Air in coolant system

Had to replace the head gasket. So Im sure theres air in my coolant system. My temp gauge has always bobbed and doesn't stay steady. With the engine cold, coolant in the reservoir, fill up the rad, and let the engine run/warm up. Coolant slowly flows out of the rad opening. After about 10mins or so of warming up coolant starts to gush out of the opening. Is this trapped air pushing it out? Thermostat opening?

You still have head gasket or cracked head issue if coolant flows out rad cap opening while engine is cold and idling.

Water pump isn't a "pump", it just circulates coolant, so there is no internal pressure in a cold cooling system, so nothing to push coolant out of radiator, except for cylinder pressure leaking in from bad head gasket or cracked head.
As "air" from a cylinder is pushed in to the head it displaces the coolant in that head, so coolant in rad starts to overflow making room for the new "air" coming in.

Do Glove test to confirm or to take it off the table
Cold engine
Rad cap off, coolant level down at least 1" from top of rad
Overflow hose off and its port on rad plugged, use vacuum cap, gum, putty, hose with bolt in it, ???
Put Latex glove over rad cap opening and seal it with rubber band, you can also use a balloon or even a condom in place of glove.

Disable spark, unplug coil pack or coil, you want a no start

Crank engine over and watch the Glove
If it bounces up and down you have a leak
If it just lays there then you do not, move on

If it bounces then remove 1 spark plug at a time and crank engine
When glove stops bouncing last spark plug removed was from leaking cylinder, put it back in to confirm

You can have more than 1 cylinder leaking, if Glove bounces less then keep going to find other leaking cylinder


Easy to miss a cracked head unless heads are pressure test when off the engine
4.0l OHV heads were prone to cracking between valve seats, if overheated for any reason they will crack, for sure
They don't just crack out of the blue, but if that temp gauge gets above 3/4 then they will
When heated all metals expand, this is normal, heads expand and contract every time you run the engine
Over heating expands head metal more than "normal", 4.0l heads have a weak spot, between valve seats, so crack when they expand too much

Yes the gushing out after a few minutes warm up is the thermostat opening, this is allowing the air that was pumped in from leaking cylinder to get out to radiator
The air is trapped in the top of the head until t-stat opens, the air is what causes the random temp swings as it moves around cooling system, air doesn't transfer heat as well as a liquid so engine gets hot spots around temp sender, then air gets push out of the way and sender cools down again, but same thing is happening all over top of the engine

Already performed glove test. Everything is fine. It only started coming out of the rad after idling for 10min or so. Truck is a bit higher in the rear than front so today I parked on a decent incline and "burped" the system. My temp gauge doesn't work well as the connection on the top of the sensor is loose. When i turned off the engine when I was sure it was warmed up (about 10min) coolant gushed out of the rad opening. Im assuming this is normal. Ill keep an eye on the temp gauge now that I've fixed the connection.

Takes about 4 to 5 minutes for coolant to start to expand in the engine as it warms up so yes, if there is a delay after startup for rad to start overflowing with cap off that is normal.

With cap on that coolant stays in the Radiator and pressure in the cooling system starts to go up, as it should, higher pressure raises the Boiling point of the coolant.

Rad cap rating sets pressure limit, usually 14-16psi cap rating.
When hot coolant expands enough rad cap spring is pushed up and hot coolant flows out to overflow tank, also normal, and a good thing.

The overflow setup PURGES AIR from the cooling system.
Air in the cooling system will find its way to the highest point in the system, and that point is the top of the radiator.
So when the coolant gets hot and cap opens......any air in the top of the rad is the first to go out to the overflow tank.

If you look at the setup of the over flow tank the hot coolant, or air, comes in at the BOTTOM of the tank, if it's air then it bubbles up to top of tank and is Purged from the system.
As engine/coolant cools down coolant flows back from overflow tank into the Rad not air, so self-purging of air happens every time you drive for over 20minutes or so.

If you check coolant level in rad after engine is cold and it is not topped up, then check overflow hose, it may have a leak, also check overflow tank it may have debris at the bottom clogging/stopping coolants return to rad

Rad caps have 2 valves, the larger one with big spring, and smaller one in the center, smaller spring.
When engine coolant is cooling down is shrinks back to cold volume, so pressure in the system drops down, it will get to -1psi this pulls the smaller valve open and sucks coolant in overflow tank back into the radiator