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  #1  
Old 06-05-2015, 04:40 PM
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Help 2005 grand cherokee random misfire

I have a 05 grand Cherokee with the 3.7 random misfire code. Sometimes the cel is on and sometimes it's off. It's stumbles at idle only on start up after it sits. It clears up if u put in neutral or let it act up for 15-20 seconds then stops then it's fine for the trip. I've put new copper plugs, new crank sensor , seafoamed , new grounds, new battery ends, Checked fuel pressure it's 60 psi and stays at 60 even after sitting for 5 mins... Jeep gets between 15.5- 17 mpg on the miles thing and around 300 miles to tank. Runs good just has the flashing cel at idle on startup after it sits for few hours and sometimes it stays solid. This is really getting annoying. Please help me.

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  #2  
Old 06-05-2015, 04:41 PM
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Re: Help 2005 grand cherokee random misfire

Also changed the asd relay and checked all fuses and relays
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  #3  
Old 06-07-2015, 11:13 AM
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Re: Help 2005 grand cherokee random misfire

Just to be sure whats happening, does it give the misfire code when starting up cold, or when its already warm? Do you have a scanner which can show live data? I would have a look at the fuel trim figures, when the problem occurs on a warm/hot engine. Also your model still has the idle air control stepper motore(IAC). I would remove the IAC and clean the plunjer and air canal.
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Old 06-07-2015, 11:49 AM
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Re: Help 2005 grand cherokee random misfire

Actually I took it to autozone this morning and now it's showing a O2 sensor bank 1 sensor 1. Not enough oxygen. So is that the culprit?
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Old 06-07-2015, 12:32 PM
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Re: Help 2005 grand cherokee random misfire

If its misfiring when warm, yes it could be. What is the exact code number you have?
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Old 06-07-2015, 12:56 PM
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Re: Help 2005 grand cherokee random misfire

Po300 random misfire. Po132 bank 1 sensor 1. I just ordered the new ngk O2 sensor today. Hoping it fix it. It does it at random times but more so when u let it sit over 10-15 and hop back in and just throw it in gear and take off in it it misfires
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Old 06-07-2015, 01:04 PM
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Re: Help 2005 grand cherokee random misfire

Ok, this code is for O2 sensor voltage too high. That normally means or a broken wire, or shaved or melted wiring isolation (resting against hot exhaust part), which causes a short between the heater wire and the sensing wire. It can be that the new O2 sensor will fix the problem, otherwise its most probably wiring related.
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Old 06-07-2015, 01:10 PM
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Re: Help 2005 grand cherokee random misfire

I looked wires connected and plugged in. I didn't see a shield over the wires just the wires. But all look intact. Could it be the O2 causing all this mess?
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Old 06-07-2015, 01:35 PM
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Re: Help 2005 grand cherokee random misfire

Could be yes. I presume that it takes a bit for the PCM to see that an O2 sensor is bad, and in the meantime is using the output from the O2 sensor. A high O2 sensor voltage means a low O2 level, which means a rich mixture (as seen by the PCM, eventhough faulty). The PCM will downtrim the fuel injector pulse and that can go that low, that the mixture will simply not burn anymore, causing the misfire. After some time, the PCM is seeing that the O2 sensor voltage is just too high, and will not use the sensor output anymore, and goes into open loop mode. From then on, the idle will be normal again, eventhough the fuel consumption will be higher then normal, due to the fact that the PCM is not trimming the fuel anymore for the best air/fuel ratio. That would also explain why on a cold engine the problem is not there, because then the PCM works in open loop mode and doesn´t use the O2 sensor output. Before the sensor is hot enough for the PCM to use its output signal (and go into closed loop mode), the PCM already saw that the O2 sensor voltage is too high and will simply not go into closed loop.
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Old 06-07-2015, 01:38 PM
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Re: Help 2005 grand cherokee random misfire

Thanks man. Sometimes it does it cold but a lot of times it acts up after it has sit for 15 mins and u take off. It acts up bout 15-20 seconds and its fine rest of trip till u turn off
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  #11  
Old 06-07-2015, 01:45 PM
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Re: Help 2005 grand cherokee random misfire

Ok, lets wait and see what the new O2 sensor will be doing. Keep us informed please.
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