06 E350 v10 pushing coolant out of tank. need advice.
 

06 E350 v10 150K miles. Van is pushing coolant out of overflow tank. Runs fine, but periodically burps fluid out of tank. Doesn't seam to overheat. Since last week it pushed about a gallon of coolant. I just replaced thermostat, but after a good test drive tank is wet again. Anyone ideas? I'm thinking head gasket, but trying to see what else it could be? looking for ideas. Engine light has been on for a while, misfire in cyl 5 is the code I read tonight. I did replace all my coils and plugs (Motorcraft) a few thousand miles ago.

How new is the radiator cap?

Cap on my overflow tank is original, radiator has no cap.

Update: So I went out and swapped caps with my 01 Super duty V10,(has no issues cap looks exactly the same) and took it for a drive. Van ran fine till I pushed it up a big hill, then CHECK GAUGE flashed over my odometer, Heat suddenly went cold check engine light on....pulled over, coolant tank had puked again. Drove it home and took it easy, heat came back and ran fine. Code is PO305 Cyl 5,(this has been on for a bit) (I also got a PO303 cyl 3 misfire detected, after pressure blew) I do have all new coil packs and plugs with only a few thousand miles.

OEM cooling system caps have a smallish spring.
Stant aftermarket caps have a larger spring.

My cap is OEM, I swapped it for another one (off my other truck) last night but it did the same thing.

Anyone have some advice? What about radiator or cooler? I'm grasping at straws here.

Test your coolant for exhaust gasses. It's probably a blown head gasket, the test will tell you with about a 95% confidence level.