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?
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
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
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.
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.
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
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.
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
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.
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
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.
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