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Old 07-25-2014, 09:16 PM
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4.7 Rough Idle Mystery Continues

Ok so I've had this dang rough idle since I bought the Jeep a year ago. It's a 05 4.7 Laredo. I cannot cure this rough idle.

Here is what I've tried:

New Throttle Body
Spark Plugs
EGR Valve
Throttle Position Sensor
Idle Air Control Valve
Drivers side Exhaust Manifold
Seafoam treatment


At this point I'm leaning towards PCM or something transmission related. Any help is greatly appreciated.

Note - it idles great right after startup. The problem is coming up to a stop at red lights, stop signs, fast food, etc. one other thing I have noticed is I have to let it warm up before driving because it seems like it just won't go into overdrive or something and it sounds like it is working too hard. Difficult to explain but maybe the issues are related.


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  #2  
Old 07-27-2014, 12:04 PM
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Re: 4.7 Rough Idle Mystery Continues

Did you already check the fuel trims with a scanner? You will need a scanner which can show live data, or an android phone with torque app and a bluetooth or wifi dongle.
Right after startup the fuel injection works in open loop (O2 sensor input is not used) and it uses a preprogrammed fuel map. After a few minutes it should go into closed loop and then the O2 sensor inputs are used and the PCM will add or reduce a bit of fuel to the standard value of the map. A not properly working O2 sensor could cause a too negative fuel trim with as a result no good combustion or even misfire and rough running.
The fuel trim figures for both banks should be around the 0 and normally jump a bit from -4 to +4. If the trim figures go into the double digits, then there is something wrong. You should look at the STFT (Short Term Fuel Trim) figures for this.
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Old 07-27-2014, 12:43 PM
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Re: 4.7 Rough Idle Mystery Continues

Thanks. Interesting info. Wouldn't it throw a code if a 02 sensor was bad?


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Old 07-27-2014, 01:27 PM
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Re: 4.7 Rough Idle Mystery Continues

I think there is a TSB for the 4.7 (rough idle at stop light) you would need to take it to your dealer service.
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Old 07-27-2014, 02:36 PM
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Re: 4.7 Rough Idle Mystery Continues

I found this tsb but it's from 05 and you would think the owner at the time or dealership that was selling it brand new would have flashed the PCM.

http://www.wkjeeps.com/TSB/tsb_wk_1801205.pdf


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Old 07-27-2014, 03:10 PM
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Re: 4.7 Rough Idle Mystery Continues

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Originally Posted by 2Sh3a View Post
Thanks. Interesting info. Wouldn't it throw a code if a 02 sensor was bad?
Not necessarily a code will be set for a not well working O2 sensor. If the heating is affected yes. If the fuel/air mixture would be downtrimmed too much, misfire(s) will occur and a fault code for the misfire (P0300 for random misfire or the last digit will show which cylinder is misfiring) will be set.
For this reason its best to check the STFT to be normal and go from there.
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Old 07-27-2014, 03:15 PM
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Re: 4.7 Rough Idle Mystery Continues

I'll definitely mention this and have the dealer perform a test when I take it in. Is there something I can do in my garage to test this without a scanner?


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Old 07-27-2014, 03:32 PM
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Re: 4.7 Rough Idle Mystery Continues

Without a scanner or torque app you could do the following to see if the idle will be stable and smooth, but will end up with a check engine light, which needs to be cleared with a scanner or torque app.
If you have an android phone i would strongly recommend to buy the bluetooth or wifi dongle (approx USD 30 for the bluetooth dongle) and install the free torque app. With this you can read the fault codes, reset them and also visualize many live data, which will help in trouble shooting or just let you monitor the engine operation.
If you disconnect both upstream O2 sensors (the 1-1 and 2-1, which are used for the fuel trim), the PCM will not do any fuel trimming and will just work in open loop. The PCM will set two fault codes for the O2 sensor heaters, which you will need to reset after connecting them again, but if the idle will be smooth, then at least you know that the problem is fuel trim related. In that case i would not directly say that the O2 sensor(s)is (are) bad, because an exhaust leak could also cause this problem. When removing the O2 sensor connectors, take care that the wires will not touch any exhaust part, because it will melt the isolation and can cause shorts.
Also the exhaust ducts and especially the cats will get extremely hot in a short time, so watch your hands/arms.
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Old 07-27-2014, 04:36 PM
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Re: 4.7 Rough Idle Mystery Continues

I replaced my drivers side exhaust manifold last week hoping that would cure it but the rough idle came back. I am hearing some more ticking from under the jeep though. Maybe I have another exhaust leak.


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Old 07-27-2014, 04:50 PM
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Re: 4.7 Rough Idle Mystery Continues

Quote:
Originally Posted by 2Sh3a View Post
I found this tsb but it's from 05 and you would think the owner at the time or dealership that was selling it brand new would have flashed the PCM.

http://www.wkjeeps.com/TSB/tsb_wk_1801205.pdf


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The dealer only does the flash if the owner complains about a problem,also i found the tbs i was thinking about(18-045-04 rough idle after warm-up) and its for 02,03,and 04s I see you have a 05 o well.
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Old 07-29-2014, 04:16 PM
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Re: 4.7 Rough Idle Mystery Continues

I have an appointment to go in next Tuesday. Charging me $90 for the diagnostic. The guy on the phone said it is probably PCM related.


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