Idle difference Grand Cherokee v. Commander: 3.7L - Jeep Commander Forums: Jeep Commander Forum
General Commander Discussion General Discussion involving the Jeep Commander goes here. If it does not fit into a more specific category below, post it here.

Reply
 
LinkBack Thread Tools Display Modes
post #1 of 21 (permalink) Old 08-29-2008, 10:07 AM Thread Starter
Member
Points: 1,780, Level: 24 Points: 1,780, Level: 24 Points: 1,780, Level: 24
Level up: 80% Level up: 80% Level up: 80%
Activity: 0% Activity: 0% Activity: 0%
 
Join Date: Aug 2008
Location: Chicagoland
Posts: 64
Idle difference Grand Cherokee v. Commander: 3.7L

I'm driving a 3.7L 2005 Grand Cherokee as a loaner while the Commander gets the water leak repaired (hopefully).

Sure seems like the 3.7 in the GC idles smoother than on my Commander. I've noticed the Commander is a bit rough at idle. I barely feel the Grand Cherokee.

How rough is too rough for idle on the Commander? Just wondering if there is something I should troubleshoot.

Thanks!!

AR
artmanr is offline  
Sponsored Links
Advertisement
 
post #2 of 21 (permalink) Old 08-29-2008, 10:25 AM
Senior Member
Points: 11,816, Level: 71 Points: 11,816, Level: 71 Points: 11,816, Level: 71
Level up: 42% Level up: 42% Level up: 42%
Activity: 0% Activity: 0% Activity: 0%
 
robby's Avatar
 
Join Date: Jun 2008
Location: Desplaines IL.
Posts: 3,262
Less power per cylinder firing= less vibration at idle. Also, woudn't be surprised if it idles a bit faster which will tend to mask the firing impulse.

Where in Chicago area are you? I'm over in Desplaines.
................Rob


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
|
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
|
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
robby is offline  
post #3 of 21 (permalink) Old 08-29-2008, 12:03 PM Thread Starter
Member
Points: 1,780, Level: 24 Points: 1,780, Level: 24 Points: 1,780, Level: 24
Level up: 80% Level up: 80% Level up: 80%
Activity: 0% Activity: 0% Activity: 0%
 
Join Date: Aug 2008
Location: Chicagoland
Posts: 64
Does the 3.7 have less power on the Grand Cherokee? I'm not sure if I understood the part about less power per cylinder. I'll have to check the rpms at idle to see if they are higher.

I'm in Downers Grove.

AR
artmanr is offline  
 
post #4 of 21 (permalink) Old 08-29-2008, 12:59 PM
Senior Member
Points: 11,816, Level: 71 Points: 11,816, Level: 71 Points: 11,816, Level: 71
Level up: 42% Level up: 42% Level up: 42%
Activity: 0% Activity: 0% Activity: 0%
 
robby's Avatar
 
Join Date: Jun 2008
Location: Desplaines IL.
Posts: 3,262
Oop's, Guilty of not putting glasses on while reading your post. I thought you were comparing 4.7 to 3.7. My bad.
So, I guess were comparing 3.7 to 3.7 eh?
That one I don't think could be explained. Both vehicles use the same chassis with minor suspension variations. Engine mounts would be the same. I would be inclined to write it off as varience in mass production. That is, of course. if you are not experiencing a vibration that can rattle things off the dash.
Usually that type of vibration is related to a exaust system bound on its hangars.
On a six, often a/c compressor, when on, will cause a bit of roughness, but thats about all that comes to mind. And that should be about the same between vehicles as well.
Again, sorry about my misinterpetation of engines.

