Your 1st question mark: How did I come up with the 20% fuel consumption?
Answer: It's about the voltages. Each cel has a voltage trip point, for its light to come on and this one is 20% from spec. This p420 cel is based on a voltage change and it has nothing to do with oil. The ecu uses O2 voltages, as one of many parameters to determine the fuel to air mix. The ecu compares the O2 voltages to preset values. When they are off by 20%, the cel comes on.
As the voltage mismatch (O2 Primary vs O2 Secondary) becomes greater the percentage, from the preset tolerance (spec) value will increase.
Again not correct statement. There is no change in fuel consumption with gutted cats on MDX
FYI:
What causes the voltage changes?
Answer: The ability of the cat to have enough surface area to convert exhaust to something that meets federal regulations. When the surface area is reduced or damaged the voltages change.
FYI:
When oil gets past the oil rings, where does it go?
Answer: It's blocked by the cat, burnt by the cat, and a residue is built up inside the cat, as it tries to go out the exhaust pipes. If the (residue) restriction is enough to cause a 15% voltage difference between the primary O2, when the ecu compares it to the voltage at the secondary O2, the ecu will correct those voltages by adjusting the fuel to air mix. Adding fuel to make the mix less rich and to correct the O2 voltage ratio (between the sensors), of 15% will not throw the p420 cel. When you look thru a good cat you can see daylight, no so with a clogged cat.
You keep repeating same thing, question was at what rate you think oil should be burning to clog the cat. I just worked on RL with high oil consumption, according to owner 3-4 quarts per oil change. Cat was in perfect condition (like NEW). O2 readings have nothing to do with oil being burned in cylinder...
Another CAT pic of my oil burner MDX. Do you see any oil damage, or traces of engine oil byproduct?
2nd question mark: See answer #1.
Still not answered
3rd question mark: How come with a straight pipe you get a p420, but you get very little fuel consumption?
Answer: You are incorrect and that is why a fouler is used to pull the secondary O2 sensor out of the direct flow of exhaust, which prevents the p420 cel. Without a secondary O2 voltage the ecu will try to change the voltages that it has received, by changing the fuel to air mix.
again you do understand that P420 code is for downstream oxygen sensor which is after cat. It measures pollutants not air/fuel ratio....This job is for upstream sensor...
4th Question (no question mark): O2 sensors do not read oil, they send oxygen levels back to the ecu via a voltage value. The ecu is programmed to change the fuel to air mix based on several factors and the voltage difference, between the Primary and Secondary O2 sensors is one of the factors.
In your photo, that shiny goo will coat everything that it comes in contact with, including the guts of a cat. The cat will burn it, but there is still a residue left inside the cat.
Here's a question for you: What causes a cat to become clogged... Is it air, spark, or petroleum?
Clogged cats are more common with severe oil consumption cases (like one quart of oil every gas fill-up or leaky intake/exhaust valve steams) witch is not the case with j37 engines.
So have you ever seen dead j37 or j35 CAT? I did, few of them failure was never caused by oil consumption, ceramic core was destroyed. Bad fuel, water in fuel and bad a/f ratio will damage it for sure.
You never answered what is first symptom of failing cat in j37 engines? It is not a p420 cel, but a misfire. Cylinder bank with bad cat will start throwing misfires. Especially after steady drive like interstate and sudden stop. No hard cel were produced, sample pic of the scanner:
And there is no goo in the pic . Piston is little washed with fuel, because there was misfires and misfires was cause by burned/destroyed cat. Gases were backing up in to combustion chamber and there was no combustion...
I am just trying to explain how everything works in real life, actually experience, I gathered working on Acuras, Hondas Some times google cannot produce correct answers...