Isuzu Repair: 89 Isuzu p/u, blown head gasket, coolant tank

Question
I have an 89 Isuzu p/u with a 2.3, a/c, p/s and a standerd trainy. It just started to missfire realy bad and there is a lot of smoke comming from the muffler. It smells really bad. After work I checked to see if the plug wires were tight and started it up. It ran a little rough but it was ok. I noticed after it warmed up that there was air being pushed past the radiator cap and into the coolant tank. Can you lead me in a direction so I can fix it. Thanks  

Answer
Sorry Erid, I hit the wrong button! Depending on the color smoke you have coming out of your exhaust, your question makes a strong case for a blown head gasket, or a cracked head. If the smoke is white in color, and smells sickly sweet; if you are loosing coolant, with no sign of a leak, then this would be your problem. Good luck!  Phil