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Unread 05-17-2011, 07:27 PM   #1
PFCLinscomb
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ECU Throwing trouble codes -_-

Okay i have a 1994 Jeep Wrangler 4.0L Inline 6 and the check engine light came on so i did the key rock 3 times to get the ECU to flash out the trouble codes and i get code 51-Oxygen sensor-lean condition and i don't know whats causing this because its running fine. So i wana know if its a bad sensor or what because I'm at a loss, seeing the damn check engine light is driving me up the wall

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Unread 05-17-2011, 07:38 PM   #2
MJMPC
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Do you have a cracked ehaust manifold?
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Unread 05-17-2011, 07:45 PM   #3
PFCLinscomb
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Do you have a cracked ehaust manifold?
Nope, That's the first thing i checked
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Unread 05-17-2011, 09:34 PM   #4
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Clear the code runn it see if it comes back.
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Unread 05-17-2011, 09:53 PM   #5
PFCLinscomb
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Clear the code runn it see if it comes back.
Already tried that, it ran for about 20 min and sure enough, code 51 came back again. Any ideas?
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Unread 05-17-2011, 10:32 PM   #6
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Ok so your running lean... this meains you have too much air, or not enough fuel exiting the combustion chamber. There are two ways that you could be receiving a false reading and thus a false CEL... if you have a crack anywhere in your exhaust system between the O2 sensor and the engine or, the sensor is bad. If you are 100% sure that you checked arround your collector, and around each pipe for cracks and there isn't one... then you have the posibility of a bad sensor, or your not getting enough fuel into the combustion chamber.

Not enough fuel can be caused by a failing fuel pump, a clogged fuel filter, faulty/dirty/clogged injector(s). Either of these will give you a lean condition every time. If your exhaust checks out, I would HIGHLY recomend replacing the fuel filter, if that doesn't work, you will need to run a fuel pressure test. This can be done with a rental fuel pressure test kit from Autozone, Advance Auto Parts, etc... Connect the fuel pressure tester to the fuel rail and see what you are reading and compare that to average operating PSI of our motors.

I would start with an extremely thurough inspection of the exhaust manifold, then consider replacing the fuel filter, then consider a fuel pressure test... If all of those lead nowhere, then replace the sensor. I say replace the sensor last because it's the most expensive.

1. Check exhaust system 100% between O2 sensor and block. (FREE)
2. Replace fuel filter and re-check trouble codes... (~$15.00)
3. Rent fuel pressure tester and verify fuel system performance (Rental..l FREE)
4. Replace O2 sensor if the above 3 turn up alright... (~$50+)

IMHO, those are the steps I would start with to diagnose a lean condition with the jeep still functioning symptomless.
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Unread 05-17-2011, 10:43 PM   #7
PFCLinscomb
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Quote:
Originally Posted by Apollo12227 View Post
Ok so your running lean... this meains you have too much air, or not enough fuel exiting the combustion chamber. There are two ways that you could be receiving a false reading and thus a false CEL... if you have a crack anywhere in your exhaust system between the O2 sensor and the engine or, the sensor is bad. If you are 100% sure that you checked arround your collector, and around each pipe for cracks and there isn't one... then you have the posibility of a bad sensor, or your not getting enough fuel into the combustion chamber.

Not enough fuel can be caused by a failing fuel pump, a clogged fuel filter, faulty/dirty/clogged injector(s). Either of these will give you a lean condition every time. If your exhaust checks out, I would HIGHLY recomend replacing the fuel filter, if that doesn't work, you will need to run a fuel pressure test. This can be done with a rental fuel pressure test kit from Autozone, Advance Auto Parts, etc... Connect the fuel pressure tester to the fuel rail and see what you are reading and compare that to average operating PSI of our motors.

I would start with an extremely thurough inspection of the exhaust manifold, then consider replacing the fuel filter, then consider a fuel pressure test... If all of those lead nowhere, then replace the sensor. I say replace the sensor last because it's the most expensive.

1. Check exhaust system 100% between O2 sensor and block. (FREE)
2. Replace fuel filter and re-check trouble codes... (~$15.00)
3. Rent fuel pressure tester and verify fuel system performance (Rental..l FREE)
4. Replace O2 sensor if the above 3 turn up alright... (~$50+)

IMHO, those are the steps I would start with to diagnose a lean condition with the jeep still functioning symptomless.
Okay ill try that first thing tomorrow morning
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Unread 05-18-2011, 12:45 AM   #8
Apollo12227
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By the way you are looking at 30psi when priming the pump (turning key forward and hear pump engage). I believe while idling and driving you should see a consistent 36-40psi. If your running higher than 45psi you have a pinched fuel return line which causes the pressure to rise at the fuel rail, a pinched return line will burn up a fuel pump. If the pressure is low, only priming to 1-12psi and idling less than 32psi your pump is probably going out.

The only other thing I can think of besides cracked exhaust before the O2 sensor, a faulty fuel pump, clogged filter or lines would be an injector or two clogging, you would see a pretty obvious hit on performance if this were the case. I'm hoping the filter will do it for you of it's been a while since it was changed.
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Unread 08-15-2011, 02:17 AM   #9
PFCLinscomb
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Quote:
Originally Posted by Apollo12227 View Post
By the way you are looking at 30psi when priming the pump (turning key forward and hear pump engage). I believe while idling and driving you should see a consistent 36-40psi. If your running higher than 45psi you have a pinched fuel return line which causes the pressure to rise at the fuel rail, a pinched return line will burn up a fuel pump. If the pressure is low, only priming to 1-12psi and idling less than 32psi your pump is probably going out.

The only other thing I can think of besides cracked exhaust before the O2 sensor, a faulty fuel pump, clogged filter or lines would be an injector or two clogging, you would see a pretty obvious hit on performance if this were the case. I'm hoping the filter will do it for you of it's been a while since it was changed.
Btw, i fixed it. My muffler was all cracked to hell and it was like i did not have one so there was much less back pressure causing the code 51. $120 bucks later and a trip to the muffler shop no more codes

-Linscomb
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