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Old 09-01-2012, 10:10 AM   #1
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Rough idle

My jeep has a rough idle. I'm not sure how to describe it. I swap out the motor cause it was only running a 3 cylinders thinking that was the problem. But no luck it still has a rough idle. Can someone please help. The jeep has lots of power and runs good it's just the rough idle

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Old 09-01-2012, 10:53 AM   #2
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First thing I would do is clean the throttle body. Spray some throttle body cleaner that is safe with O2 sensors into the throttle while the engine is running. Sometimes it is necessary to take the throttle body off and clean it really good. After your gas is almost gone in the tank, fill it up with premium gas. That higher octane will help to further clean everything up.

Also check and make sure all rubber hoses going to and from the throttle body are in good condition. Actually check this first bc a broken or cracked vacuum hose can cause rough idle.

Good luck!

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Old 09-01-2012, 06:24 PM   #3
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Run a vacuum test and post your results. You have to look at the needle on the gauge and observe what the needle is reading and if it is fluctuating. It helps to diagnose the problem.

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Old 09-08-2012, 04:43 AM   #4
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I haven't had time to do a vacuum test. I just notice that when I'm shifting onto second from first it bogs out a bit. Could this be a fuel filter issue
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Old 09-08-2012, 08:20 AM   #5
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Originally Posted by Jdusome View Post
I haven't had time to do a vacuum test. I just notice that when I'm shifting onto second from first it bogs out a bit. Could this be a fuel filter issue
It could be a fuel filter issue. Check your fuel pressure with a fuel pressure test gauge.
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Old 09-09-2012, 08:24 AM   #6
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It could be but generally fuel filter issues are consistent throughout the rpm range, the exception being higher rpms... when the vehicle has a great demand for fuel. Switching from first to second gear is "yo young"... you rev up in first gear, then you shift into second gear which immediately drops down rpms and puts more of a load on your engine. I would think if this was a fuel related issue this "bogging" would be more prevalent when you were switching from forth to fifth on the highway. If it isn't doing it there (under the greater load) I wouldn't think it was a fuel filter problem. Of course, this would apply to both carburetors and fuel injection systems. If you haven't done a vacuum test yet, I would assume you haven't had a chance to check the fuel pressure in the fuel rail yet?

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Old 09-09-2012, 08:50 AM   #7
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What engine?
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Old 09-09-2012, 09:14 AM   #8
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It's a 2.5 liter. I've gotta check the fuel filter.
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Old 10-13-2012, 05:12 PM   #9
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Ok so the rough idle is there more so when it warms up and idling almost like the idle needs to be turned up. If I keep my foot on the gas alittle bit it sounds good.
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Old 10-28-2012, 12:21 PM   #10
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Would an o2 sensor effect the way my jeep runs. Right now I've got a rough idle and my exhaust stinks. The muffler is really black on the inside.
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Old 10-28-2012, 01:40 PM   #11
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Your running rich for some reason. The operating temp should be 210 degrees. The you have several sensors back to ECM. So if yoiu have tuned up and cleaned the throttle body make sure the wires are all connected to the other sensors. O2 is is just as important as any other part it reads the exhaust stream and tells the computer what it finds and the computer will compensate the mixture.

Here is a site that deals with smog problems but in every section it explains how all your sensors work and what they do it will help you figure out why it is running rich.

Failed Emissions Test. My car failed the emissions test - SmogTips.com.
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Old 10-28-2012, 04:04 PM   #12
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Thank you very much
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Old 10-28-2012, 04:20 PM   #13
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Mine idled rough, and would also rev up and down, as if I were feathering the gas pedal. It also would die once in a while. A bad Throttle Position Sensor was the culprit.
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Old 11-09-2012, 06:18 PM   #14
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Ok I changed the o2 sensor. The jeep still runs rich on idle doesn't when it's being drivin. Mechanic says it seems like the engine seems loose? What does that mean? Need help I've gotta fix this its the only problem left.
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Old 11-09-2012, 06:21 PM   #15
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Oh and the mechanic put it through a dianostics test and he could figure out why it is running rich when idling.
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Old 11-09-2012, 07:41 PM   #16
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Has anyone looked at your engine temp?? It should be running at 210 degrees F. If not and it is running cool that would be a cause for running rich.
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Old 11-09-2012, 08:16 PM   #17
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How do I check the engine temperature? I don't think the mechanic checked that. He said I should take it to a jeep dealer.
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Old 11-09-2012, 08:41 PM   #18
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Yeah he is confused. The temp gauge on the dash should be working at least as a guide. 210 is the optimal running temp for the 2.5L. It is achieved with an OEM 195 degree thermostat. It is all designed to work together. When it is cold it will have a high idle until the ECU senses temp from one of two sensors. One sense air temp in the intake and the other senses water temp in the intake. As the temps change the resistance readings change and are sent to the ECU to adjust the fuel air mixture.

