Question about 1999 Isuzu Rodeo

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Unburned fuel blowing out the exhaust

I just failed my emissions test. The computer showed an O2 Sensor error. I just replaced an idle air control valve last week and thought that was the final issue. The Check Engine light is still on too. The guy at the emissions testing facility (Littleton, CO) said I'm blowing unburned fuel out the exhaust which I had suspected but the repair shop (Pep Boys) denied that was happening. I noticed my gas mileage dive sharply from the 17/18mpg I was getting, down to 15mpg I get now. No noticeable loss of power though. Can this really be caused by an O2 sensor? Or is it possible I have a spark plug not firing, or valve adjustment problems? Thanks for any help some one can offer!

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  • Rick May 27, 2009

    Thank you! I'll try that tonight. Do you know if there's a way to test for which O2 sensor(s) that are failing? I have 4 total and they're about $45 each.

  • Rick Jun 02, 2009

    Replaced 2 of the Oxygen (O2) sensors ($53 each at Pep Boys) before the catalytic converters. Problem fixed!

  • Rick Feb 12, 2010

    Back again... This problem has resurfaced. I replaced the finaly 2 O2 sensors that I hadn't replaced the first time, 4 in total. After resetting the power (unplugging the battery) the Check Engine light went out. I passed emissions and my gas mileage improved considerably, about 3-4mpg. However, about 2 weeks later the light came back on and I can smell unburned fuel in the exhaust again. Is there another component that can give the same symptoms and the 02 Sensor code from the computer?

    Thanks to anyone who might help.

  • Rick Feb 12, 2010

    Hmmmm....hadn't occured to me. I haven't noticed one, but I'll check that out. I've never replaced any of the exhaust system yet and am well over 200k miles. I'll have to check that out after work. Thanks.

  • Rick Feb 12, 2010

    Thank YOU. I will post my findings as soon as I have a chance to look into it. Right now though, I'm doubtful that I have a leak. But I've been wrong before and found the correct guidance here, so I'll give it a try.

  • Rick Feb 23, 2010

    Couldn't find any exhaust leaks to my surprise. I could have missed one I guess, if it's not leaking very much. I will check again near the engine block but I don't hear anything that sounds like an exhaust leak.

  • Rick Feb 24, 2010

    Hmmm. The MAF had been suggested by a mechanic as one of a laundry list of parts they wanted to replace but I didn't do it because the error codes never indicated it, and it was kind of expensive. I'll check my repair manual and see if there's a way to test it. On visible inspection it looked fine, but I know that can mean nothing. I will also look for vacuum leaks but wouldn't those cause a pretty noticable lack of power? I drive up the mtns here in CO every weekend and don't notice any lack of power, other than what would be expected for an engine with 240k miles or so.

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Yes, that can be caused by an O2 sensor.

Valves, fouled plugs, etc., would usually cause "missing", not necessarily rich mixture.

Posted on May 27, 2009

  • 3 more comments 
  • Samuel Charles Spriggs Jun 02, 2009

    Great! Glad the problem is sovled. Thanks for the update.

  • Samuel Charles Spriggs Feb 12, 2010

    First, do you think you might have an exhaust system leak? -- like a quiet, low pitched "rumbling" noise.



    Charlie

  • Samuel Charles Spriggs Feb 12, 2010

    Great !! If the system has 200k on it, a leak wouldn't be a surprise. A leak can cause the O2 sensor to signal the Electronic Control Module (computer) that the system is running lean and the ECM will inject more fuel.



    Please let me know what you find.



    Charlie



    Charlie

  • Samuel Charles Spriggs Feb 24, 2010

    No exhaust leaks -- I would next suspect vacuum leaks or the Mass Air Flow Sensor (MAF) is not functioniing properly because of bad seals or failed electronic components in the MAF.



    Charlie

  • Samuel Charles Spriggs Feb 25, 2010

    Re vacuum leaks: some would cause a lack of power and some not so much -- it's difficult to say for sure.



