420A Backfire, rough idle

[SameOldDSM]

1999 Gs Auto. 185k

"Mods" cold air intake, headers and full exhaust with a brand new catalytic converter.

In the past few months the car has seemed to slowly idle worse and worse. Hard to explain but it idles normal until it misses once then immediately idles normal again.

On acceleration from a stop with over maybe 20% throttle it backfires through the exhaust and occasionally through the intake (loud) until it's above maybe 2500rpm then it goes away or until i let off and accelerate very slow. Once I'm moving I can floor it whenever and it runs perfectly to redline.

I've replaced, idle air control, throttle position sens, intake temp sens, fuel pump, fuel filter, new plugs, new plug wires, it had a new distributor when I bought the car. The only code that it ever shows is egr but that's been deleted long before the issues started. It burns a good amount of oil but only after sitting idle for minutes at a time never while driving not even detectable by smell. Compression test came back good on all cylinders.

Very frustrated because just a few months ago it ran perfectly. Any help is much appreciated

 

[VelocitàPaola]

I'd be curious to know what the plugs look like and/or what a monitor shows from the O2 sensors while this is happening. Also, I'm a little concerned about your comment about the car having a new distributor, since these cars don't have distributors.