Back to the rough idle after all has been checked and confirmed good you still have the other parts of the emission system to check. TPS on the TBI and MAP on the firewall operated by vacuum. The PCV system should be working along with the egr valve and O2 sensor. So there is a lot that can contribute to a rough idle not an easy problem to vercome.

You can pull your spark plugs and read the plugs to tell you how the engine is running. While they are out a engine temp warm compression test will tell you a lot more about the condition of the motor. Check out the picture and see what the pplugs tell you no mechanic needed just remove and match to the picture.
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Old 11-09-2012, 09:13 PM   #19
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It only runs rich on idle when it's driving it runs perfectly good
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Old 11-10-2012, 08:56 AM   #20
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It could also be running rich due to a lack of air. When the airflow is reduced it creates a higher fuel/air ratio mixture. How many miles does your air-filter have on it? Check it out maybe it's over-dirty.
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Old 11-10-2012, 10:05 AM   #21
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It doesn't have many km on it maybe 2000 it clean just has alittle bit of oil on it nothing major
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Old 11-14-2012, 05:54 AM   #22
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Ok I know this thread is older but did you ever find yer cure? I went throught some muddy water today after changing my set up to a bigger throttle body, ran great except for the High rpm on start up. How ever after getting her all muddy she is idling rough, sounds like a cam was put in. After about 20 second of it idling like this at 6-700 rpms it will kick it's self to 1k and sound perfect just idling higher than normal. Check vacuums and they all look good. Air filter is clean and throttle doesnt look bad. Going off this thread id assume TPS or IAS? Thanks!
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Old 11-14-2012, 06:27 AM   #23
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I haven't figured it out yet. I've gotta check the tps still.
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Old 11-18-2012, 09:10 AM   #24
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Ok yesterday I took the throttle body off and I gave it a real good cleaning. I took the tps off and gave that a real good cleaning I've change the o2 sensor. Now the jeep is idling at 1k. It doesn't smell like its running rich anymore. What is the normal idle speed for a 2.5?
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Old 11-18-2012, 01:50 PM   #25
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Ok I just drove the jeep today now I've got a new problem. The jeep is now starting to hesitate on acceleration. What could cause this
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Old 11-18-2012, 08:34 PM   #26
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Thats my exact issue!!!! does yours hesitate around 3-4k? Got my rough idle fixed however it still moves rpms while idling and i cant figure out what it is. BTW FOR PEOPLE READING THIS THREAD: I walked into oreillys today to warranty an 02 sensor that was clearly falling apart at the tip and learned something. Many of the o2 sensor from the last few months have been getting warrantied for this issue. I like Bosch products so i dont wanna bad mouth them however as a head up; if you bought one in the past few months you may want to take a look at it since they only have a 1 year warranty on them!
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Old 11-18-2012, 08:50 PM   #27
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was doing some research and found this from a tj guy on this same forum. Him and a few guys agreed it takes a few days for the cleaner to work its way out.


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After reading my post I can see how it was misunderstood. I meant that I bet his problem is his TPS. On a completely different note, after doing a thorough throttle body cleaning it is normal to get some stumbles and have a little weird jump here and there until all the cleaner is burned out.
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Old 12-20-2012, 03:11 PM   #28
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Ok so I've by-pass the vacume line that goes from the throttle body to the map sensor. I put a vacume line from the manifold to the map sensor and I've minimized the rough idle. But I'm still running rich.
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Old 12-20-2012, 11:08 PM   #29
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So I have been having ALL of the same issues. then all of a sudden as soon as it hit Operating Temp it would shut off. Once it started back up after cooling for a little while it would idle rough and backfire as soon as you gave it throttle. Did my research and headed to the auto parts store. They argued that it was either an intake leak or vacuum line. I said to hell with them and bought an ignition coil as that is one of the last things for me to change anyways.. now it runs PERFECT again. The coil has three separate pieces inside of the One coil. I apparently had 1-2 of those burned and the 3 was what kept it somewhat alive. Found that out on several jeep forums. Maybe this will help?
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Old 12-21-2012, 12:09 AM   #30
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When I worked at NAPA, the owner would only allow us to sell the OE brand o2 sensors... for instance, if BOSCH was the OE brand for Jeep... (I dont know off top of my head what is..) we would only sell you the BOSCH, even if say they listed a DENSO part number too and we had the DENSO in stock... we would order you the BOSCH... same with spark plugs...

Of course, if the customer wanted another brand, we would give it to them, with them understanding that they couldn't return it defective...

It's weird... with electrical parts, sometimes manufacturer matters...

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I walked into oreillys today to warranty an 02 sensor that was clearly falling apart at the tip and learned something. Many of the o2 sensor from the last few months have been getting warrantied for this issue.

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