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Old 10-29-2012, 09:58 AM   #1
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Hard start, won't start when hot, rough idle

My 2000 4.0L TJ takes a few cranks to start up when cold, and it has a very rough idle for a few minutes. Usually settles down after it warms up but experience occasional loss of power. When I shut down and try to restart while the engine is hot it won't restart. Have to wait at least an hour for it to cool down. CEL codes P0304 (misfire in cylinder4) and P0171 (o2 sensor).

So far I have replaced the plugs, replaced the coil pack, replaced the TPS and Crank Shaft Position Sensor. I've cleaned the IAC.

I'm at a loss as to what's causing the starting issues. Thinking of replacing #4 fuel injector next. The 02 sensor couldn't cause these kinds of issues, could it? Any other thoughts?

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Old 10-29-2012, 01:13 PM   #2
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Anybody?

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Old 10-29-2012, 01:25 PM   #3
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I'm having issues with my 99 where it turns over, but will bog out unless I keep goosing the gas. I can eventually get her going, but it takes some time. The issue comes and goes, so I'm in the process of diagnosing too. Go to replace the CPS and look into the fuel pump... Good luck, I'll be watching this thread to see if anyone else has some ideas.
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Old 11-01-2012, 11:01 AM   #4
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I used an ohm meter to check the injector and it's within parameters. Of course, it could still be mechanically broken or clogged and still be electrically sound.

I think I have to give it a go and replace it this weekend.

Any other thoughts?
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Old 11-01-2012, 01:47 PM   #5
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I have a similar problem in my 2001 4.0L Wrangler. It always starts fine on a cold start, but sometimes it acts up after driving it and restarting. Let's say for example I drive to a store. When I get back in the Jeep and start it, it may start fine or it may have this problem: it will start quickly like normal, but the idle will be very rough, like it's running on 4 cylinders. If I put the automatic tranny in drive to drive away, it doesn't have enough power to pull itself more than a few inches before stalling. However, if I just sit there and let it idle for 30 seconds, it will smooth itself out and all is well. Like I said, this problem is very intermittent, it may happen 1 out of every 4 warm start ups, but never on a cold start. I've pretty much been ignoring it, but it bugs me. I'd love to hear ideas and what you find is wrong with yours. I bought my Jeep 6 months ago with 49,000 original miles, so it obviously spent most of it's life sitting and not being driven.
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Old 11-01-2012, 04:11 PM   #6
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My 2001 Wrangler had the rough idle poblme frequently and times takes couple cranks when the engine is warm, (like if I stall on the road from lack of clutch ha ha). My self and my dad not being able to figure it out, I too it to a mechaic and he did a tune up. He found nothing wrong but it only idles rough once in a while now. Wish I could help you guys with this, I have heard alot of this from Wrangler owners including a friend and there never seems to be an definite solution.
GOOD LUCK!!!
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Old 11-02-2012, 10:25 AM   #7
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I'm leaning toward an oxygen sensor. I've experienced some wierd stuff with O2 sensors in other vehicles. I had a girlfriend with a little 4 cyl car that intermittenly acted like it dropped a cylinder going down the road. I'm talking about the whole car would shake and the engine would spit and sputter. Sometimes it would do it, sometimes it ran fine. I replaced the O2 sensors and it ran like a dream after that. The problem with our Jeep is there are 4 of them and they ain't cheap.
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Old 11-02-2012, 10:37 AM   #8
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Exact same issue here in my 01'. Well, not when cold but when hot (especially when I run the a/c). Had a tune-up done (including all the O2 sensors changed) & $1600 later it is still intermittentantly an issue. "Only" thing left to try, that can be thought of, is a sleeve on the 3rd or 4th injector. I'd LUV to hear what others have done to actually fix this once and for all. Btw, 85,000mi on the odo.
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Old 11-02-2012, 10:46 AM   #9
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I think I would probably change the O2 sensor if that code was coming out
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Old 11-02-2012, 10:49 AM   #10
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Been having the same issue. Upon start up it acts like its misfiring severely but after about 30 seconds, problem gone! If anyone figures it out, let us know!
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Old 11-02-2012, 10:53 AM   #11
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Local parts stores want $50 to $60 each for the O2 sensors. I found them on Ebay for $20 each so I got all 4. It will take a week for them to get here and probably another week for me to change them, so give me 2 weeks and I'll post my results. And before you go bashing cheap Ebay O2 sensors, they fixed my ex-girlfriend's little car I was talking about and they outlasted the relationship!
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Old 11-02-2012, 03:51 PM   #12
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Quote:
Originally Posted by tride000 View Post
Exact same issue here in my 01'. Well, not when cold but when hot (especially when I run the a/c). Had a tune-up done (including all the O2 sensors changed) & $1600 later it is still intermittentantly an issue. "Only" thing left to try, that can be thought of, is a sleeve on the 3rd or 4th injector. I'd LUV to hear what others have done to actually fix this once and for all. Btw, 85,000mi on the odo.
I saw the repair bulletin, but it was specifically for the 3rd injector, and I'm not getting a code for the 3rd but the 4th. While I have the fuel rail off I'll insulate all the injectors, but I'm not too confident that's the issue.

