Nutty O2 Oscillation. WTF?

[BigBirdTA]

Been chasing this gremlin for a while now and I can't figure this one out. The car runs fine during the South Florida heat during daytime hours. But when I run the car after it cools off in the evening hours, it farts, pops and backfires. Checked for the obvious like wires, plugs, even swapped O2's TWICE with brand new ones. Tried brand new AC/Delcos twice and now third time around, tried Bosch 13111's to see if it made any difference.

So, finally found my HP Tuners cable and slapped the scanner on it and took these two logs today. 1st one is the one taken at 9:00pm after things have cooled off - farting, popping and backfires. Car sometimes feels like it's dropped a cyl or is firing less than 8 holes.

Notice how the driver side O2 oscillates but stays in the upper range of the mv. Pegging the high side of each oscillation and only coming down to about 350mv. No amount of driving will make if finally correct and start oscillating through the whole range.

The second is for reference, it was taken in 95* F around noon, with about 1,000,000% humidity here in the swamp. Car ran fine, no issues.

It's been doing this for a few weeks. Under the hood is a TSP 408, MS3 with a FAST 90/90. Don't pay too much attention to the LTFT's as I started messing with the tune to see if I could tune it out, so they may be off cuz the car hasn't completely learned the trims.

What I need is someone to give me a logical explanation of what the hell is up with that Driver side O2 sensor?

[BigBirdTA]

Wow! So this one was a stinker.

Anyone with any idea? Even if this a wild one?

[BigBirdTA]

Ok, here goes another stab at trying to get someone who has seen this before or has an idea of what might be going on.

Took another log. This time later in the evening 11:15pm after the car had sat for over 6 hours. Looking at the log, it clearly gets wierder still.

Someone has suggested it might be a header or intake leak. I had been thinking along the same lines, with a header leak being my primary suspicion. But if suspecting it's a leak (header or intake), it will either leak or it doesn't. How is it that it runs fine during the day, but goes downhill as soon as it gets dark?

Will be pulling header and inspecting O2 harness next week.

Anyone seen this before? And thanks in advance to anyone who might want to share the knowledge...

[Kris_LS1]

Ok, here goes another stab at trying to get someone who has seen this before or has an idea of what might be going on.

Took another log. This time later in the evening 11:15pm after the car had sat for over 6 hours. Looking at the log, it clearly gets wierder still.

Someone has suggested it might be a header or intake leak. I had been thinking along the same lines, with a header leak being my primary suspicion. But if suspecting it's a leak (header or intake), it will either leak or it doesn't. How is it that it runs fine during the day, but goes downhill as soon as it gets dark?

Will be pulling header and inspecting O2 harness next week.

Anyone seen this before? And thanks in advance to anyone who might want to share the knowledge...

Before doing anything else, get a can of carby clean and spray around the intake to head flanges and the injector mounts, the throttle body and all that area as I suspect you have an intake manifold leak. If it is a leak the revs at idle should pick up (or if it is misfiring the revs will stablise). Also have a look at your PCV hoses, these may have perished.

[Harry Stone]

I've seen where humidity will cause a faulty plug wire or coil to short to ground, causing a condition like what you're seeing. Would only happen at night or while it was raining.