Coolant recovery system not working

[13matsc]

Hi guys,

Over the last 4-5 months I’ve noticed a small puddle on top of the coolant expansion tank. Didn’t think much of it, as the truck had no cooling issues and never had.

But this weekend I pulled the radiator cap and saw that the coolant level was about 10cm low. When I checked the expansion tank the level was about 2cm above «low».
I transferred the excess coolant into the radiator and then the radiator level was correct.
I inspected the gaskets, spring and valve in the cap and could not see anything wrong with it. Checked my books and saw that I replaced it in 2022 along with all the coolant hoses.
Blew through the hose from the radiator to the expansion tank, and it was open.
Drove the truck some, and next time I checked, the level was back to 2cm above «low» in the expansion tank and 10cm too low in the radiator.

Seems like there is some sort of vacuum issue as the radiator does not pull the excess coolant back into the radiator?

Any ideas?

 

[Smtty58]

Lack of vacuum to make the recovery system work is signs of a coolant leak. That 10cm you're missing is going somewhere. Does the exhaust smell like antifreeze? How's the oil dipstick look?

 

[13matsc]

The 10cm is ending up in the expansion tank, slowly but steadily.

Dipstick is fine, truck has about 180 psi of vacuum on all cylinders.
I’ve never refilled coolant except when flushing.

Edit: Whar could be the cause of coolant escaping through the expansion tank cap?

 

[Smtty58]

You might still have a leak. The coolant is flowing to the overflow via the pressure release in the radiator cap. With a sealed system, the coolant should be drawn back into the radiator when the engine cools. Could just be a bad cap or the overflow hose leaks somewhere.

Edit: Coolant on top of the expansion tank could be caused by excess coolant, or a plugged overflow tank vent.

 

[13matsc]

Thanks. I was thinking of replacing the cap just to check. I think I’ll do it.

 

[H2OnSnow]

It could also be a wrong type of cap. Any chance it was replaced at any point that fits with your problem?

 

[13matsc]

Its the correct 16 psi cap. Replaced it in 2022, and have not have any issues until the last 4-5 months when I noticed the coolant on top of the expansion tank.

Went for a drive today and behold, when I came back and opened the hood there was a coolant leak. Seems like it came from either the radiator cap or the filler neck itself. There was wetness around the filler neck. It had been leaking down on to the radiator support/bracket below the headlight but the traces had probably evaporated because of running down the rad. Wondering if the mating surface on the filler neck might be worn out on this 41 year old original rad.

 

[Frankenchevy]

Do a pressure test on the cooling system. You’ll find the leak either by observing the coolant escaping or with some soapy water. At least you hope you find it on the outside after pumping it up, if it’s losing pressure.

 

[Dejure]

Thanks. I was thinking of replacing the cap just to check. I think I’ll do it.

They do wear out, but we tend to run them for 40 years. . . .

A bad cap can make a good system broke and diminish efficiency, just as a thermostat can.

Easy, cheap stuff first.