level in the reservoir at /least/ once a month, and do the check TWICE: once COLD, and once HOT. Take note of what the level should be at each instance. This way, when you do your monthly check, you'll know what to expect to see.
For instance, say you check your oil at the gas station, and while doing so, you glance at the reservoir. You discover that the HOT level is the same as the COLD level. Now you know there's a problem. And you know this because you've built up a history on the fluid level.
Due to the age of my car, I check all the levels every week or so, and I usually pull the rad cap just in case.
--
Tegger

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It's a better idea to do both checks, but generally speaking, you can just rely on the reservoir level. There are one or two somewhat-obscure problems which can result in the rad getting low while not appearing to affect the reservoir level.
But you MUST take close note of BOTH the cold AND the hot levels in the reservoir. The level will raise a fixed amount when hot, and fall back to its original level when cold. This will be constant. If you see any deviation outside what may be attributable to ambient temperature swings, then you know there is a problem.
--
Tegger

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On 04/25/2012 12:09 PM, cameo wrote:

depends on whether there's a leak. if everything is leak-free, then the level of the expansion reservoir is the only thing that changes. but if there is a leak, then the radiator level can change independent of the reservoir.
bottom line, both need to be checked to make sure. but if you monitor the vehicle regularly, then your method should be good enough to catch early signs of leakage before there is any issue with the radiator level.
--
nomina rutrum rutrum

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On 4/25/2012 6:26 PM, jim beam wrote:

OK, thanks for all the responses. They were mostly reassuring and where they were not, I'll adjust my method of checking per your suggestions. I alway learn something new on this NG.
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