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Unread 02-20-2009, 11:10 AM   #1
Rollin
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Slight tremble at idle

Both my TJ and this XJ have had a slight idle issue. It will idle fine other than the occasional "tremble". Just a slight shake for maybe a quarter second and then go back to normal. any ideas?

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Unread 02-20-2009, 12:02 PM   #2
4.0H.O.XJ
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Yeah, like an idle hiccup. Doesnt affect anything else, power, acceleration are good, no hesitation,MPG OK, starts right up.......just a little tremble at idle.

4.0's have this. It's rather common. I spent dollar after good dollar trying to track it down, and nothing made it any better or worse. Jeep ran a whole lot better with lotsa new parts on it though!

I am a part tiime mech, and I see this in about 90% of the 4.0 Jeeps that I look at. It'll be fine. If it bothers you, look at the usual suspects: Vaccuum leaks, sensors, plugs/wires ignition etc and see if that helps it improve.
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Unread 02-20-2009, 12:12 PM   #3
wessker96
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Got the same issue just started it last month) was told that new plugs will take care of it, when i do it i will let you know
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Unread 02-20-2009, 12:16 PM   #4
theducksguts
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Hicup

I just assumed this was normal of all 4.0's. I have heard this in everyone I have driven or rid in. I never thought twice about it.
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Unread 02-20-2009, 12:19 PM   #5
Rollin
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All the newer ones that Ive ridden in (04 rubi, 01 cherokee) were smooth as could be which raised the question
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Unread 02-20-2009, 03:05 PM   #6
tock172
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I think it is a 4.0 thing. My engine did with my old plugs, horrible with NGKs, and pretty mild with my new champions, so I guess it is a 4.0 thing. No other issues, great accel, starts right up, MPG is fine, all that, just hiccups when I am at a light and my wheel shakes a tad.
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Unread 02-20-2009, 04:30 PM   #7
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You didn't mention if this is a new symptom or one that you've always had? I've seen some do this, others smooth as silk at all points.

Always time to review tuneup hardware when symptomatic. Plugs, wires, cap, rotor, bottle of Techron fuel injector cleaner in a half a tank of gas. Cleaning of your idle air control and throttle body is never a bad idea.....
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Unread 02-20-2009, 05:36 PM   #8
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smooth idle when startup/cold is great indicator of how engine is mechanically. that's were noises/shakes/ticks are noticed the most.

a warmed up engine in closed loop (computer is controlling all functions during idle) mode that idles a bit rough/has hiccup or random "idle stumble" is very normal for a 4.0 that is HO and has mopar computers.

after driving myself nuts on this one...stumbled across one of many threads on the forum regarding spark plugs, yaddah yaddah.

suffice it to say, due to the way the 4.0 is set up (210'F operating temp, etc) the standard copper plugs work best...and the old school, autolite 3924 type work best (slighter colder heat range than the 895 plug)

on the 4.0, the long reach/long porcelain tip results in very hot plug at idle, and you get a lean (computer controlled) miss that goes away the moment you place it in D or let the clutch out in 1st, or turn a/c on, a motor that idles uphill perfect without a stumble...will stumble in N once fully warmed up.

Cause? spark plugs that are in the too-hot heat range. The long nose/porcelain tip moves less heat from the tip to the cylinder head, and run very hot at idle. Go to a parts store and compare the two- you will see they are identical except the 3924 have less porcelain.

Next, look up old part numbers for the early 4.2/4.0 AMC spark plugs...you will see the CORRECT part number- from years ago- that is NOT what is currently listed for our XJ's. go figure!

The 895 autolite/champion RN12 plugs are TOO HOT heat range for our 4.0's.
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Unread 02-20-2009, 09:10 PM   #9
4.0H.O.XJ
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A few remedies:

There is no "magic bullet" to cure the stumbling Idle problem. It is charchteristic of the H.O. Motors and is seen often in other 4.0 motors as well. The "stumble" is usually so insignifigant that it does not even register on the tachometer. Most people do not even hear it except for the most critical and maintenance-minded 4.0 Owners. It is more audible in engines with a slightly louder aftermarket exhaust.

If power, acceleration, start up and MPG is fine, and you do NOT have a CEL light on, and you are still having the minor occasional "hiccup" or "stumble" at idle, there is no real cause for alarm. If you are encountering other issues succh as hard starts, overheating, smoke, and/or STEADY mengine miss throughout the entire RPM range....then you have an actual PROBLEM that must be addressed that is totally unrelated to this.

A few things I have donehave lessened the severity and frequency of it to the point of no one else but myself noticing it.

- QUALITY ignition components: Copper terminaled cap and rotor, good high quality plug wires, and CHEAP Champion truck plugs or Autolite 3294's. (Thanks, Bruno.).....Do NOT think you will improve power, economy and idle stumble with multi pronged, platinum spark plugs. You WILL have problems. Stick to copper terminaled ignition componenents, and basic, cheap plugs as outlined above.

