AFASTYZFR1

iTrader: (55)

Down about a cup of coolant after I drive car and let it sit overnight. But the overflow bottle is almost at the full hot mark so why isn't the radiator pulling the coolant back in from the overflow bottle? Car doesn't overheat and it runs perfectly.

LS6427

iTrader: (2)

Maybe a clogged overflow line. When it gets pushed into the overflow tank its under alot of pressure. The vaccum that sucks it back in isn't that strong. Take the over flow tank out and clean it real good and make sure the lines are clean.

Also, it could be leaking somewhere else, then when it cools off it sucks air in from that leak. Like a slightly leaking water pump gasket. Maybe a pinhole leak in a hose or the radiator too.

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AFASTYZFR1

iTrader: (55)

I checked out the overflow bottle yesterday and it was clean and the line was not clogged. I cannot find any leaks. So I guess I am going to get a new radiator cap and pressure test the system.

wssix99

iTrader: (5)

Yep - time for a new radiator cap. Unless you have a hole somewhere - that's most likely the culprit.

BTW - The AC Delco cap has a special pressure relief mechanism that lets air in to the system in the event you have a clogged overflow line. (So, the suction doesn't crush the radiator.) You may want to go to a dealer to get your new cap, if you want this feature. (To the best of my knowledge, the aftermarket caps don't have this mechanism.)

AFASTYZFR1

iTrader: (55)

Got a new gm cap, still doing it. Any other ideas......I can't find any leaks and all fluid is accounted for, it just stays in overflow bottle.

fleetmgr

The only good answer is it's sucking in air from somewhere. The question is where?

LS6427

iTrader: (2)

You're saying that the coolant that you are "losing" is ending up in the overflow reservoir????? You sure?

There's only one way it can get there...through the overflow hose in the filler neck right under the radiator cap.

Pop that hose off, top off the coolant, start it up and let it get hot. See if coolant starts coming out of that port. It might take you 20 minutes of idling. Maybe go for a ride for 15 minutes and come back. Takle the hose off first before you go for the ride so its not hot when you get back and go to pull it off.

If it comes out...you bought a "bad" new cap.

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wssix99

iTrader: (5)

The GM cap will also let in air if there is too much suction. (its a safety feature)

I'd suggest checking out the connection between the overflow bottle and the radiator to see how its performing. In an ideal world, I'd use a vacuum brake bleeder system to see if you can suck fluid from the reservoir and through the line to the radiator and would also check to make sure the line can hold suction.

If the overflow line and tank test out good, you may have your full system pressure tested to see if there is a leak anywhere.