Would a malfunctioning pressure cap cause the level in the coolant tank to decrease but not empty? It seems that when I fill it up and take it for a drive that the coolant level goes down to about a third of the “full” level but stays there. I can’t imagine where else it’s going.

 

My truck does the same thing. For whatever reason the extra coolant I was adding was venting out the overflow tube and I would smell coolant, making me thing i had a problem. I finally stopped worrying about it and for the last 1,700 miles the level hasn't dropped a millimeter.

 

@greisinb and @ahab there 3 basic reasons but as noted it may settle to a level and be fine for along time

• Over pressure
• Bad rad cap
• Pin hole leak throttle body heater is typical example.

If you replace the cap and the problem remains it is over pressure or leaking

Test for over pressure by or leak (after replacing the rad cap) :

• Adding an empty GLASS bottle to the overflow line to catch coolant
• Extend the line with some rubber hose and put the bottle in the jack container- a clear long neck beer bottle works well
• Go for a drive then check the bottle if it has significant coolant, even a quarter bottle is significant, in there you have over pressure
• If the bottle remains empty you have leak - a tiny bit in the bottom is not an issue due to the cap and return line design

The problem is figuring out why you have over pressure here a couple of reasons:

• Reduced coolant flow from a clogged rad or heater core
• Pinhole leak in the head gasket
• Bad AF mix
• A clog somewhere in the cooling systems other than the rad or heater core

 

I put a piece of electrical tape on the overflow bottle so I could monitor the level and I wouldn't expect to see anything in a catch bottle since the level isn't moving off the tape line. I did the headgaskets 1787 miles ago and after I stopped trying to keep the level on the exact line molded into the plastic my problems went away.

 

A close inspection suggests a pinhole leak in my throttle body heater. I’ll add it to the quarantine/summer todo list. Thanks for your response!

 

Is it reasonable to assume that a pinhole leak around my throttle body would also suck air back in to the system once it has cooled down?

 

@greisinb Yup your cooling system will pressurise and as the pressure drops it will stop leaking and draw air in.