2003 chevy trailblazer misfiring

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 fourty. Now my SES light is flashing almost consistantly 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? Tommorow 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 commpression check (even though I should have done that when I changed the plugs, but live and learn).

If the Check Engine light is flashing it indicates a major misfire. And each time the Check Engine light flashes it means damage to catalytic converter could be occuring.

You need to find out what’s causing the misfires ASAP before you end up replacing the cat(s).


I agree 100% with @Tester

The plugged cat (if that’s what you have) is the symptom, NOT the cause

I would also investigate other causes of misfire, besides the ignition components:

Fuel pressure test
Fuel injector balance test
Compression test . . . I’ve seen a fair share of misfires caused by low compression, FYI

Well im hoping it is not the cat as we all cring when we hear that price tag. Will wait and see what the scanner says tommorow. Then take it from there. Got more info from the regular driver of the truck. It didnt start loosing power until it started misfiring so I dont think the cat is plugged. It would have had lost power before. But he added seefaom additive to the gas and it seemed to help for a day or two. So my thinking is it may be fuel related and now with better spark (the gap was way off on old plugs) its causing the major misfires that is happening now. But im not a mechanic just a chic that pretends like she knows things. But am not driving truck anymore till I get it fixed dont want to have to fix the problem and replace the cat.

Just an update. Turns out it was a dud spark plug. Brand spanking new plug but it was no good. Took all spark plugs back out of truck, number two you could tell was not firing, no tell tale blue mark on the plug. All the rest was firing fine. Took it back, got another new one, put it back together again and it is running like brand spanking new. Not one misfire, it has power, not throwing any codes. Raised Cain with the store manager, got myself a $50 gift card. Not fair that my mechanic should have to do the job twice because of a faulty part. And I’m not one to make the mechanic eat his labor, even if it is a backyard mechanic, if its something that is not his fault. Did a compression check while we had plugs out again, everything was reading fine. Now I am going to change the valve cover gaskets and the gaskets that the spark plugs sit on (don’t ask me what they are called). It’s been happening alot here where I live, you either get the wrong part or the part is no good and it’s getting annoying. But I thank each and everyone of you for all your suggestions. Do the simple things first…



At least it was something relatively simple

Was the bad plug an AC Delco?

BTW . . . I really admire you for getting that $50 gift card. Most store managers will exchange bad parts, “as a favor”, but will tell you straight to your face, that they don’t really believe the part is bad

A thermostat I installed (as maintenance, NOT because of overheating) actually CAUSED my car to overheat, which it wasn’t doing before. The manager exchanged the part, and he told me to my face that he thought I was just flat out wrong. He told me I most likely have a blown head gasket, and I was just grasping at straws. After putting in the second thermostat, all was back to normal.

Next time that happens, I might ask for something to compensate my lost time . . .

Yes, it was an AC Delco…think I’ll stray away from that brand from now on…My 2009 Suzuki SX4 will be needing a tune up soon as it’s nearing 90,000 on it, and would rather tune it up before it starts having problems. I drive alot of miles with this car. Is there a specific brand that has a better reputation that I should ask for?

Had that happen with a Theromstat too. Learned to boil the thermostat’s first before installing them to make sure it will open when needed. Trick my grandpa taught me…What’s for dinner ma? A thermostat…

As a Japanese car, Suzuki will likely have NGKs. I’d stick with those.