- SENSORS. Ensure that all of your sensors are clean and up to par. Most specifically your Idle Air Controller and 02 sensors. Either of these in a worn or dirty state can contribute to a crappy idle condition.

- Vaccuum system. Even a small vaccuum leak can cause an erratic idle. Look for cracked hoses and loose connections, then look at the intake manifold and TB base for possible vaccuum leaks.

- THROTTLE BODY: Dirty throttle bodies can contribute to an erratic, stumbling idle. Disassemble, clean and torque to spec if it is dirty. Pauy attention to getting all of the crap out around the idle circuit. Dental picks, wire brushes, compressed air, and throttle body cleaner are your friends when doing this job.

-CCV System: The orifices on the return side of the CCV (think PCV system) are smaller than the intake side, and can clog prematurely, especially at the elbow on the valve cover. Disassemble, clean, re-install ensuring there are no leaks.

- Electrical Grounds/System. Make sure all electrical grounds underhood are snug and corrosion free. Ditto for battery terminals. Clean, consistent voltage helps systems signal better. Ensure your alternator is putting out proper voltage and your battery is holding the same. Weak electrical systems can play havoc with computer-controlled vehicles.....to include an erratic, stumbling idle.

- Proper TEMP Range. 4.0's NEED to be operating in the 210 degree temp range. If you have anything other than the proper thermostat in there, get rid of it ASAP and get the proper temp T-stat. The computer will throw excess fuel and operate in "warm-up" mode until it reaches warmup temp. If it never makes it to 200 - 210 degrees, idle quality and MPG will suffer as a result. DITTO if it is running too hot (over 225 degrees.) Maintain thayt cooling system within the 200-215 degree range and the computer will operate properly resulting in a proper idle.

- Exhaust System. A plugged catalyctic converter and/or crimped/crushed exhaust components can affect idle quality, resulting in stumbling at idle. Inspect and replace as necessary.

- Fuel system: When was the last time you replaced your FUEL FILTER?

- AIR filter. Same as above. It matters. Really.

.......These are the basics. Even the most seasoned ogf mechanics can overlook a basic or two. Once youve checked all of the above, your idle should be a little better. About the best you are going to get it using conventional tune-up and maintenance teqhniques. If you want to further your attempt, use old school hot rodding techniques:

- 1999 and up intake manifold: Re-designed in 1999 as an equal-length intake to increase HP, reduce emmissions, and smooth intake charge, this intake flows smoothly without restrictions and is a basic junkyard upgrade that improves stumbling idle...a little bit.

- Free flowing exhaust. Quality aftermarket mufflers and free-flow cat converters have helped increase HP and clean up the idle quality a bit. The problenm is....now that it is slightly louder, it will be more audible as a result, though it will do it less frequently.

Damn if this didnt get longer than it was supposed to. Basically, keep your engine in top form, maintained and use quality replacememnt components, and it will lessen the severity of a minor problem that is inherent to most 4.0 motors.
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Unread 02-22-2009, 09:54 AM   #10
Rollin
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Wow thanks guys.

I hadnt changed the spark plugs so I figured Id start with that. Wouldnt you know, I found my problem.


Cylinders 2 and 6 had their plugs finger tight and leaking compression. The threads on those plugs were completely sooty along with their spark plug boots. 1,3,4,5 were clean and had clean threads. I put some new plugs in when I had them out (champions) and what do you know, a completely smooth idle and I now have some power back and no vibration at rpms above idle.

Idk why the computer didnt pick up the fact that it was loosing compression. Maybe because the plugs were still firing?
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Unread 08-12-2013, 08:36 AM   #11
Svenlylethor
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My XJ just started acting like that. It's got about 240,000 miles on it and I haven't really driven it much these past 2 years but I let a friend of mine borrow it and now it's starting to tremble, it was low on oil, and the fan doesn't work. I guess I'll pull out the spark plugs and see what they look like. The fan is a whole other story...

Remind me to never loan my Jeep out to anyone again.
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Unread 08-12-2013, 07:13 PM   #12
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Quote:
Originally Posted by Rollin View Post
Wow thanks guys.

I hadnt changed the spark plugs so I figured Id start with that. Wouldnt you know, I found my problem.


Cylinders 2 and 6 had their plugs finger tight and leaking compression. The threads on those plugs were completely sooty along with their spark plug boots. 1,3,4,5 were clean and had clean threads. I put some new plugs in when I had them out (champions) and what do you know, a completely smooth idle and I now have some power back and no vibration at rpms above idle.

Idk why the computer didnt pick up the fact that it was loosing compression. Maybe because the plugs were still firing?
Simple stuff. How's the cap, rotor, and wires?
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