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Discussion Starter #1
Hello,

I drive a 2011 that is affected by the oil consumption issue (two top ups needed before change time). Since I switched jobs and started driving 86 miles on the highway daily, fuel consumption has become horrid: ~16 mpg down from 18+ mpg.

I confess my combined driving habits are not the best (leftmost lane speed, ride set to Sport, and I rely on the ACC for most of the drive). I already experimented with the driving variants for a week a time (right lane drive a week, ACC off another week, Comfort ride a third week etc.) but there was no significant change.

I am trying to pinpoint if the oil consumption issue causes bad fuel economy in general or if something else needs to be taken care of to regain a tolerable mpg. Of if 16 mpg is the norm in which case I would think of getting something smaller as a daily driver and keep the beast for whenever the whole family is in.
 

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Realtime highway MPG for my 2008 MDX when going on cruise at 75-80 mp/h (120-125 km/h) with moderate traffic is 23-25 mpg (9.2-10 l/100 km), so 18 seems way too low already, let alone 16.

Sport/comfort has nothing to do with mpg (well, almost nothing), so that's out of the equation. If you are saying that changing driving habits didn't help, I'd say something is wrong with the car. Do you have any check engine lights? Are you using at least 91 octane?

What's your average (B) mileage?
 

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You know that you have a problem.

1. oil getting past the oil control rings will be recognized by the emissions system.
2. the differences in what the emission O2 sensors report back to the ecu will cause the fuel to air value to be changed.
3. if the ecu thinks that the fuel to air mix is too rich (meaning burning too much oil) it will attempt to thin out the mix, by adding more gas to the mix.
4. as more fuel is added to the fuel to air mix your mpg is reduced and once the ecu has increased the fuel by 20% a check engine light will appear.

Using the factory estimated value of HWY mpg = 20 plus an additional mpg reduction of 20%... is a 4 mpg loss and your 16 mpg hwy is to be expected.

Is oil getting in to the catalytic converters good or will it cause them to fail?
 

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You ride in sport, you mean transmission set in sport? IF so there will be increase fuel consumption especially on highway (last gear is eliminated).

carpayment4life
Could you please educate me how do you come up with that information? This is kid a opposite what I know/have learned. 20% increased fuel consumption is due to oil consumption?
O2 sensors are there to monitor OXYGEN. They dont read any kind or any form oil in the combustion gases. What they could see is change in combustion mixture due to carbon buildup in combustion chamber. Smaller chamber-> spark is firing not at the right moment->not the most optimal air/fuel mixture burning process>preignition/detonation.
 

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The ecu recognizes the oxygen mismatch in the emissions path, by tracking O2 sensor voltages.
When mismatched voltages are recognized the ecu changes the fuel to air mixture, for each fuel injector.
Certain voltages indicate that the fuel to air mix is too rich and the ecu "leans out" the mix with more fuel.

The oil is going somewhere and there are no oil leaks on the driveway.

The oil that is getting past the oil control rings has to go out the exhaust gas path.
The oil begins to clog up the cat and (over time) prevents the cat from doing its job. (Eventually it will be so clogged that you won't be able to accelerate up an expressway's on-ramp.)
The oxygen levels being seen by the secondary O2 are not what they should be, because the cat isn't doing its job.
The ecu compares the values reported by the primary O2 and the secondary O2, then changes the fuel to air mix; lean vs rich.
Once the ecu has changed the mix to correct the "mismatch values" by 20% it displays a check engine light (p420 cel).
When you use 20% more gasoline your mpg goes down by 20%.

The reason for Emissions monitoring goes back to the early 70s, when 20 year old cars from the 50s were blowing blue smoke out of their exhaust pipes, because they needed a ring job. The government couldn't mandate a ring job. Instead, they found a way to make it so expensive to keep old cars on the road that people would have to get rid of them. Car makers loved it.
 

