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Old 04-16-2012, 04:05 AM   #1
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Rough to start, then runs fine?

Since the Jeep motor was replaced, been driving it more and more to work the kinks out. I think most are worked out, save the one that seems to have plagued me before the motor replacement and now it's back with a vengence.

When I first attempt to start the Jeep, it takes a while to start and a while to idle properly. Usually, if I turn the starter for 10-15 seconds, give it a little gas with the pedal, pause for a few seconds and then give it another 5-10 seconds of starter, it will start up. Seems like it starts to catch throughout the starting cycle but not quite there. When it does start, idle is rough, often idling up to about 1500 RPM for 2-3 seconds and then back down to a stall. Sometimes it stalls out all together and I have to repeat the process (with only 3-5 second of starter time) and it fires right back up. Other times, it will idle very low (300-400 RPM), then cycle back up to 1000 or so RPM, then back down to 300-400RPM.

This RPM cycle process goes on for 30-45 seconds usually. If I let it sit and idle, it generally idles up to speed eventually and I have no problems. I can drive it all day without any real issues. Performance once it's warmed up is sharp and as expected. If I let it sit for 4-5 hours, it will hesitate a little to start up but not like it has sat overnight. If it first starts and I try to give it gas to compensate for the wild idle, it will hesitate to respond, choke out a bit and then sometimes force a stall. Other times, it will act as if it's choking (not enough air?) and will spit and spudder to a higher idle.

So, what's the next step to diagnosing this?

I was going to buy a mean green starter and install my dual batteries in parallel in hopes of fixing it. Not sure if that will work or not. Any ideas?

Denis

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Old 04-17-2012, 12:52 PM   #2
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Anyone on this?

It's a bit better today than it was yesterday but it's still driving me crazy. Should I just start replacing vacuum lines? the starter? the battery? What else could cause it to be so hard to start and then even out as it warms up?

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Old 04-17-2012, 02:11 PM   #3
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Well first of all, do you have compression readings from the cylinders? Fuel rail pressure?

When were the vacuum lines last replaced? O2 sensor? MAP sensor? There could be quite a few things at play here.
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Old 04-17-2012, 02:17 PM   #4
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Xpress - Haven't done any fuel pressure tests. As I said above, once it idles for 30-40 seconds, it runs great all day, that's why I was thinking it wouldn't have to do with fuel delivery. That said, I'll definitely test it.

As for vacuum lines, they're all the original lines. Haven't swapped them yet.

O2 sensor is also original. I'll be doing that tonight.

MAP sensor was replaced about 6 months ago.
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Old 04-17-2012, 02:19 PM   #5
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Well replacing the vacuum lines and O2 sensor should be done anyways if they're OEM, even if they aren't the problem. If you say it idles fine after it warms up then I suspect that you may have a compression issue, as I've heard of it happening before. Doesn't hurt to test it.
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Old 04-17-2012, 03:33 PM   #6
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If it takes that long to start you may have a fuel pressure problem, it sounds like the fuel has drained back into the tank, you end up with some air each time you start it.
A bad manifold temp sensor will affect the idle too......
have you checked the fuel pressure regulator?
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Old 04-17-2012, 09:57 PM   #7
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definitely replace those vac lines. im guessing you havent done the nutter? either way check your idle tubes, if they are clogged up it will never idle right because it will allow extra fuel to drip. just a suggestion, hope it helps.
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Old 04-17-2012, 10:37 PM   #8
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Quote:
Originally Posted by Cavemans89YJ View Post
im guessing you havent done the nutter?
I know for fact he hasn't, he has a 2.5L...
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Old 04-18-2012, 04:57 AM   #9
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Quote:
Originally Posted by Xpress View Post
I know for fact he hasn't, he has a 2.5L...
You are correct, sir. No nutter to check, as far as I know.
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Old 04-28-2012, 08:12 AM   #10
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Replaced O2 sensor last night. Drove about 40 miles since. No noticable difference. Ordered a bunch of "cut your own" vacuum lines. Will replace those as soon as they arrive. Going to do the MAP sensor, fuel pressure regulator and manifold temp sensor tomorrow.
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Old 04-28-2012, 09:52 AM   #11
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O2 sensor wouldnt be it. I would check the TPS throttle positioning sensor. or the CPS crank positioning sensor. TPS is on the throttle body, and CPS is on the bell housing. Those 2 are the main sensors that would be a reason for a hard start and stalling. Unless you have a fuel rail problem such as a bad filter, or a failing pump, or clogged injectors. When you turn on your key before the start, how long does the fuel pump run? 3-5 seconds is a normal build pressure time. longer than that means you have a fuel rail problem. There is a 1way valve that goes bad and lets the pressure bleed off.
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Old 04-28-2012, 10:00 AM   #12
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0III0forlife - New fuel filter 3 weeks ago, so I think I can discount that.

I'll add CPS and TPS to the list of things to replace as they are both older/original.

Any idea what the name and location of that one way valve might be? I've seen people talk about it on other forums but haven't seen any kind of official name or where to look for it to replace it.
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Old 04-28-2012, 10:34 AM   #13
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I'd get a vacuum gauge on it it its above 17lbs, its doubtful you have a leak. I'd spray some starter spray in the intake and see if she fires right up. If it does, you know you have some sort of fuel delivery issue.
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Old 04-28-2012, 10:36 AM   #14
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P0234 - Thing is, it does eventually start. Sometimes, it takes 10-15 seconds to start, then runs rough. Once it's warmed up and running a minute or two, runs right until it stops and sits again for another few hours.
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Old 04-28-2012, 06:25 PM   #15
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Right, which is why I'm saying spray the starter spray. If it fires right up, even for a second or two, you know you have an issue where fuel isn't making into the motor for some reason. Not sure if the YJ's have check valves, but I've had this exact issue in other cars. The fuel drains down and you have air in the lines. The next step is to manually prime the system.
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Old 04-28-2012, 06:39 PM   #16
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On my 4.0, I had a similar preoblem and it turned out the the Crank Position Sensor, (or rather on of it's wires) was bad. I replaced it and no problems since.
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Old 04-30-2012, 05:46 AM   #17
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Ordered Crank Position Sensor, fuel pressure regulator and throttle position sensor. Will be replacing those as soon as they come in.
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Old 05-02-2012, 11:36 AM   #18
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Have a similar issue, I thinks its the drain-back valve...I turn the key on and off several times (usually doesn't take as long the warmer it is outside) until I hear the fuel pump and fuel pump fuze under the hood popping for several seconds, then the "check engine" light comes on for a few seconds, then it starts right up and runs fine. From the research I've done, I think the drain-back valve is a part of the fuel pump in the tank. I just turn the key on and off a few times until I hear the pump. Haven't looked into fixing yet, cause I don't have the time/money to drop the tank and replace the whole pump assembly.

long post short, i vote drain-back valve is bad.
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Old 05-02-2012, 12:22 PM   #19
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My new to me 89 is doing the exact same thing....once it has been idling it runs great but have to "play" with it to get it going and then it idles rough at first. Keep us posted!
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Old 05-03-2012, 04:57 AM   #20
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Quote:
Originally Posted by matnic View Post
Have a similar issue, I thinks its the drain-back valve...
If it is the drain back issue, what parts would I need to replace?

Isn't that a component of the fuel pump in-tank? and or is it part of the fuel regulator?

If I have to drop the tank, I'm thinking of doing everything I need to do while it's down (seals, pump, etc).

Denis
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Old 05-03-2012, 11:38 AM   #21
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http://www.wranglerforum.com/f5/looo...rt-150955.html

its a TJ, but similar issues/resolution.

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