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Old 02-11-2010, 03:32 PM   #1
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Rough Idle

I started to get a rough idle this afternoon, in line for some food, check engine light came on, got it read, another misfire code on 1,2 and 3. It only seems to do it when its been idling for a minute or so, and gets rougher the longer it sets, but when you give it gas it "clears out" and runs fine, also runs fine while driving only. Just changed out the plugs and wires a month ago. Any suggestions??

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Old 02-11-2010, 06:18 PM   #2
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Possible moisture... Might need to replace the injectors for those.

I had a misfire on #1 2yrs ago and replaced the injector and all was good.

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Old 02-11-2010, 06:44 PM   #3
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seafoam it
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Old 02-11-2010, 06:57 PM   #4
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Is it an early model year TJ like a '97 or '98? If so, they have a problem with weak valve springs from a bad batch that can cause that kind of misfire.
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Old 02-11-2010, 09:56 PM   #5
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yep its a 97
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Old 02-11-2010, 09:59 PM   #6
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when it was doing it i noticed that it would start misfiring, then catch itself and stop then start misfiring again.
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Old 02-12-2010, 06:57 PM   #7
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Could be a bad TPS as well ...
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Old 02-13-2010, 12:44 AM   #8
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Where do you think i should start?
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Old 02-13-2010, 10:20 AM   #9
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Cracked Distributor? If its warming up and drying out then the problems would disappear when the moisture had evaporated from the distributor. Take off the cap and look at the rotors?
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Old 02-13-2010, 11:29 AM   #10
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Hah, great name redneckpilot. That name would fit me too.
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Old 02-15-2010, 06:19 PM   #11
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CE light came on again today after driving 20 minutes on the highway and stopping at a red light, its started missing out. Any suggestions on what to tackle first? distributor cap?
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Old 02-15-2010, 06:40 PM   #12
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I already made my suggestion, #4 above. Surely you didn't miss it.
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Old 02-15-2010, 07:04 PM   #13
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Nope didnt miss a thing. Is there any way to check that? If that is the culprit how do you take care of it? new springs or an all new head?
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Old 02-15-2010, 07:04 PM   #14
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Is there anything i should look into before that requiring less $$?
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Old 02-15-2010, 07:26 PM   #15
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Nope, not really. The only way to test it to see if it is the springs is to perform a compression test while the engine is having the miss. If it is weak valve springs from the bad batch that is causing the missing, they are all you replace, not the entire head. I don't recall what I paid for my replacement valve springs (I had this issue with my '97) from the dealership but it wasn't much. I paid $150 to have them installed, it's not a tough job but I wasn't willing to break my back leaning and stretching over my lifted TJ to work on the engine.

BUT... if this problem goes on long enough as a result of the weak valve springs so common in 97-98 4.0L engines, it can burn the valve seats which would then require a valve job in addition to the replacement valve springs.
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Old 02-15-2010, 08:02 PM   #16
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hmmmm ok then, ill look into those spings, this is the second time ive noticed it, ill run to the dealership tomorrow and look into some springs. Thanks Jerry.
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Old 02-15-2010, 08:06 PM   #17
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I would check the compression first when the engine is missing before you jump into buying the springs. The engine isn't running during the compression check but it should be having the problem at the time you test the compression.
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Old 02-15-2010, 08:27 PM   #18
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10-4
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Old 02-23-2010, 07:07 PM   #19
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Got a compression tester last night....haynes maunal give a broad how to, says something about disconnecting the fuel system but just tells you to go to the fuel chapter which says to drop the tank and undo the electrical wires to the pump, is there any other way?

And does anyone have any advice on how to do this and what the compressions should be on the cylinders??

Jeep hasnt been missing bad enough lately to throw another code but it still misses when it sits at idle for more than 45 seconds
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Old 02-23-2010, 07:26 PM   #20
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Pull the ASD relay inside the PDC (power distribution center) located on the passenger-side fender well in front of the relay. That will disable the fuel pump and spark. Next, relieve the fuel pressure at the fuel rail on top of the engine via its pressure check valve stem, on the driver's side of the valve cover. The fuel pressure check valve is physically on the fuel rail. It's like a big tire valve stem and is under a big black protective cap that is like a giant valve stem cap. Once the cap is off, lay an absorbent cloth underneath when the engine is cold & push on the pin inside the valve. That will allow the pressurized fuel out of the fuel rail. This is similar to airing your tires down by pushing on that pin.

