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What's been done, this started last season took it to a recommend shop. They said had water in the fuel. No codes etc. Fuel filter changed and treatment added. The claim was water tested ran fine . I went from there to the lake. No difference still was skipping. End of season winterized. What I've done this year. Plugs changed all were carbon black , signs of running rich. Checked for spark on all wires, good spark. When idling there is an occasional pop in the exhaust like it's firing unburned fuel. I haven't tried isolating coils and injectors yet just frustrating I'm older reason for hiring it out in the first place. Hard to get into motor compartment. Have to check fuel pressure yet too. Just looking for conformation I'm headed on the right path.

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So glad you found us GTO (btw do u have one?)

We had a very similar issue.
Went down a few rabbit holes but it ended up being a bad coil pack. We shut them down one at w time via the ECU and isolated the bad one.

Hope this helps.

Any pics?

And welcome aboard mate!
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I've had several GTOs since my first one in 74 . Currently have a 67 post since 76, 64 factory tri power 4 speed convertible since 77, another 64 convertible, a 65 factory tri power 4 speed hardtop since 09. Several parts cars 64-67 . Not sure how to post pictures.

Maybe take a fuel sample from the bottom of the tank to check for more water.