What would cause coolant to come out of the cap on the resovoir? The cap is on fine, i got a new one and it still does it...Any advice would be great

 

Possibly a blown head gasket. The combustion presure enters the coolant system through the blown gasket.

 

a sticking thermostat will do it....

a collapsed lower hose will do it...upper as well...

both with cause the pressure to go somewhere and in the res is where it goes...

 

A cracked reservoir will do it. Had that one happen on my 04 F350 with 6 Point Uh Oh diesel. Thought it was head gaskets, they are known for dislodging them, but it was just the degass bottle was cracked at the neck.

 

I believe the head gaskets are fine, my coolant system is full of rusty stuff too so i am gonna replace the t stat before i do anything else.

Sometimes when your driving it the temp gauge will drop to cold and the heat will turn to cold then it slowly comes back up and its fine...

 

Start with a thermostat. 10$ I would even flush my system since I would be in there

Maybe coolant temp sender, could be corroded or gunked up as you said. 20$

Check your hoses? as bowtie mentioned

Sometime the drop in cold/heat could be coolant too low or air in the system.

 

the rusty stuff is from the inside of your radiator, your going to have to end up replacing it eventually, a new thermostat will help for a while, but will soon be clogged with the iron oxide particles coming off in the coolant...I had a supra that did this, after replacing three thermostats, i finally sprung for the damn radiator, and it fixed it right up. Mine would just overflow into the overflow tank, to the point it overfilled that and dumped it all on the ground, it was a PITA...