    Charlie

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Idles rough and low intermittently. Stalls occaisionally. Error code P1137 pulled off of the computer. 2001 Alero, 2.4L Automatic trans


P1137 is HO2S Bank 1 Sensor 2 Lean Or Low Voltage. Oxygen sensor is reading a "lean condition". This code probably is stored from the rough and low idling. So it sounds like you maybe having other issues causing this P1137 code to come up. For lean conditions read this:
1. Vacuum leaks - check for failed or loose vacuum lines, leaking intake gaskets, intake air tubes loose or any other source of un-metered air leaks (leaks after the Mass Air Flow Sensor)
2. Restricted fuel filter or bent/pinched fuel system lines
3. Incorrect input from other sensors, such as the Mass Air Flow Sensor/cam or crank sensors, which may not always drop a separate code
4. Engine misfire ? Yes I know this one may seem weird. You might think that if there is a misfire then you will have all that unburned fuel and it should read rich; right? Well the O2 sensors read only oxygen content in the exhaust, so if you have all that unburned fuel from incomplete combustion then, you guessed it, you also have all that unburned oxygen. High O2 content in exhaust equals a lean reading!

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I have a 2003 Mercury Sable with engine code 1031 what is the problem,I need to get this corrected to have the car inspected.Thank you.


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Its refer to the Oxigen Sensor; the most common issues for lean codes are:
1. Vacuum leaks - check for failed or loose vacuum lines, leaking intake gaskets, intake air tubes loose or any other source of un-metered air leaks (leaks after the Mass Air Flow Sensor)
2. Restricted fuel filter or bent/pinched fuel system lines
3. Incorrect input from other sensors, such as the Mass Air Flow Sensor, which may not always drop a separate code
4. Engine misfire – Yes I know this one may seem weird. You might think that if there is a misfire then you will have all that unburned fuel and it should read rich; right? Well the O2 sensors read only oxygen content in the exhaust, so if you have all that unburned fuel from incomplete combustion then, you guessed it, you also have all that unburned oxygen. High O2 content in exhaust equals a lean reading! There are also some other possibilities such as an internally leaking EGR system, (but this will typically set a separate code). A leak in the exhaust system before the O2 sensor will also cause incorrect readings. And always check for after- market modifications. These can throw a wrench into the works! The only other possibilities (however unlikely), are wiring issues, computer concerns or a bad O2 sensor! There now that I’ve said it, on to rich codes.

The possible causes of rich codes are:
1. A leaking or faulty fuel injector
2. Fuel injector driver in computer shorted, or wiring short for injectors (likely a ground short)
3. Leaking or faulty fuel pressure regulator or restricted return line
4. Faulty evaporative emissions system - bleeding fuel vapors into engine (not commanded by computer)
5. On newer models a faulty fuel pump or fuel pump driver module
6. Faulty readings from other sensors such as a Mass Air Flow Sensor. You may actually be getting more air than the MAF tells the computer
7. Exhaust leaks before the sensor will cause erratic readings
8. After market components or performance chips
9. And yes, if I dare say it, possibly a computer, wiring issue or even a faulty O2 sensor!

The other codes we should address are those related to the sensors located after the catalytic converter. Though these may appear identical to the oxygen sensors pre-converter, they perform an entirely different task and are known as Monitors. The only job of these sensors is to “monitor” the efficiency of the catalytic converters. The readings from these sensors should be much more stable and not fluctuating like the front O2 sensors. The computer compares the readings from the oxygen sensors (pre cats) and the monitors (post cat) to determine if the catalytic converters are doing their job and “cleaning” the exhaust. You never want to replace a monitor for a rich/lean concern as they have no bearing on these codes. As the converters begin to fail, you will see the monitors voltage readings follow the oxygen sensor readings. Technically these are all “oxygen sensors” but it is important to distinguish the difference between pre-converter & post converter sensors, so I find it easiest to stick to calling the back ones monitors.

