Since I replaced the radiator, I have been checking the coolant level after every ride. Last night, it seemed like it did not need any more coolant. This morning, same thing - it is at the lower line. Then I go to work, come back, and figure- let me check again. The coolant is a bit below the lowest line. Hm, maybe I should add some?
So I open it, and swoosh - air comes out, almost looks like its boiling (it isn't I think it was just trapped air, but my wife was the one who opened it, and that's how she described it), and coolant rises all the way to the top! Above the top line!
Is that normal?
Obviously, the car was hot, I was driving for 45 minutes in hot weather and slow traffic.

 

The reservoir is basically an expansion tank. It gives the fluid a place to go as it expands under normal operating temp. It should be full when hot and somewhat empty when the car is cold. As long as the reservoir isn't empty you're fine.

 

That's what I thought, but the coolant would NOT have expanded had I not opened the cap! That's what confuses me - shouldn't it breathe and the level should go up and down on its own? Is the cap making too tight a seal or something??? I haven't noticed this before BTW, so it's a new thing and I don't quite get it.
Actually, every time, since I changed the radiator, when I open the cap, I get a "PUF!" of air coming out of the coolant expansion tank. Why?

 

No it shouldn't breathe, you don't want any air in there. The level does go up and down on it's own when it gets hot and cools off. The cap needs to have a good seal to keep the pressure in the system. With all that pressure, that's why it's also important to have good hoses. (not dry rotted or too soft) Ya don't want a hose blowing when it gets hot.
When the coolant gets hot the system gets pressurized and pushes coolant into the tank. When you loosen the cap when it's hot that pressure only has one place to go, out the open cap. With the cap closed the coolant should only expand to the hot mark on the tank.
I had a 74 Camaro which I opened the radiator cap when it was overheating and as soon as I twisted it the cap almost went straight through the hood. Biggest dent I had in the car. NEVER open the cap when it's hot. That's why it has the warning right on the cap. "Warning. DO NOT open when HOT"

 

Thank you deanshark, much appreciated!
(can you tell I feel dumb as a doorknob?)

 

 

... and a little more info to add to the accurate info above ...

(based on how pretty much all coolant systems work - although some have the radiator cap on the radiator and a vented expansion tank, many of these principles below apply to both types)

The radiator cap on the expansion tank (overflow tank / coolant tank / all names for the same thing) has a tight seal and a spring in it that is set to release at a certain pressure. If the system is filled properly, bled properly, the engine does not overheat, no head gasket problems, etc., then that release pressure will never be reached and the system stays sealed. The air gap between the MAX level on the coolant tank and the top gives the system the leeway it needs.

The release pressure (which is a carefully chosen design parameter, typically between about 1-2 atmospheres) and seal allows pressure to build within the system (can be felt as hard radiator hoses at pressure) which significantly raises the boiling point of the coolant (by about 2-3 *F per psi), allowing them to design the engine to operate at a higher temperature without risk of boiling over. That's why they don't just have cooling systems operate as vented to the atmosphere.

If something is wrong and the pressure exceeds the radiator cap release, the seal in the cap will press against the spring and lift up slightly, allowing air/coolant/steam/vapor to exhaust through that outlet through the black hose, safely draining onto the ground without making a mess in your engine bay. So this should not happen under normal conditions.

^^^ So that's how things are supposed to work. You should never leak coolant, never need to replenish, etc.

You may just be going through some initial bleeding process, if your system is not yet working like this.

But, that puff of air you say you always get ... thay may be an issue ...

It depends on the temperature. If your engine is hot and you release the cap, you *should* get a rapid bubbling and release - that is because by removing the cap, you instantly bring system pressure down to atmospheric, which instantly reduces the boiling point throughout the whole system to the normal one, probably around 230*F depending on your coolant mix (vs. 212 *F for pure water at atmospheric pressure / sea level). So all throughout your system, if things are hotter than that, even locally, the coolant will instantly boil, creating bubbles that can rush through your system and out your loosened cap. That's why it is dangerous to open a cap on a hot radiator / tank.

But if your car is stone cold and you release the cap there should be no residual pressure at all. One time might be nothing to worry about, but if you get it consistently, there is something wrong. One thing (the only thing ?) that can put gas + pressure into the cooling system that does not subside over time is exhaust gas leaking past a failed head gasket / cracked head / warped head. These engines generally are not known for these problems at all. But any engine that is abused by running it with a failed cooling system can overheat and cause these problems, which are permanent until repaired.

 

A little off track but IF it was a head gasket fail and repairing it exceeded value of car.....Subaru dealers carry a head gasket sealer that works.Subies had a real problem with head gaskets for a while.

BUT....I would never use that product on our cars except only as a choice between sending it to the junkyard or using it.Those sealer type products can be real bad news clogging things up.At that point only downside to trying it is the cost of the product.

Just food for thought....

 

Aftermarket radiator hoses are thinner wall thickness and will flex a bit more.

See; https://www.clublexus.com/forums/ls-...rmarket-p.html

While you are improving your cooling system;

See Also; https://www.clublexus.com/forums/ls-...-in-ls400.html

And;

Where to top off your cooling system;

https://www.clublexus.com/forums/ls-...hermostat.html

 

Great stuff oldskewel and yodaone, thank you!

 

1999 LS400 265K miles. Had something happen today that never happened before.
My wife was using the car, called me saying that she parked and while the car was still idling first heard a noise ( expansion cap under pressure?) and steam coming from under hood and then saw coolant on the ground.
I came over, inspected and found no hose leaks, looks like that coolant escaped through the expansion tank.
Last time I messed with coolant was last fall when I changed the starter and everything was fine since.
I may have over-filled a bit - it was slightly over the max line. Could that have caused the problem?
The cap is original too, replace it?

Thank you,

Val

 

That puddle of coolant suggests more than a little purge from the overflow reservoir.

If no visible leaks in the obvious places, check your water control valve & hoses on firewall, heater core, and water pump.(especiallly if aftermarket waterpump)

 

[QUOiTE=peterls;10544687]Great stuff oldskewel and yodaone, thank you![/QUOTE]

If it were me I would top it off fire it up and check for leaks.
Most h2o pumps have a weep hole. When the pump goes bad it leaks out the hole.it usually located on the pump housing bottom.

If it was a head gasket it would blow white smoke especially on first start up.

Could be a hose. Lots of fluid there possible lower radiator hose.

 

I first let it cool, then started and looked, no leaks, not overheating, added a little distilled water, drove like 10 miles, level in expansion tank not dropping...raised it on lift, lower radiator hose seems ok, Upper is new ( last fall) nothing obvious...
will look more, take intake out to see better...
meanwhile, shall I get a new expansion cap?

 

I guess you could.

But i would try to reproduce it.
Stuck thermostat?

Top it off let it run with the ac on .watch the temp gauge

Take it to a radiator shop and pressure test it.
I think you can rent them from most auto parts stores if not there not very expensive. .