96 Cherokee 4.0 Complete Loss of Power
02-08-2018, 07:09 PM
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Model: Cherokee(SJ)
96 Cherokee 4.0 Complete Loss of Power
I have a 1996 Cherokee Sport High Output 4.0 that’s all stock. I’m pretty sure it has the California emissions deal as well. Today I was driving up to Thompson Pass which is a somewhat steep grade for several miles. The Jeep was running decently on the way there but on the way up the grade it seemed that the power wasn’t keeping up. Once I neared the top there was a dramatic loss of power with stuttering. I pulled over and put it in park to take a look and it seemed to idle ok. Fearing a breakdown I turned around and headed back to town. On the way down the stuttering and power loss continued to worsen. After about ten miles the Jeep was running extremely rough with low RPMs at idle and black smoke coming from the tailpipe. It got to the point where I could only drive at 5mph so had to get a tow. Also, the fuel economy was horrible and I checked the oil a few times and it appeared ok. Any ideas would be helpful. And let me know if additional information is needed. Thanks.
02-08-2018, 07:17 PM
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Is there currently a check engine or check gauges indicator showing?
The black smoke and loss of power hints at the possibility that your upstream O2 sensor has failed or is indicating a lean condition and is causing the PCM to run as rich as possible. Typically, this will result in a check engine light if it continues on long enough.
Is your 96 an OBD2 model? If you are unsure, the OBD2 port would be right above your left shin when sitting in the driver's seat (if it is a left hand drive model). It is a black socket connector, somewhat trapezoid in shape, kind of hard to miss.
The black smoke and loss of power hints at the possibility that your upstream O2 sensor has failed or is indicating a lean condition and is causing the PCM to run as rich as possible. Typically, this will result in a check engine light if it continues on long enough.
Is your 96 an OBD2 model? If you are unsure, the OBD2 port would be right above your left shin when sitting in the driver's seat (if it is a left hand drive model). It is a black socket connector, somewhat trapezoid in shape, kind of hard to miss.
02-08-2018, 07:35 PM
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Thank you for the reply. It is an OBD2 model. And the check engine light has been on for years with no adverse effects. I assumed it was for California emissions. I have had an average of 20 mpg and a solid idle, oil pressure etc. I live in a tiny town at the end of a long road with limited resources. How might I be able to check the O2 sensor on my own?
02-08-2018, 11:24 PM
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If you or one of your friends has an android phone or tablet, the first thing I would get would be a BAFX bluetooth OBD2 scanner. They can be had on Amazon for about $20 these days. Then get the Torque app on Google play for about $5. It will show short and long term fuel trims, which is what the computer is doing to try and keep the air/fuel ratio at the proper amount. It will also show you O2 sensor voltage readings and you can confirm that your upstream sensor is properly cycling between something like .1 and .9 volts.
If the sensor is failing and reading a constant .1 volts (or similar), it will fool the PCM into running as rich as it can. Which will present itself as a high positive long term fuel trim. Fuel trim readings are usually between -35% and +35% but when things are working properly this value typically hovers around 0 (zero). If it gets higher than more than about 25% in either direction for a period of time, the PCM will throw codes for lean/rich respectively (P0171 too lean, p0172 too rich, or p0170 mixture out of spec).
The BAFX OBD2 scanner will also read and reset codes for you.
If the sensor is failing and reading a constant .1 volts (or similar), it will fool the PCM into running as rich as it can. Which will present itself as a high positive long term fuel trim. Fuel trim readings are usually between -35% and +35% but when things are working properly this value typically hovers around 0 (zero). If it gets higher than more than about 25% in either direction for a period of time, the PCM will throw codes for lean/rich respectively (P0171 too lean, p0172 too rich, or p0170 mixture out of spec).
The BAFX OBD2 scanner will also read and reset codes for you.
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