Coolant Reservoir

[Brandon_Kelsey]

I am losing coolant out of my reservoir and i dont know why. it only drips, theres no steady stream and it holds for a little while so i assume its not cracked. i need some help, could this be because of air pockets in the system? do i need a pressure test done? help me out. i just dont know what to do

 

[RonD]

Unfortunately it reads like a cracked head.

Quick how it works:
With rad cap in place engine cooling system is sealed
Water pump just circulates coolant, it does add pressure inside system.

As the engine warms up the coolant it expands in volume, this expansion puts pressure in the system and on the rad cap.
Rad caps are rated for a specific PSI, like 14psi(pounds per square inch)
At 15psi this cap's Main spring is pushed up and coolant is allowed to flow into the overflow tank, at 14psi it closes again.

When engine/coolant cools off the coolant shrinks in volume.
When PSI in cooling system reaches -1psi the small center valve in the rad cap is sucked open and pulls coolant from overflow tank back into the rad.
This type of system is self purging for the most part, any air trapped in the engine will usually find its way to the top of the rad and then be purged when pressure gets to 15psi.
(You can get an "air lock" in the heater hoses, but usually only when refilling the system after replacing coolant)

Pretty simple system really, but relies on sealed system, no leaks and sealed overflow hose, overflow hose must always be covered by coolant in the tank.


Engine cylinders are surrounded by the cooling passages, that's the point of the cooling system, to remove heat from the cylinders.
Cylinders have 170psi just when cranking, 1000+ psi when firing
So even a small crack or head gasket leak will force "air" from cylinder into cooling system.
This extra pressure is sometimes seen as too much coolant going into overflow tank.
The "air" being pumped in raises pressure to above 14psi quickly and that pushes coolant out of the rad, eventually the "air" will reach the rad and be purged, bubbling in overflow tank, not boiling just bubbling.
Engine temp gauge may go up and then come back down as this purging is taking place.

Or you could just have a bad rad cap


Remove rad cap and start cold engine, watch for coolant overflowing out rad cap opening and watch for constant stream of bubbles in opening, both signs of cylinder "air" being forced in to cooling system.

Search for Glove Test here, that's the next step if you see bubbles