Coolant overflow bottle

Quick question about the coolant overflow bottle on 2000 3.4L van.
What are possible reasons for the coolant to move from the rad to the overflow bottle during normal operation, but not returning to the rad after cooling down? I tried blowing through the vent hose and it seems fine. Could it be a bad radiator cap or something else?

Thanks
Mark

Bad head gaskets, bad radiator cap, etc.

Bad head gaskets, bad radiator cap, etc.
agree if it doesn't return when engine is running. if the head gasket is worn, it should have also a leak out of the gasket and engine overheat randomly.

when you start the engine after it cools down, the coolant returns to the rad?

Thanks for the replies

"when you start the engine after it cools down, the coolant returns to the rad?"

I will check that tonight. Does anyone have a picture of the two bleed screws? (I think I can see the one near the thermostat.) I want to try to make sure there is no air in the system.
Thanks,
Mark

There's one in the metallic hose of the thermostat housing. It's a big flat screw.
And another at the top of the water pump. It's kind of small bolt.
It's like that in my Oldsmobile, it may be similar.

There's one in the metallic hose of the thermostat housing. It's a big flat screw.
And another at the top of the water pump. It's kind of small bolt.
It's like that in my Oldsmobile, it may be similar.

Bingo.

I've been trying to bleed the system to get all the air out. I understand it will take multiple attemps to do this, however, approx how long should it take to bleed out all the air and have coolant come out of the bleeder screw from a cold engine?

It takes about 2 seconds. It makes a pshhhh sound, and it's over.
Yes, a bit of coolant may leaks, it indicates that you finish.

Also, the rad hoses should be rough when air is gone. Also remove the bleed bolt.

You will need to run untill the coolant comes out, usually on the stat side it will be when it opens to get alot out if you dont have one with the hole in it. (engine warms up.)

I think I have all the air out of the system. I replaced the rad cap with a new GM cap. I topped up the cooling system as needed. I test drove around the block a few times (about 10 to 15 minutes run time and about 3 to 5 km) Let the system cool and checked the rad and overflow bottle twice. All seemed okay. I let it sit overnight, and went for a 15 to 20km test drive. About 2/3 of the way through and the low coolant level light came on and off (on for about 2 seconds then off for maybe 30 seconds and then repeat on and off) The whole time the temperature gage operated perfectly, halfway between 1/4 and 1/2. When I got home, the overflow bottle was about 1/4 inch higher than when I started out, so I assume the coolant went there. I am waiting for it to cool and to see if the coolant goes back in the rad, and to see the level in the rad, but based on what I have seen so far I will assume it will not.
What would be causing this? Leak in the overflow hose, leak in one of the other rad hoses?

Thanks

A leak in the head gasket or intake gasket can cause that. When engine is running, it pumps the collant in the rad if it's cold. But it it has a leak, it'll not have enough pressure to pump it. Instead, it will pumps air. I had this random issue with my oldsmobile.

Do you loose collant under the engine sometime?

It doesn't appear to be losing any coolant. I'm trying to figure out why the coolant does not go back to the rad. The next test drive, the low coolant light went on and off in the same type of cycle. The level in the overflow bottle went up about 1/4 inch. After cooling down, the overflow bottle level went down to the line. I went out for another test, this time no light for low coolant, and the overflow bottle level did not change. I will let it cool off and see if the overflow level changes, and then go for a longer test drive. Could this no flow back to the rad be caused by not getting all the air out of the system?

Thanks

Could this no flow back to the rad be caused by not getting all the air out of the system?

No. If there's too much air in the rad, it will go to the bottle when it heats up.

The long as the temps of the engine is good and doesn't fluctuate too much, you're all right. Don't be afraid of the low coolant indicator.

The Coolant as it heats expands, thus forcing some out into the overflow jug, notice there is a cold and a hot mark? The hot mark is higher than the cold mark. If you arent loosing coolant and the temp guage is behaving you probably have a flakey level sensor, could try replacing it if you verify your levels are ok. Just my .02....

Thanks for all the replies so far. It looks like I finally bled the system of all the air and it seems that all is working properly. As the van heats up, the level in the overflow bottle will rise slightly, about a 1/4 inch, and the level will go back to the cold level after sitting overnight. I think the new rad cap may have helped. I don't dare open it now as i am afraid of introducing air in the system again. I think the coolant level sensor was/is working properly as before when the light went on intermittently, the coolant level in the rad was a little low. I'll keep my eye on it for a while.

Thanks again.