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 Post subject: why do 3.7L's have a rough idle?
PostPosted: Sat Oct 11, 2008 6:53 am 
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anybody know what causes the 3.7L to idle rough?

here are some possible reasons i found while searching lost:

1) 90 degree v-6's idle rough

2) motors were manufactured slightly out of balance

3) the 3.7L is an odd fire v-6 and when the odd cylinder fires it causes vibration (rough idle)

i don't know if any of the above are true? but what do you all think?

i also have gotten the standard answer from the dealerships that all 3.7L/4.7L motors have the rough idle and that it is a characteristic of the motor? but they don't know what causes it?

any help is much appreciated. mike

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PostPosted: Sat Oct 11, 2008 8:53 am 
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Fuel and ignition, tested with a scantool -

have rail pressure tested - could be the regulator, could be the in-tank fuel pump, but rail pressure must be in-spec for smooth idle

run a balance test on the injectors - erratic idle is first symptom of dirty or failing injector

Faulty coils and boots usually give first symptoms in cold damp weather as erratic idle,even though there sunk into the block they can crack and arc.

Lastly, sometimes firstly, slightly-leaking EGR valve gives most noticeable symptom at idle, as can dirty PCV valve

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PostPosted: Sat Oct 11, 2008 9:11 am 
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LIV42DY wrote:
Fuel and ignition, tested with a scantool -

have rail pressure tested - could be the regulator, could be the in-tank fuel pump, but rail pressure must be in-spec for smooth idle

run a balance test on the injectors - erratic idle is first symptom of dirty or failing injector

Faulty coils and boots usually give first symptoms in cold damp weather as erratic idle,even though there sunk into the block they can crack and arc.

Lastly, sometimes firstly, slightly-leaking EGR valve gives most noticeable symptom at idle, as can dirty PCV valve

thanks for your response! but there is nothing wrong with my liberty. i have new plugs, freshly cleaned throttle body. fuel pressure was just checked. top engine clean done, misfire rate monitored with the drb3 (none found). all this done under warranty. clean bill of health!

my question is more of a general question of why this engine line (3.7L/4.7L) have a characteristic of a rough idle. almost all 3.7/4.7's i have rode in seem to have the same rough idle? thanks mike

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PostPosted: Sat Oct 11, 2008 9:24 am 
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I'm a chevy guy, had quite a few 4.3L V6's over the years, worst was the 95's with the CPI injection systems. I had two 95's and threw everyting in the book at them, always had rough idles, one even had an occassionally miss that never showed up on scanners/codes. It is wierd that the V6's were rough, but the V8's run smooth as the 4.3L was a 350 with 2 cyls knocked off. I did have a 350 in my camaro that ran rough though, oh wait, that would have been due to the larger cam :wink: :D
Our '05 liberty does seem to run a little rough, maybe it's just the nature of the beast to say.

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 Post subject:
PostPosted: Sat Oct 11, 2008 9:26 am 
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sorry :( reading without my glasses. Anyway good question, i have owned a xj 4.0 currently have a TJ and my KJ and they never realy run 100% steady. I quess I have grown use to it. Now that you mentioned it. Ill have think awhile.

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PostPosted: Sat Oct 11, 2008 10:43 am 
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i've being wondering the same thing for a while, i keep my jeep in tip top condition, and the motor idles slightly rough i guess you could say. Some people may not even notice it, others will...

I thought my motor or trans mounts were going bad... is this unlikely?

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 Post subject:
PostPosted: Sat Oct 11, 2008 11:03 am 
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They all run slightly rough I think but its one of those things that if you sit and think about it long enough it will drive you crazy, :shock: PLUS when you are out on some rocky twisted up trail you never notice it, so go out and 4 wheel it and don't worry, lifes too short the way it is! :wink: :lol: :lol:

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PostPosted: Sat Oct 11, 2008 11:44 am 
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tommudd wrote:
They all run slightly rough I think but its one of those things that if you sit and think about it long enough it will drive you crazy, :shock: PLUS when you are out on some rocky twisted up trail you never notice it, so go out and 4 wheel it and don't worry, lifes too short the way it is! :wink: :lol: :lol:



agreed haha

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 Post subject:
PostPosted: Sat Oct 11, 2008 7:36 pm 
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well according to wikapedia the 3.7 is an odd fire v-6 with a three pin crank shaft making it a odd fire v-6. the lack of a split pin crank makes the rough idle. you can read about it in the cut and paste below! mike

Link in full:
http://en.wikipedia.org/wiki/V6_engine

90 degrees
90-degree V6 engines are also produced, usually so they can use the same production-line tooling set up to produce V8 engines (which normally have a 90-degree V angle). Although it is relatively easy to derive a 90-degree V6 from an existing V8 design by simply cutting two cylinders off the engine, this tends to make it wider and more vibration-prone than a 60-degree V6. The design was first used by Buick when it introduced its 198 CID Fireball V6 as the standard engine in the 1962 Special. Other examples include the Maserati V6 used in the Citroën SM, the PRV V6, Chevrolet's 4.3 L Vortec 4300 and Chrysler's 3.9 L (238 CID) Magnum V6 and 3.7 L (226 CID) PowerTech V6. The Buick V6 was notable because it introduced the concept of uneven firing, as a result of using the 90 degree V8 cylinder angle without adjusting the crankshaft design for the V6 configuration. These engines were often referred to by mechanics as "shakers," due to the tendency of the engine to bounce around at idle speed. More modern 90-degree V6 engine designs avoid these vibration problems by using crankshafts with offset split crankpins to make the firing intervals even, and often add balancing shafts to eliminate the other vibration problems. An example is the 90-degree Mercedes-Benz V6 which, although designed to be built on the same assembly lines as the V8, uses split crankpins, a counter-rotating balancing shaft, and careful acoustic design to make it as smooth and quiet as the inline-6 it replaced.