What you're looking for when you check the cylinder compression is a lower compression level at a particular cylinder indicating a leaky valve or ring problem.
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Old 02-23-2010, 07:32 PM   #21
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Alrighty, thanks Jerry. I dont have any classes friday so ill be doing this then, ill be back with the results.
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Old 02-26-2010, 05:46 PM   #22
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Ok just did my compression check, cylinders 1,2, and three were all at 150 psi give or take 3-5 pounds. Number 4 was at about 160-163. But none of them were lower, when i pulled the plugs, they all looked fine, no oil, no carbon, no discoloring. What do you guys think i should try next???
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Old 02-26-2010, 06:27 PM   #23
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also when i put my plugs back in i let it run to see if it would start doing its thing again, and i let it run for a good 10 minutes just idling and it never started it. I switched the plug order around.
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Old 02-26-2010, 06:34 PM   #24
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That's the problem, it can't be easily troubleshooted when it's running ok as it is now. I used to be a field engineer for mainframe computers and I hated hearing the customer tell me the problem wasn't happening right when I would arrive. I'd sometimes have to park my butt inside the computer room for a few days to catch it in action. You just can't troubleshoot a problem very effectively when it's not happening.

The next time it starts misfiring again, try to do the compression test right away while it's still misfiring. Mine came and went all the time.
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Old 02-26-2010, 07:10 PM   #25
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I made sure it was was doing its thing BEFORE i did the check.

It just wasnt doing it after i let it run for 10 minutes after putting the plugs back in after performing the compression check.
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Old 04-04-2010, 08:19 PM   #26
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My 97 was doing the same thing, I did the ohms check on the crank shaft position sensor replaced it because it read 000 ohms and it cured it. A year later it's doing it again with a too lean CE code. Good luck
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Old 04-04-2010, 11:28 PM   #27
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Jerry - Re valve springs

Jerry - if I remember right Chrysler first told you about the weak valve spring problem, and I think there's a TSB on it. It did the trick for yours. Since then you've told countless others, and it's worked for them too. So it's a viable fix that works.
Not that I'm doubting you at all - too many people have verified the fix. IT WORKS!

But - I seem to have trouble understanding the “why.”
All the weak valve spring ailments I've encountered have been at higher speeds, not at idle.
A weak spring will not allow the valve to close fast enough, letting it leak a little. We call that valve float (but float isn't always a weak spring.) It only manifests itself at the higher speeds Low speeds are fine.

At idle it has far more time to close than at any other speed. Thus it should only affect it at the higher speeds (and it's not an easy diagnosis,) usually a vacuum gauge gets you pointed in the right direction.

If the valve stem/guide has a problem, like a slight bend, then it could tend to slow the closing too - but again, at high speeds more than low speeds.
Unless the weak spring is so weak it won't let the valve close hard enough - with a snap. It may be hanging up slightly open? A compression test should show that. But if it's that weak it should really give trouble at the higher speeds too.

If it's so weak it can't close fast enough at idle, at higher speeds it should rattle and possibly jump off.

It must be on the exhaust side, since on the intake side it should cause backfire in the intake manifold.

The fact it's intermittent is odd too - weak doesn't come and go.

I'm wondering if it can be felt by hand with a lever type spring compressor - would it “feel” weaker? Comparing cylinders may show it, comparing with an engine that has normal ones would be good too.
Modifying a lever to test it with the rocker still in place shouldn't be hard.

I wonder it there's someone that had the problem and replaced the spring - and measured them? I'd sure like to know.

Is it possibly something else that is getting inadvertently changed in the process that actually was the culprit?

Where I'm going is I wonder if it's a lifter adjustment problem. When replacing the springs, the rocker is backed off and removed, allowing the lifter to come up all the way. Then when we replace the spring, we reposition the lifter's piston back to the center as we tighten it down.
Wondering if it's a factory adjustment error? When we service it, it gets put back on right. If the lifter had been positioned right at the top of the travel, then I wonder?
Would a simple loosen and tighten do the trick?

Thoughts and comments?

As always, I'm looking for the simple easy solution - sometimes there just isn't one!

I'd sure like to get hold of one that's acting up like that.
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Old 04-06-2010, 02:23 PM   #28
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No comments or further information?

Jerry?

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