This morning, I started my car, and it ran really rough for about a minute. There was some white smoke coming out the back. The problem went away, and everything seemed normal. This afternoon, I tried to start it, but it turned over, then died. When I tried again, it stayed running, but ran rough again, and I saw more white smoke. Again, it ran fine after that. My coolant has been disappearing, with no leak. Do I have a bad head gasket, or could something else cause this? Would that also explain some of the misfires I've had around 3000 rpm?
The symptoms you have described are classic signs of a head gasket leak. Do not operate the vehicle any further until repairs can be made, as continued use of the vehicle can cause extensive engine damage.
When the head gaskets fail it allows coolant to become introduced into the cylinders. When coolant is burned it gives off a white smoke. A qualified technician, such as one from YourMechanic, will be able to diagnose your car’s smoke issue and determine what repairs are necessary to get you back on the road.