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Old 08-14-2013, 07:23 PM
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1999 Jeep grand Cherokee laredo 6 cyl.

Hi guys, I have a problem with my jeep.
when is in idle running ,truck start showing less RPM than normal,and smelling gasoline not burn.
take to the mechanic and he found out all spark plugs were deep black color. his diagnostic bad coil rail, changed, same not solved problem.
then crank sensor changed , not solved.
distribuitor rotor changed ,not solved.
He said that injectors are working perfectly,fuel filter is good.
check engine not light up never.
Any other advices?
Thank you.
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Old 08-14-2013, 08:39 PM
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Re: 1999 Jeep grand Cherokee laredo 6 cyl.

Put a scanner on it which can show live data and check if the O2 sensors are working ok. You should see the voltage output of the O2 sensors fluctuating a lot, this is ok. When the indication is more steady, then the sensor is bad.
If this is ok, have a look at the fuel trim values. Is the jeep not consuming more fuel then usual?
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Old 08-15-2013, 05:53 AM
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Re: 1999 Jeep grand Cherokee laredo 6 cyl.

Thank you for you fast help.
I will do the Live data test today to the O2 sensors and let you know the results.
And yes ,looks like is consuming more gas than before.
I will replay again as soon I make the test after come home from work.
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Old 08-15-2013, 12:08 PM
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My actron scanner showed two sensors,while I run the jeep the O2sensor#11 fluctuation is from 0.00 v. To 780 v.
O2SENSOR#12 FLUCTUATION 700V TO 780V. At ALL THE TIME WHILE RUNING. Looks like sensor 12 is bad.
How can I know where this sensor is located? Thank you.
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Old 08-15-2013, 03:32 PM
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Re: 1999 Jeep grand Cherokee laredo 6 cyl.

Does your jeep only have 2 sensors in total? Sensor 1-1 is the upstream sensor(in the forward pipe), which is used for fuel trimming. Sensor 1-2 is a downstream sensor in the same pipe after the catalyser, which is only used for catalyst performance and will not change a lot.
If the jeep has 4 sensors in total, then the 2-1 sensor is the upstream sensor in the aft pipe, which is used for fuel trimming and the 2-2 is the sensor in the same pipe after the cat.
Your scanner didnīt show any actual stored faults?
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Old 08-15-2013, 03:45 PM
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My scanner showed me. 3 historic codes that I erased already.
Something like piston 5 misfire
And piston 2 misfire.
Looks like I have 2 sensors only.
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Old 08-15-2013, 04:27 PM
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Re: 1999 Jeep grand Cherokee laredo 6 cyl.

Ok, then it looks like the O2 sensors are working fine. Did you see what the fuel trim values where?
Also check with the scanner the air inlet temperature value and the MAP sensor value. Both give an input to the PCM for calculation of the fuel injector pulse width
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Old 08-15-2013, 04:42 PM
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Re: 1999 Jeep grand Cherokee laredo 6 cyl.

But the Downstream sensor is not fluctuating a lot! Do you really think that is ok?
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Old 08-15-2013, 05:16 PM
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Re: 1999 Jeep grand Cherokee laredo 6 cyl.

Yes, that is ok. The upstream sensor receives the exhaust gasses directly from the cylinders and by constantly finetuning the injector pulse width, the O2 content fluctuates while the fuel/air mixtures surges around the 14.6 and because of this the O2 sensor output also fluctuates.
The downstream sensor however is after the cat and the cat is converting toxic byproducts in the exhaust gasses in less toxic substances by a cathalitic combustion which uses O2, thus reducing the O2 content after the cat and making the O2 level after the cat more or less stable (and low). For this reason will the downstream O2 voltage almost not change.
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Old 08-15-2013, 05:30 PM
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Re: 1999 Jeep grand Cherokee laredo 6 cyl.

Ok, now how can I see the fuel trim values?
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Old 08-15-2013, 05:35 PM
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Re: 1999 Jeep grand Cherokee laredo 6 cyl.

Quote:
Originally Posted by Frango100 View Post
Yes, that is ok. The upstream sensor receives the exhaust gasses directly from the cylinders and by constantly finetuning the injector pulse width, the O2 content fluctuates while the fuel/air mixtures surges around the 14.6 and because of this the O2 sensor output also fluctuates.
The downstream sensor however is after the cat and the cat is converting toxic byproducts in the exhaust gasses in less toxic substances by a cathalitic combustion which uses O2, thus reducing the O2 content after the cat and making the O2 level after the cat more or less stable (and low). For this reason will the downstream O2 voltage almost not change.
On the 4.0 ZJ, in addition to the excellent advice already provided, check for hairline cracks if you have either of the two factory manifold variants. Extremely common problem and can mess with the upstream sensor if memory serves.
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Old 08-15-2013, 05:43 PM
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Re: 1999 Jeep grand Cherokee laredo 6 cyl.

Quote:
Originally Posted by rocaloro View Post
Ok, now how can I see the fuel trim values?
Your scanner should be able to show them if it has live data capability.
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