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Great Mpg with underlying problems???; Hypothetical
Topic Started: Nov 22 2011, 08:29 PM (673 Views)
chubart
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This next tank i'm sure i'll be right around 55 mpg. Right now i have all but 400 miles on the current tank of fuel and it's not even to the red line yet. I'll fill up tomorrow i can calculate it w/out running too low.

My question is can i still get great mpg with a potential underlying problem existing? Right now i'm experiencing a idle issue that i'm looking into. I'm also getting a bit of blowback of oil in the air fitler housing. Also i got the old P0420 code...o2 sensor, however it may be a bad cat. So what does the GMF know about all this?
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Johnny Mullet
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Fear the Mullet

Clogged PCV valve may be the oil issue. As long as it runs good at cruising speed, MPG's are good.
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mwebb
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FOG

chubart
Nov 22 2011, 08:29 PM
This next tank i'm sure i'll be right around 55 mpg. Right now i have all but 400 miles on the current tank of fuel and it's not even to the red line yet. I'll fill up tomorrow i can calculate it w/out running too low.

My question is can i still get great mpg with a potential underlying problem existing? Right now i'm experiencing a idle issue that i'm looking into. I'm also getting a bit of blowback of oil in the air fitler housing. Also i got the old P0420 code...o2 sensor, however it may be a bad cat. So what does the GMF know about all this?
p0420 is not 02 sensor
it is cat converter efficiency tool low
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Bad Bent
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Facetious Educated Donkey

From http://www.obd-codes.com/p0420
The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.
Causes: A code P0420 may mean that one or more of the following has happened:
* Leaded fuel was used where unleaded was called for
* An oxygen sensor is not reading (functioning) properly
* The engine coolant temperature sensor is not working properly
* Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
* Retarded spark timing
* The oxygen sensors in front and behind the converter are reporting too similar of readings
Possible Solutions: Some suggested steps for troubleshooting a P0420 error code include:
* Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
* Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
* Inspect the downstream heated oxygen sensor (HO2), replace if necessary
* Replace the catalytic converter
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Woodie
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Burning oil from the air cleaner has poisoned your cat. If it hasn't been going on too long, your cat might burn up all the oil and recover it's functionality after you eliminate the oil supply. Replace the PCV valve and clean out the T shaped tubing that connects the system together. This is very often blowby past worn rings though.

mwebb is right on this one P0420 is almost always the cat itself. It means that the second sensor is mimicking the first too closely (wildly fluctuating), cat is supposed to smooth this out. I can't think of any failure mode that would cause the rear O2 sensor to swing more than it's supposed to. It's going to fail way rich or way lean.
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KY Metro
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Woodie
Nov 23 2011, 06:12 AM
Burning oil from the air cleaner has poisoned your cat.
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