Section 1: Universal Testing Methods pg8 Low Power

The WOT o2 test has helped me nail down quite a few complaints that other techs had given up on. I'm wondering about variables to this test however.

Are there any variables that would result in a rich upstream o2 reading @ WOT that is NOT a restricted exhaust?

None that I have run into Noah. I'm sure some type of timing chain or belt issue but you would have other symptoms.
I have seen a shorted rotor on a distributor style ignition system cause the O2 to drop lean at WOT, thought for sure it was a fuel pressure problem because it only did it under load.

Thats cool, thanks for the response.
That rotor must have been a fun diagnosis!

Man I've got a great case study coming up. I really want to make a video on it. 96 Tahoe rich exhaust codes, no activity on O2 sensor codes, and no power? Guesses?

Sounds like fun, I'd watch that.
Wild guess... leaking CFI?

To answer your question, I think it's normal for the O2's to go rich at WOT. More power more fuel, last thing you want is a lean condition under load.

Yup, agreed. I'm wondering if there's a scenario in which there may be a low power complaint, when under WOT the upstream o2s go full rich (as to be expected), but the low power is not the result of a restricted exhaust.
In light of the recent discussion about fuel contamination, I guess that junk fuel could cause this, but would likely exhibit other symptoms.

Maybe aftermarket injectors with higher flow rates? Plugged air filter? Incorrect timing as Paul said can cause problems. That's about all I can think of off hand.

Ooh, good thinking with the plugged air filter. Some kind of intake restriction sounds reasonable.
I know it's kind of a weird question.

A restricted intake could cause what you're describing. I actually experimented with this once, just to prove the idea. I'll see if I can find the scan data captures.

EDIT: Found 'em on ScanShare. Don't forget to hit 'Open PID List'

Here's a known good WOT run, Chevy 5.3L:

www.scanshare.io/share/PthKNw1SfUGJnZW7SEl4Og

And here's the same run with an exhaust cone stuffed in the intake tube:

www.scanshare.io/share/usHlQlFdnkiOTKEZYiTR8w

The giveaway is the MAP sensor showing vacuum in the intake while at WOT.

Wow, look at that MAP voltage drop off! The MAF is is significantly lower as well.
Looks like I can add "restricted Intake" to my notes on that page.
Thanks Tyler, Chris and Paul!

I wonder if a stuck closed intake manifold tuning valve would show similar results...

Haha, read my mind! You might remember a thread about an '09 Camry on the old forums, guy swapped an engine and had no power afterwards. His A/F sensors were stupid rich during a WOT run.

I read somewhere that California emissions engines came with intake runner "butterflies" while Fed engine didn't. Sure enough, he was sold a California engine, and used the intake that came with the replacement engine. Swapping the intakes fixed it.

About the ScanShare.io captures, yeah, isn't that crazy? I really wish more cars had MAP sensors after that experiment, lol.

Just thinking out loud, do you think the location of the restriction would be significant? As in, before the MAF, or after? The installed restriction in those captures was after the MAF, just not sure if a restriction before the MAF would have the same O2 behavior.

I don't recall that particular thread, but it makes sense. So if you were measuring vacuum with a gauge, the results would be what, the opposite of what a plugged exhaust would look like?

I think so? I'm kinda curious now, thinking about doing some experimenting on my guinea pig. Maybe take some video of the results?