91 TransAm GTA, 350 tpi, 114,000. I try to do as much as possible myself but I'm still a newbie.

It's been gradually losing power since I bought it a year ago, at first not noticeable. I thought it was associated with getting hot, because when it was hot it would lose power and sound airy, kind of like in neutral, but never *really* obvious, couldn't nail it down.

In order of what has happened : (about 3 or 4 months ago the alternator died but this is not the problem because the alternator was a straightforward case, and clearly electical.)
The only mod is a welded flowmaster muffler, got it about 3 or 4 months ago, about the same time the alternator went bad. It gives me really great mileage and I am not eager to take it off, because I like it and I can't really believe it might be making my engine run lean... But the car really hasn't been the same, at least, it made the power loss more noticable. & slight missing.
I would have changed spark plugs *then*, but then the temps started going up. My mechanic suggested I have a dealer check the codes. That was a month ago. There were no codes but the dealer said power loss could be a fuel filter (he did that) and a thermostat would fix the temps. I put in a 180 which fixed the temps and the car should love the new lower temps but there was no effect on performance. It is missing more than ever and all the time, not due to taking longer to warm up.
I bought an oxygen sensor but I haven't changed it because I don't think it will help and the old one looks ok. I've also recently had changed spark plugs & wires but I think they screwed it up.
I've decided to take back to the dealer for a different diagnostic, or at least a diagnosis but I have gotten really good advice from the message boards and I am not fond of the dealer but at least I get free snacks when they screw me. I am feeling very frustrated with the car, my ignorance, and mechanics in general. Any advice is welcome.

 

I'd think a lean, or rich, condition should light up the check engine light. Anytime I've had an O2 sensor go bad, I've gotten a code for it. On the 83Z it didn't seem to run different, but on the 81Z it would start pinging when I'd hit it hard. As far as the miss goes I'd check plugs first, a lock up torque converter (automatic transmission) or manual transmission makes a miss more noticeable. Make sure you don't have a plug wire that is burnt and arcing to the exhaust manifold. I once had a hard miss at high rpms in a 4 cyl. car. My uncle told me to pull the plugs and see if one had a greenish tint on the electrode end. He said that indicates a blown head gasket. #3 did have the green tint, I pulled the head and sure enough, there was a crack/burned spot through the metal ring in the head gasket around the cylinder. Check those plugs.

 

Oops I made 2 posts...go to the other post, while I try to sort this out.

 

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