[edit] Other angles
Narrower angle V6 engines are very compact but can suffer from severe vibration problems unless very carefully designed. Notable V6 bank angles include:

[edit] Odd and even firing
Many older V6 engines were based on V8 engine designs, without altering the V angle or using a more sophisticated crankshaft to even out the firing interval. One characteristic of these engines was a notorious odd-firing behavior.

Purpose-built V6 engines use one crankpin per cylinder for an even 120° ignition pattern. In contrast, most V8 engines share a common crankpin between opposite cylinders in each bank. That is, the crankshaft has just four pins for eight cylinders, and a cylinder fires every 90° for smooth operation.

V6 engines derived from V8 engines often have three shared crankpins arranged at 120° from each other, similar to an inline 3-cylinder, with two pistons per crankpin. If the cylinder banks are arranged at 90° (as they commonly are in V8-derived V6s), this leads to a firing pattern with groups of two cylinders separated by 90° of rotation, and groups separated by 150° of rotation.

An example is the Buick 231 odd-fire, which has a firing order 1-6-5-4-3-2. As the crankshaft is rotated through the 720° required for all cylinders to fire, the following events occur on 30° boundaries:

Angle 0° 90° 180° 270° 360° 450° 540° 630°
Odd firing 1 6 5 4 3 2
Even firing 1 6 5 4 3 2

Nissan uses the firing order 1-2-3-4-5-6 in some of the V6 engines they make.

In 1977, Buick introduced a new "split-pin crankshaft" in the 231. Using a crankpin that is 'split' and offset by 30° of rotation resulted in smooth, even firing every 120°. However, in 1978 Chevrolet introduced a 90° 200/229 V6, which had a compromise 'semi-even firing' design using a crankpin that was offset by only 18°. This resulted in cylinders firing at 108° and 132°, which had the advantage of reducing vibrations to a more acceptable level and did not require strengthening the crankshaft. In 1985 Chevrolet's 4.3 (later the Vortec 4300) changed it to a true even-firing V6 with a 30° offset, requiring larger crank journals to make them adequately strong.

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 Post subject:
PostPosted: Sat Oct 11, 2008 7:42 pm 
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Well I'm glad someone made this post. Because this would have eventually driven me crazy trying to chase down the rough idle in my engine. I was already pulling plugs and checking lines not getting anywhere.

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 Post subject:
PostPosted: Sat Oct 11, 2008 9:31 pm 
My KJ idles smoother than any V8 I've owned :lol: Good 'nuff for me


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 Post subject:
PostPosted: Sat Oct 11, 2008 9:58 pm 
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thunderbirdjunkie you are the reason i got started on the odd fire path! i saw this previous posting of yours and started looking for proof. i found the proof in wikapedia.
Link:
http://www.lostjeeps.com/forum/phpBB3/vie ... fire#30097

i also found some help at allpar, in the bio of the 3.7L. it says (at the end) of the bio the rough idle of the motor is trying to be cured by engineers. but until they change the crank (so that each piston has its own pin in the crank) the rough idle is here to stay! at least nothing is wrong!

Link to 3.7 bio! rough idle mentioned at the end of bio:
http://www.allpar.com/mopar/37.html

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 Post subject:
PostPosted: Sat Oct 11, 2008 10:56 pm 
I was right...

That's scary :shock: :shock:


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 Post subject:
PostPosted: Sat Oct 11, 2008 11:48 pm 
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My 2007 KJ 3.7L runs smooth, but maybe thats because I drive diesels :lol: :lol: :lol:

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 Post subject:
PostPosted: Sat Oct 11, 2008 11:52 pm 
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my 3.7 is very smooth and a slight shudder makes its way in every now and then

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PostPosted: Sat Oct 11, 2008 11:58 pm 
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KJ4ME wrote:
my 3.7 is very smooth and a slight shudder makes its way in every now and then


It just quivers at your touch..... :shock:

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 Post subject:
PostPosted: Sun Oct 12, 2008 2:25 am 
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Once I changed to Mobile 1 pure synthetic, the 3.7L smoothed out A LOT!!!

I also go about 6-7k per oil change so the extra cost is not that bad. 8)

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 Post subject:
PostPosted: Sun Oct 12, 2008 7:56 am 
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USPLibby wrote:
Once I changed to Mobile 1 pure synthetic, the 3.7L smoothed out A LOT!!!

interesting, i will have to try the mobile 1.

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PostPosted: Sun Oct 12, 2008 10:39 am 
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I never knew the 3.7 idled rough.. Have any of you all ever been in a 4.0L zj? Thats what I like to call idling rough! I kinda wish my Jeep idled that rough. :D

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 Post subject:
PostPosted: Sun Oct 12, 2008 10:53 am 
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Jeepjeepster wrote:
I never knew the 3.7 idled rough.. Have any of you all ever been in a 4.0L zj? Thats what I like to call idling rough! I kinda wish my Jeep idled that rough. :D
if you have an automatic kj you are insulated from the rough idle!!!! (since there is no direct contact between the drive train and the cabin) but i have a manual transmission and the gearshift sticks right up into the cabin. and you can just watch the manual gear shift just vibrate around, because it is directly connected to the tranmission. but on the auto models you are isolated buy all the rubber mounts,and the fact that (on the automatic) the auto gear shift is not directly connected to the drive train (it is connected to the unibody)!

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