My Impala club meets at Fudruckers in your burb. We've probably passed from time to time.
............Rob


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
|
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
|
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
robby is offline  
post #5 of 21 (permalink) Old 08-29-2008, 01:54 PM
Points: 6,704, Level: 53 Points: 6,704, Level: 53 Points: 6,704, Level: 53
Level up: 77% Level up: 77% Level up: 77%
Activity: 0% Activity: 0% Activity: 0%
 
Join Date: Oct 2005
Posts: 1,397
Generally, larger engines (more cylinders) idle more smoothly than smaller engines with fewer cylinders. Think about the 4 strokes in the combustion cycle and it is easy to understand this concept. However, there are a number of different reasons why one engine may feel more smooth at idle than another, despite the engine size or number of cylinders. If you've ever driven a late model vehicle with liquid filled motor mounts, you would probably have a hard time determining if the engine is off or on by feeling for the vibrations.
TR4Runner is offline  
post #6 of 21 (permalink) Old 08-29-2008, 10:32 PM
Senior Member
Points: 11,816, Level: 71 Points: 11,816, Level: 71 Points: 11,816, Level: 71
Level up: 42% Level up: 42% Level up: 42%
Activity: 0% Activity: 0% Activity: 0%
 
robby's Avatar
 
Join Date: Jun 2008
Location: Desplaines IL.
Posts: 3,262
Well, we could go for hours on this one, but consider the following:
V-8-Idle speed-500 r.p.m. 500 divided by two will equal 250 combustion cycles per cyl per minute. Now x8 cyl. equals 2000 combustion cycles per minute crankshaft.
V-6-Idle speed-650 r.p.m. 650 divided by two will equal 325 combustion cycles per cyl per minute. Now x6 cyl. equals 1950 combustion cycles per minute crankshaft.
Pretty tough to pick up on 50 combustion cycle difference. Less than 1 impulse per second.
To keep it interesting,
4 cyl. Idle speed-700 r.p.m. 700 divided by two will equal 350 combustion cycles per cyl per minute. But, because its a 4 it only has 1400 combustion cycles per minute crankshaft. You can sure see why they need those classy motor mounts.
Where am I going here?
Well on my 4.7 Jeep- 5.7 Chev truck- Both my 5.7 Impala's, all automatics, and frankly every V-8 I,ve been around, at idle there is a distinct sense of combustion impulse while idling in gear. It is subtle but always present.
Now, my 08 Malibu-3.6 V-6 is without any feeling at all. If it didn't have a tach, like
TR4runner said, you would think it quit running. And ya know, I dont think its the mounts on this one. I can put my hand on the plenum and feel almost no vibration of any sort. Nice balance of reciprocating components I'd say.
So, prior to my figuring out I was comparing an apple to a orange, I was of the belief that artmanr had dropped off a v-8 and had a v-6 loaner. And I assumed [you know what that'll get ya] he was picking up on the lack of firing impuse at idle.

.............Rob


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
|
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
|
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
robby is offline  
post #7 of 21 (permalink) Old 08-30-2008, 07:30 AM
Points: 6,704, Level: 53 Points: 6,704, Level: 53 Points: 6,704, Level: 53
Level up: 77% Level up: 77% Level up: 77%
Activity: 0% Activity: 0% Activity: 0%
 
Join Date: Oct 2005
Posts: 1,397
robby,

I think you're missing a couple of important points. First of all, there are four strokes in the combustion cycle: Intake (piston down) Compression (piston up) Power (piston down) Exhaust (piston up). So the first thing you should consider is that the piston goes up and down two complete times for just one "power" stroke of the engine. Now imagine having just 4 cylinders in an engine going through the combustion cycle. How many of the pistons will be in each part of the stroke cycle? Okay, now apply the same concept to an 8 cylinder engine. Do you understand the point I'm making? That's why the firing order of an engine is intended "balance" out by having two opposing cylinders on each side of the engine fire at the same time. With a V8, two cylinders will always be at the same point in the combustion cycle, which allows them to balance each other and creates a smoother running engine.

Forget actual engine displacement for a moment and just consider the amount of cylinders in two given engines. The fact of the matter is the engine with more cylinders will be inherently smoother at idle (and across the rpm band, for that matter) than the engine with fewer cylinders. This applies mainly to engines in a V configuration. Straight engine configurations are a slightly different story. BMW is still very big on straight 6 cylinder engines even today, and one of the major reasons is because they are incredibly smooth. But for our debate, we're discussing V engines.

