Pressurized coolant reservoir 2005 mustang

Have a 2005 Ford mustang with the 4.0L V6. Was having problems with coolant being pressurized in the reservoir and pushing out of the cap. Changed radiator cap as well as putting a new thermostat and thermostat housing in the car because it had a small crack in it that was very slightly leaking coolant. Also changed the water pump because the bearing was going out on it so decided to kill a whole bunch of bird all at once while working on it. Drove it into town a couple of nights ago and back and everything was working just fine. Drove my brother to the airport yesterday and put 4 gallons of coolant before i made it home during the 3 hour drive. Don’t believe it to be a air pocket as this was already a problem before i did this work. Have been told that it could be a cracked head gasket pushing air into the system. Any ideas? The nearest dealership or mechanic is at least an hour away but i am very confident in doing the work myself just not sure what things i can do to check or if i should just order gaskets and replace them. Thanks!

Take a look at the oil … is it sludgy and crappy looking ?If the head gasket is leaking coolant will get into the oil when the engine is shut down .

Already checked that. Ive blown a head gasket before on a different car that dumped coolant into the oil and this is nothing like that. its definitely pushing air into the coolant system somewhere.

Only way I know how to do that is a blown gasket, from the combustion chamber to the coolant. One way to check is to have the gases coming out of the coolant checked for exhuast fumes.

Big repair shops will have a sniffer that can detect exhaust gas in the coolant reservoir…That will remove all doubt…It’s worth having this done so you don’t tear the engine apart for nothing…

You can try taking the radiator cap off when its cold, starting it up and see if you see air bubbles heading to the surface. When the gaskets went on my diesel, I limped it home for 50 miles with the radiator cap loose. You should be seeing some spiking in the temp guage though I would think. Just don’t see any other way you’d get that kind of pressure in there though. If you are doing head gaskets, you have to have the heads checked for flatness and magnafluxed for cracks at a machine shop. Is that a problem if its an hour away?