I was just thinking too that if one of the spark plugs looks cleaner than the rest then you may have popped your head gasket.
That's actually not as daunting as it sounds, usually you can undo the timing belt, unbolt the head, and pry it up 1/4" or so and slide the old one out, then slide the new one in provided nothing stuck. That avoids you unplugging and disconnecting everything from the manifolds.
The 2.2/2.5 engine suffered from someone's screwup at the gasket place, there should be a shim at the corner of #4 that stops the gasket from crushing, but it ain't there. People have added their own before, made from a little round piece of shim stock ruber cemented into a cut hole in some dead space. Without it, over time the steel sealing ring will squeeze into the cylinder and burn out.
Newer gaskets don't exhibit this problem. If you suspect that a gasket is the old type, just don't crank down so hard on that corner's head bolt when you torque-to-yield.
If what you do this weekend doesn't work, btw, check on the timing belt. These motors can hop back or forth one tooth and still run, just badly. It's even funner on the turbo models where the pressure masks it, all you notice with them is some power loss.