Did you know that manufacturers had to start putting balance shafts in their 4 cylinder (and some 6 cylinder engines) many years ago to try and attain the smoothness that people were used to with V8 engines? Did you ever wonder why?

Last edited by TR4Runner; 08-30-2008 at 07:33 AM.
TR4Runner is offline  
post #8 of 21 (permalink) Old 08-30-2008, 09:05 AM
Senior Member
Points: 11,816, Level: 71 Points: 11,816, Level: 71 Points: 11,816, Level: 71
Level up: 42% Level up: 42% Level up: 42%
Activity: 0% Activity: 0% Activity: 0%
 
robby's Avatar
 
Join Date: Jun 2008
Location: Desplaines IL.
Posts: 3,262
Good morning!
I think we are saying the same thing two ways. I'm saying one combustion cycle for every two crank revolutions and you're saying one power stroke for every two tdc moments.
Keep in mind the original discussion revolved around my interpetion of impulse sense. This sensitivity is at idle/in gear only.
Balance shafts in v-6's are to combat rocking couple torsional vibration and are a band-aid to allow a 90 degree v spread v-6 to behave as a 60 degree v spread. Of course, a 60 degree v-6 requires no balance shaft as they already have perfect primary balance.
On a 4 cyl inline, the balance shafts are to combat rocking couple[a result of those long 180 degree spans between firing impulse] that occur above 2000 r.p.m. and are generally found in engines 2.0 litre and above.
By the way, on a v-8, there is only one piston at combustion cycle at a time.The other t.d.c. piston is at the top of a exaust stroke.The inherent smoothness of a v-8 are the result of each firing impuse only being 90 degrees crank rotation apart.
Your one of those folks I would love to talk engine design to. Trouble is I type with only one index finger and I'm kinda slow at it.
By the way, whats your biz?
I'm a contract failure analyst for G.M./Ford/Volvo/Volswagen/Honda/Toyota and whoever else contracts me.

..........Rob


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
|
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
|
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
robby is offline  
post #9 of 21 (permalink) Old 08-30-2008, 09:25 AM
Points: 6,704, Level: 53 Points: 6,704, Level: 53 Points: 6,704, Level: 53
Level up: 77% Level up: 77% Level up: 77%
Activity: 0% Activity: 0% Activity: 0%
 
Join Date: Oct 2005
Posts: 1,397
Right, of course only one of the two pistons is actually firing (power stroke) at the same time, otherwise there wouldn't really be much of a firing order. But there are two pistons in a V8 engines at the very top of their stroke at the same time all the time. Think of them as pairs to balance each other. 1-8-4-3-6-5-7-2 is a typical firing order for GM V8 engines. No two cylinders fire at the same time, however each cylinder is paired with another on the crankshaft, which greatly affects balance/smoothness of the engine.

BTW, I started out years ago as an auto tech and then got into automotive engineering. Currently I work in management, but I still love to get my hands dirty because I'm not really the kind of person who is happy sitting behind a desk all day.
TR4Runner is offline  
post #10 of 21 (permalink) Old 08-30-2008, 10:20 AM
Senior Member
Points: 11,816, Level: 71 Points: 11,816, Level: 71 Points: 11,816, Level: 71
Level up: 42% Level up: 42% Level up: 42%
Activity: 0% Activity: 0% Activity: 0%
 
robby's Avatar
 
Join Date: Jun 2008
Location: Desplaines IL.
Posts: 3,262
Very cool,
I would have the same problem being a desk driver.
My gig is a bit of a stitch cause you get to see amazing engineering errors. Amazing cause you can't believe it got into production.
The inspections are at dealer level on in service units.
I,m sure you were aware of Toyota's sludging and unwritten recall debacle.
That kind of thing.
Anyways, we'll talk again.

.......Rob


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
|
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
|
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
robby is offline  
Sponsored Links
Advertisement
 
Reply

  Jeep Commander Forums: Jeep Commander Forum > Jeep Commander Discussion > General Commander Discussion

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Jeep Commander Forums: Jeep Commander Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in











Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome