Radiator overflow not returning fluids - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**landarts** · 09-18-2009, 03:35 PM

I have a 72 1/2ton Cheyenne that I am in the process of restoring. It had the original radiator overflow tank in the truck when I bought years ago. When I drive for a bit it dumps fluid from the radiator into the tank. I guess thats normal from what I have read on the forum. However that fluid never returns back to the radiator at any given time. So usually after a few days of driving I siphon that fluid into a used anti-freeze bottle and dump it back into the radiator. Do you think there could be something wrong with the cap on the overflow tank causing it not to return. Or am I missing something else ?

**Stocker** · 09-18-2009, 03:39 PM

Assuming the radiator cap is for a closed system, it's probably bad. If it's not for a closed system, it should be. The cap on the overflow tank shouldn't affect anything.... unless it's not vented and the tank is sealed.

hayhauler71 · 09-18-2009, 04:00 PM

Also make sure your overflow hose is not kinked or pluged.

69GMCLonghorn · 09-18-2009, 04:18 PM

Also, I believe that the overflow should have some coolant in it even when the engine is cold and the radiator is full. There should be a "cold" and "hot" line. This may have some effect on what you are talking about.

powell72 · 09-18-2009, 05:14 PM

Is the hose long enough to reach to the bottom of the overflow bottle so it can pull the coolant back into the radiator when it needs it?

cdowns · 09-18-2009, 05:22 PM

you need a closed system recovery type radiator cap

**landarts** · 09-18-2009, 05:54 PM

I started with the level marked cool on the side of the overflow tank. The hose is long enough because it pulls fluid from the radiator to the overflow tank. I replaced the old worn out cap a few weeks ago with a 16lbs cap from napa. Told the man at the counter what year of truck , engine etc . How do you tell the difference from a closed system cap from open system?

ulakovic22 · 09-18-2009, 07:00 PM

The hose either needs to go through the top of your resevior and reach the bottom of the tank or the hole needs to be on the bottom of the resevior. If the hose isn't long enough then the radiator will be sucking air back into the radiator instead of coolant. Hose length doesn't have anything to do with coolant going from the radiator to the resevior.

Think of a straw in a glass, you can put water in the glass through the straw, but if the end of the straw isn't long enough to reach the water at the bottom you can't suck it back out. Make sense?

neonlarry · 09-18-2009, 07:04 PM

If you told the parts guy the cap was for a 72 truck you most probably don't have a closed system cap. These trucks did not come from the factory with a recovery bottle, therefore they did not use a closed system cap as OE.

mclairmo · 09-18-2009, 07:16 PM

I believe the closed system cap has two seals on it, one on the bottom part that seat against the inside of the radiator filler and one just under the cap part that seals the outer cap to the outer lip of the filler. That way, the radiator can build up a vacuum and suck fluid back in without sucking in any air from the outside.

09-18-2009, 03:35 PM	#1
landarts landarts Join Date: May 2005 Location: Star, Idaho Posts: 1,350	Radiator overflow not returning fluids I have a 72 1/2ton Cheyenne that I am in the process of restoring. It had the original radiator overflow tank in the truck when I bought years ago. When I drive for a bit it dumps fluid from the radiator into the tank. I guess thats normal from what I have read on the forum. However that fluid never returns back to the radiator at any given time. So usually after a few days of driving I siphon that fluid into a used anti-freeze bottle and dump it back into the radiator. Do you think there could be something wrong with the cap on the overflow tank causing it not to return. Or am I missing something else ?

09-18-2009, 03:39 PM	#2
Stocker 20' Daredevil (Ret) Join Date: May 2003 Location: Jefferson State Posts: 13,534	Re: Radiator overflow not returning fluids Assuming the radiator cap is for a closed system, it's probably bad. If it's not for a closed system, it should be. The cap on the overflow tank shouldn't affect anything.... unless it's not vented and the tank is sealed. __________________ - Mike - 1972 K20 LWB 350/350/205 RIP El Jay Last edited by Stocker; 09-18-2009 at 03:43 PM.

09-18-2009, 04:00 PM	#3
hayhauler71 Registered User Join Date: Aug 2009 Location: St. Paul MN. Posts: 1,996	Re: Radiator overflow not returning fluids Also make sure your overflow hose is not kinked or pluged.

09-18-2009, 04:18 PM	#4
69GMCLonghorn Registered User Join Date: Dec 2006 Location: Porter Ranch, CA Posts: 978	Re: Radiator overflow not returning fluids Also, I believe that the overflow should have some coolant in it even when the engine is cold and the radiator is full. There should be a "cold" and "hot" line. This may have some effect on what you are talking about.

09-18-2009, 05:14 PM	#5
powell72 Senior Member Join Date: Jan 2005 Location: Loveland Co. Posts: 3,098	Re: Radiator overflow not returning fluids Is the hose long enough to reach to the bottom of the overflow bottle so it can pull the coolant back into the radiator when it needs it? __________________ Chris 74 Custom Deluxe SWB 350/350/3.73’s 99 2wd Silverado RCSB 5.3/4L60/3.90’s http://www.classictrucks.com/feature...short_bed.html

09-18-2009, 05:22 PM	#6
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	Re: Radiator overflow not returning fluids you need a closed system recovery type radiator cap __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

09-18-2009, 05:54 PM	#7
landarts landarts Join Date: May 2005 Location: Star, Idaho Posts: 1,350	Re: Radiator overflow not returning fluids I started with the level marked cool on the side of the overflow tank. The hose is long enough because it pulls fluid from the radiator to the overflow tank. I replaced the old worn out cap a few weeks ago with a 16lbs cap from napa. Told the man at the counter what year of truck , engine etc . How do you tell the difference from a closed system cap from open system?

09-18-2009, 07:00 PM	#8
ulakovic22 Registered User Join Date: Aug 2008 Location: Lantana, TX Posts: 419	Re: Radiator overflow not returning fluids The hose either needs to go through the top of your resevior and reach the bottom of the tank or the hole needs to be on the bottom of the resevior. If the hose isn't long enough then the radiator will be sucking air back into the radiator instead of coolant. Hose length doesn't have anything to do with coolant going from the radiator to the resevior. Think of a straw in a glass, you can put water in the glass through the straw, but if the end of the straw isn't long enough to reach the water at the bottom you can't suck it back out. Make sense? Last edited by ulakovic22; 09-18-2009 at 07:03 PM.

09-18-2009, 07:04 PM	#9
neonlarry Registered User Join Date: Feb 2002 Location: Vacaville, CA Posts: 2,746	Re: Radiator overflow not returning fluids If you told the parts guy the cap was for a 72 truck you most probably don't have a closed system cap. These trucks did not come from the factory with a recovery bottle, therefore they did not use a closed system cap as OE. __________________ 70 C/10 Light Red 350/TH350, HEI, Duals w/40 series Flows, 91 seat, LED taillights 99 Pontiac S/C GTP, SLP Ram Air hood, GMPP Konis & springs 95 Neon ACR, MP PCM, AFX UDP, 3.0 CAI

09-18-2009, 07:16 PM	#10
mclairmo Registered User Join Date: Jan 2006 Location: Take Off Posts: 1,908	Re: Radiator overflow not returning fluids I believe the closed system cap has two seals on it, one on the bottom part that seat against the inside of the radiator filler and one just under the cap part that seals the outer cap to the outer lip of the filler. That way, the radiator can build up a vacuum and suck fluid back in without sucking in any air from the outside.



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