Bad radiator cap?

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Feb 12, 2004
Western Washington
I've noticed lately that coolant will flow out of my radiator into the reserve tank, but won't get drawn back into the radiator. I'll pop the rad. cap and look inside, and the level is low. If I pour coolant back into the radiator from the reserve tank, it will fill up the radiator, and both will be back at the same level they were before. I'm not losing any coolant at all.

I'm thinking that my radiator cap is bad, but I don't want to waste money on something that isn't the problem. What do you guys think?
If it is easier to draw air in thru a "leak" somewhere else it will. Maybe there is a hose collapsing somehere that has less resistance than the line/antifreeze flowing from the tank (not likely that a BITOGER would have bad hoses now is it) - or it really could be the vacuum valve in the center of the cap.

Have you checked the hose from the tank to the filler neck for obstructions???

My criteria for changing my rad cap is the condition of the rubber gasket. Rubber hardness and grooving from the seal ring in the bottom of the filler neck and a pressure test too.



ps: Is the venting for the reserve tank open???
I have had this problem a couple of times. The radiator cap had become weak allowing coolant to flow from the radiator to the coolant resevoir. Once I was actually overheating from this and the coolant was bubbling and steaming out the overflow from the resevoir. The radiator cap can be pressure tested but is probably cheaper to just replace it.
Palut, that sounds like a small leak someplace between the radiator cap and the end of the overflow tube in the bottom of the overflow tank.

Coolant can flow to the tank with almost no loss, but the long slow draw on the overflow tank allows air to be sucked in through the leak instead of water from the tank.

In my case, it was a slightly cracked brazed joint where the overflow tube was brazed into the radiator filler neck. A loose connection on the tube to the tank or a small crack in the tube can do the same thing.

JDP had some good advice on the radiator cap. Also check for seal swelling on the cap. If it increases in diameter it can bind in the filler neck.
I just replace the radiator cap every couple of years. When you even question the cap..just replace it.
Thanks for the advice everyone! I'll examine the tubing from the radiator into the reservoir tomorrow, and if I don't find anything, replace the radiator cap. I'll let you know what I find out!
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