Hope helps (remember rated this).


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1. Vacuum leaks - check for failed or loose vacuum lines, leaking intake gaskets, intake air tubes loose or any other source of un-metered air leaks (leaks after the Mass Air Flow Sensor)
2. Restricted fuel2_bing.gif filter or bent/pinched fuel system lines
3. Incorrect input from other sensors, such as the Mass Air Flow Sensor, which may not always drop a separate code
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The pre cat sensors running lean does not mean they need to be replaced. They are are detecting a symptom. You need to know why.

If you are using a basic scantool (code only) you need to look up the P0000 code.
I thought this an odd code. I looked it up in my Autotap unit and it does not exist. The lowest # P (powertrain) code I have listed is P0016.

A regular scan tool, won't scan transmission or body codes, mu Auto tap doesn't either. You need a Tech 2 from the dealer for transmission codes.

First some history. The first O2 sensor was introduced in 1976 on a Volvo. California vehicles got them next in 1980, then federal emission laws made O2 sensors virtually mandatory on all cars and light trucks built since 1981. And now that OBD-II regulations are here (1996 and newer vehicles), most vehicles now have multiple O2 sensors, some as many as four!
The O2 sensor is mounted in the exhaust manifold to monitor how much unburned oxygen is in the exhaust. The signal from the O2 sensor tells the computer if the fuel mixture is burning rich (less oxygen) or lean (more oxygen).
A lot of factors affect the richness or leanness of the fuel mixture, including air temperature, engine coolant temperature, barometric pressure, throttle position, air flow and engine load. Other sensors monitor these factors too, but the O2 sensor is the master monitor for what's happening with the fuel mixture. Problems with the O2 sensor can throw the whole system out of whack.
The computer uses the oxygen sensor's input to fine tune the fuel mixture for the best balance of power, economy and emissions. The engineering term for this type of operation is "closed loop" because the computer is using the O2 sensor's input to adjust the fuel mixture. The result is a constant flip-flop back and forth from rich to lean which helps the catalytic converter operate at its best and keeps the average fuel mixture in proper balance to minimize emissions. It's a complicated setup but it works.
If no signal is received from the O2 sensor, like when a cold engine is first started (more on that in a minute) or the 02 sensor fails, the computer orders a steady, rich fuel mixture. This is referred to as "open loop" operation because no input is used from the O2 sensor to fine tune the fuel mixture. If the engine fails to go into closed loop when the O2 sensor reaches operating temperature, or drops out of closed loop because the O2 sensor's signal is lost, the engine will run too rich causing an increase in fuel consumption and emissions. As you might have guessed, that will set a code and turn on your check engine light.
How does it work? The O2 sensor produces a voltage once it gets hot. The sensor compares how much oxygen is in the exhaust to the oxygen in outside air. The greater the difference, the higher the voltage reading.
If you ever replace an O2 sensor (and if you're a DIY'er this is something you will do eventually), its important to remember that the O2 sensor needs to "breath" outside air to work. So don't put any grease on the sensor because it could block this air flow.
An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen left in the exhaust. When the mixture is lean, the sensor's output voltage will drop down to about 0.1 volts. When the air/fuel mixture is balanced or at the equilibrium point of about 14.7 to 1, the sensor will read around 0.45 volts.
When the computer reads a rich signal from the O2 sensor it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean the computer reverses again making the fuel mixture go rich. This constant flip-flopping back and forth of the fuel mixture occurs anywhere from 2 to 7 times a second at 2500 rpm on OBDII vehicles, depending on what type of fuel injection system they have.
The oxygen sensor must be hot (about 600 degrees or higher) before it will start to generate a voltage signal. Many oxygen sensors have a small heating element inside to help them reach operating temperature more quickly.
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The O2 sensors are expensive, diagnose what really is going on.

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3 Answers

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