Coolant overflow tank filling

[mclasser]

So I put a new OEM rad cap on my Accord this morning. Old one was a little crusty but was still springy. Drove the car about 5 miles and everything was good. Popped the hood when I got home to do a final check and noticed the overflow tank level had just slightly risen, which I know is normal. A FYI - The tank isn't pressurized, just a regular overflow. The cold level was about 1/3rd above the low mark; it was halfway b/w L and H after the drive. I remember in the past, a similar trip around town would fill the tank almost to the high mark. Is this new cap simply holding pressure better and doesn't need to fill the overflow as quickly to vent excess pressure?

Or am I just overthinking this?

 

[Olas]

The most worrying I'd do is to drive with the heat on full for an hour or so, let it cool, check&fill if necessary.
Done.

 

[Yah-Tah-Hey]

Run your vehicle at highway speed for a few miles, let the vehicle cool down for a few hours and then check level in overflow tank. Add enough coolant to bring level up to top line.

 

[KingCake]

Did you not bleed the air? Hondas need to be bled or your level will move around for a while.

 

[CR94]

Originally Posted By: mclasser
... Is this new cap simply holding pressure better and doesn't need to fill the overflow as quickly to vent excess pressure? ...
Yes. If your old cap was leaky, that would've caused the level in the overflow container to rise more than it should've as the engine warmed up.

 

[mclasser]

Went on a 15 mile trip on the highway and the overflow level was at the halfway mark.

I don't think changing the cap warrants bleeding the system.

 

[Tosh]

Originally Posted By: HosteenJorje
Run your vehicle at highway speed for a few miles, let the vehicle cool down for a few hours and then check level in overflow tank. Add enough coolant to bring level up to top line.

Wrong: Add coolant mix to bring level up to BOTTOM line.

 

[SHOZ]

Originally Posted By: mclasser
Went on a 15 mile trip on the highway and the overflow level was at the halfway mark.

I don't think changing the cap warrants bleeding the system.
If the cap is doing it's job the air will soon be out of the system.

 

[Yah-Tah-Hey]

Originally Posted By: Tosh
Originally Posted By: HosteenJorje
Run your vehicle at highway speed for a few miles, let the vehicle cool down for a few hours and then check level in overflow tank. Add enough coolant to bring level up to top line.

Wrong: Add coolant mix to bring level up to BOTTOM line.
If the engine is at ambient and the coolant in the overflow tank is at the lower mark, why wouldn't you fill to the upper mark? The overflow tank has the capacity to hold whatever coolant is discharged when the system is hot. Leaving the coolant overflow tank level at the lower mark will allow air to enter the system sooner or later as the water component of the cooling evaporates to ambient.

 

[SHOZ]

I always fill mine to the full mark. There is still plenty of space for expansion and you can easily see how much it has gone down after a few cycles.

 

[Tosh]

Originally Posted By: HosteenJorje
Originally Posted By: Tosh
Originally Posted By: HosteenJorje
Run your vehicle at highway speed for a few miles, let the vehicle cool down for a few hours and then check level in overflow tank. Add enough coolant to bring level up to top line.

Wrong: Add coolant mix to bring level up to BOTTOM line.
If the engine is at ambient and the coolant in the overflow tank is at the lower mark, why wouldn't you fill to the upper mark? The overflow tank has the capacity to hold whatever coolant is discharged when the system is hot. Leaving the coolant overflow tank level at the lower mark will allow air to enter the system sooner or later as the water component of the cooling evaporates to ambient.

Because it's an 'overflow' tank, so when the engine heats up, hot water expands and flows into the tank, maybe as far up as the top line. When the engine cools off, the water is sucked back into the radiator, and the level in the overflow tank should be at its minimum (lower line).

 

[SHOZ]

I'd be lucky to see a 1/2" level change with my two cars between cold and hot.

 

[Tosh]

Now having read my OM in detail, I admit assuming the upper mark was a full overflow tank, when in fact at the full (upper) line, there is still room for overflow (expansion). So, yes, one should fill the overflow tank to the UPPER mark when the engine is hot.

 

[eljefino]

As long as your antifreeze doesn't spill at max temp, you're good.

I fill my surge tanks pretty full after an antifreeze change to allow for burping. I wouldn't be surprised if the assembly line does the same thing.

On a non-pressurized setup, having more cool antifreeze to absorb heat when the steam bubbles come in from the radiator means there'll be more condensing, and less evaporative loss. When the rad cap cracks at 15 PSI what comes out could flash-boil on its way down the little rubber hose since it's no longer under pressure.