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The ecu recognizes the oxygen mismatch in the emissions path, by tracking O2 sensor voltages.
When mismatched voltages are recognized the ecu changes the fuel to air mixture, for each fuel injector.
Certain voltages indicate that the fuel to air mix is too rich and the ecu "leans out" the mix with more fuel.

The oil is going somewhere and there are no oil leaks on the driveway.

The oil that is getting past the oil control rings has to go out the exhaust gas path.
The oil begins to clog up the cat and (over time) prevents the cat from doing its job. (Eventually it will be so clogged that you won't be able to accelerate up an expressway's on-ramp.)
The oxygen levels being seen by the secondary O2 are not what they should be, because the cat isn't doing its job.
The ecu compares the values reported by the primary O2 and the secondary O2, then changes the fuel to air mix; lean vs rich.
Once the ecu has changed the mix to correct the "mismatch values" by 20% it displays a check engine light (p420 cel).
When you use 20% more gasoline your mpg goes down by 20%.

The reason for Emissions monitoring goes back to the early 70s, when 20 year old cars from the 50s were blowing blue smoke out of their exhaust pipes, because they needed a ring job. The government couldn't mandate a ring job. Instead, they found a way to make it so expensive to keep old cars on the road that people would have to get rid of them. Car makers loved it.

I understand how check engine light is triggered:), but you never explained how you came up with 20% increased fuel consumption? If there will be p420 cel so that means there will be fuel consumption increase by 20%? How come with test pipes there will be code P420 but there will be little effect in fuel consumption?
And what oil consumption have to do with increased fuel consumption since oxygen sensor cant read "oil" in the mixture... and burning oil will not change mixture to much.
Now you stated that oil will clog cat, have you ever seen j37 engines clogged cat and at what rate it have to burn oil to clog the cat? Do you know what is first symptom of failing cat in j37 engines?
Here is a picture j37 combustion chamber, looking at it, can you tell me whats wrong with it, give me your wild guess:
114987
 

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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.

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.

2nd question mark: See answer #1.
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.

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?
 

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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?
114989




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:

114988

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...
 

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Discussion Starter #10
Realtime highway MPG for my 2008 MDX when going on cruise at 75-80 mp/h (120-125 km/h) with moderate traffic is 23-25 mpg (9.2-10 l/100 km), so 18 seems way too low already, let alone 16.

Sport/comfort has nothing to do with mpg (well, almost nothing), so that's out of the equation. If you are saying that changing driving habits didn't help, I'd say something is wrong with the car. Do you have any check engine lights? Are you using at least 91 octane?

What's your average (B) mileage?
I have had the MDX for two years. Fuel economy was around the 19.6 mpg. Coming from X5s, I thought this was normal for its size.

The oil consumption issue started in the last six months. Since then I have seen fuel economy go down to 18 mpg. Then in conjunction with the new job and longer drives on the highway, economy is down to 16 mpg now.
 

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Discussion Starter #11
You ride in sport, you mean transmission set in sport? IF so there will be increase fuel consumption especially on highway (last gear is eliminated).

carpayment4life
Could you please educate me how do you come up with that information? This is kid a opposite what I know/have learned. 20% increased fuel consumption is due to oil consumption?
O2 sensors are there to monitor OXYGEN. They dont read any kind or any form oil in the combustion gases. What they could see is change in combustion mixture due to carbon buildup in combustion chamber. Smaller chamber-> spark is firing not at the right moment->not the most optimal air/fuel mixture burning process>preignition/detonation.
No, I rarely put the transmission in Sport. I was referring to the Comfort/ Sport suspension button. I use that all the time.
 

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There is a solution, but you have to tamper with the emissions system, which is against the law.

Here's one, from e-bay. angled O2 adapter

Unplug the O2 wire's plug, remove the secondary O2 sensor, install the adapter, screw the O2 sensor in to the new adapter, and plug the wire back in. Then use a OBD scanner to clear the cel and your mpg will be much better, because the p420 cel is gone. You will fail a visual emissions inspection.
 
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