Radiator cap sucking in air

[ISX]

My radiator has been doing this for a long time but I have not been able to fix it. I got a new radiator cap and nothing changed. It draws in air whether heating up or cooling down. I don't know what I need to do to fix it. Any ideas? I know for sure the overflow hose isn't leaking.

[guesswho512]

could it have to do with you relocating your overflow bottle...i remember when i flushed my radiator, i loosened the drain-plug and it sucked the coolant out of the overflow bottle(this was with the radiator cap ON and tight)

[flagmanruss]

I'm confused. You write "draws in air"... Do you not have a working cap & return bottle? It should draw in PAF (Permanant Anti Freeze). Let's say you have a return type cap & overflow bottle. Where is the air? The radiator tank? Is the overflow hose clamped? Is there wear or damage to the cap or filler neck? Is the tank end of the overflow in the PAF? Have you marked the oveflow tank to see if it's increasing (PAF being pushed out & not withdrawn?) (I checked my STANT radiator test kit instructions... which giives a long general list.) I've had a lot of trouble with caps / filler necks. I've mickey-moused extra rubber gaskets into caps on occasion. Sometimes playing with the cap position will inexplicably change the result.

[Mopar1973Man]

I've come across several bad caps missing the spring on the vacuum valve of the cap. As you would hold the cap in normally mounted position the valve would hang down and not close causing the radiator not to hold pressure. I've seen where upper rubber seals in the cap start to get little crakcs in them causing air to leak in. Worse yet seen radiator cap missing them out right.. Damaged radiator necks it possible cracked, deformed, or just damaged it will not seal correctly and draw from the bottle. How is the hose to the bottle is the hose damaged? Is the bottle damaged on the draw tube if its it will spit coolant into the bottle be never draw it back up... See this one too...

[Terry C]

What do you mean "draws in air while heating up"? When the antifreeze is heating up it will expand and push into the overflow bottle.

[ISX]

There is no issue with tubing/overflow it is purely the cap. I can hear it sucking in air. The tube going to my overflow is clear and I can "prime" it and watch air come into the tube straight from the radiator cap, pushing all the coolant back. It does this after I top the radiator completely off and have the hose full of green. The cap is brand new, didn't change anything.

[flagmanruss]

I have not looked at the top radiator hose on my CTD lately but my old Chevy, I used to prime the tube with a stone cold engine but using the top radiator hose as a squeeze ball & expelling the air. When I let go, if the cap was seated just right, it would draw back PAF. Yes, I had to play with that cap a bunch. I've had caps leaking through the center rivet, new caps which would not seal, old caps with with cracked gaskets, ears not engaging right, distorted radiator fill neck. Remember, these overflow/return systems were an after thought not many years ago.

[AH64ID]

Sounds to me like you either have a damaged neck on your radiator or a defective cap.

[Terry C]

You sure you don't have a headgasket leaking? That would be the only thing I can think of that would cause the cap to expel air when it is running.

[AH64ID]

Which brings up the question is it all the time or just when the thermostat opens?

[ISX]

I will do more testing.

[flagmanruss]

Do you have a cap / radiator tester? It's a pressure guage/pump combo. Goes on in place of the radiator cap... if you have a headgasket leak the guage will spike. There's an adaptor to check caps also. Mine is a "Stant".