Quote:
Originally Posted by XKMAN View Post
I think I would probably change the O2 sensor if that code was coming out
I'm willing to try that but would the 02 sensor malfunctioning cause the engine to act like this? Hard starting and all?
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Old 11-02-2012, 03:52 PM   #13
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Originally Posted by baker3 View Post
Been having the same issue. Upon start up it acts like its misfiring severely but after about 30 seconds, problem gone! If anyone figures it out, let us know!
Sounds like the same kind of thing. Really frustrating not to be able to figure this out.

Quote:
Originally Posted by that_one_guy_steve View Post
Local parts stores want $50 to $60 each for the O2 sensors. I found them on Ebay for $20 each so I got all 4. It will take a week for them to get here and probably another week for me to change them, so give me 2 weeks and I'll post my results. And before you go bashing cheap Ebay O2 sensors, they fixed my ex-girlfriend's little car I was talking about and they outlasted the relationship!
Let us know if it helps. Are you throwing any codes right now?
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Old 11-02-2012, 04:03 PM   #14
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I checked it once about 4 or 5 months ago and it did not throw a code, but then again when I was having it checked it wasn't acting up.
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Old 11-02-2012, 06:14 PM   #15
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I thought I read somewhere that DC recommends changing the o2 sensors after 100,000 miles. Some say that they can be acting up (or not working 100%) but not throw any codes... I'm going to give it a shot on my rig. Even if I only gain 1 mpg out of it, it'll be worth the $60 and hour of my time.

I have 119k on my Jeep, so there's a bunch of stuff I want to replace just for the sake of seeing what kind of performance improvements I get. Just swapped the IAC today... Think its worth doing the injectors?

(edit: don't mean to hijack the thread here)
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Old 11-09-2012, 08:49 PM   #16
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This is typical with the 4.0.
On warm startups, if its sat fpr about 10 minutes, the fuel evaporates in the 3rd injector. It needs to be insulated from the manifold heat.

(Just from what ive read on other forums)
I'm on my phone so i may have missed some info in the post but I believe this is what you're getting.
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Old 11-10-2012, 12:19 PM   #17
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I changed all 4 oxygen sensors (even thought my Jeep only has 53000 miles) and I also changed the IAC for the heck of it. None of this fixed the problem, had absolutely no affect, and my Jeep still does it. On another note, I noticed something: For the past week here in north FLorida, we have had cooler nights like around 45 to 50 degrees. My Jeep doesn't seem to do it when the outside temperature drops like that. During the day, we're back up into the 60s and 70s again, and my Jeep will do it. Outside temperature could affect fuel evaporation, which would support BirminghamTJ's comment above. Since my Jeep sometimes does it and sometimes does not, that might be dependant on how long it is between startups. I never really paid attention to that, but that could also support BirminghamTJ's comment. You, sir, could be onto something here..........
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Old 11-11-2012, 04:18 PM   #18
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Try cranking it roughly 10 minutes after a good warm up a few times. If it runs rough 3/5 times... id say thats the issue. Mine was throwing a 3rd cylinder misfire. After i. Noticed the rough idle i started researching it. Now i notice it never does it unless i stop somewhere for 10-15 minutes. I haven't insulated mine but theres lots of posts and instructions on various ways to do it.

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