Chevrolet - Trailblazerext :: 2003 Chevy Misfiring / Spitting And Sputtering

My trailblazer started with the spitting and sputtering (misfiring) last week. Over 100,000 miles on it, ya, in need of a tune up. So for obvious reasons new spark plugs seemed to be the logical choice. If it wasn't the problem, it can't hurt to change them. Truck's SES light did not come on, but truck was running pretty crappy, more when it was cold than hot, but definetely a loss of power. There is also a slight exhaust leak starting, where abouts right now I haven't pinpointed, deaf in one ear so it's hard to tell where it is coming from.

So I had my brother in law install the new spark plugs. We took the old ones out and 4 and 5 cylinder plugs had oil on them. Ah great, valve cover gasket and the gaskets the spark plugs sit on (don't ask me, not a mechanic lol) need to be changed. Started with just the plugs for now. Started truck up, still had a bit of vibration on idle, like it would from an exhaust leak, but still didn't seem right . But an even more noticable loss of power when driving, truck doesn't seem to want to get above forty.

Now my SES light is flashing almost consistently unless I'm stopped (idled). It almost feels like the cat is clogged. Did smell a heavier than normal gas smell when changing out the plugs. I know that the flashing SES almost certainly means a misfire. But why would it all of a sudden come on after changing the plugs? Tomorrow I am going to my friends garage who has the scanner tool (not just the one that reads codes) to see if it is a bad coil pack and to see if there is a specific cylinder that is misfiring.

But there is something about that exhaust leak that is leading me to believe it is the culprit of my problems. Leak came first, then the misfiring. Could the cat be clogged, which blew a hole in the exhaust and cause a misfire? And the new plugs just made the misfiring worse to cause the SES light to come on? Is there anything else I should check besides a faulty coil pack and compression check (even though I should have done that when I changed the plugs, but live and learn).