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Discussion Starter #1
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
 

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Discussion Starter #3
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
 

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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
 

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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.
 

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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
 

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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?
 

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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
 

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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.
 

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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
 

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Yeah, Toyota's sludging problem was fairly major, considering just how many vehicles used the shared block. The only Toyota product I've ever owned has been my 4Runner, and lucky for me that no 4Runner engine ever had a sludging problem.

The ironic part is that the engines would still run well over 100,000 miles without showing any signs of sludging, but then the next owner would usually have to deal with the problem as the vehicle started to age. I believe the sludging problem only surfaced as a result of improper maintenance (no oil changes, for example) by owners who were mostly leasing the vehicles. In fact, that's one of the major reasons why BMW started offering "free" maintenance on all of their vehicles. It seems that many owners looked at their leases as a rented vehicle and did not want to pay for routine maintenace.
 

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I believe we've fallen way off the subject, but I have to throw this in.
I certainly have run into the poor maintainence issue on lease vehicles, but the sludging I was referencing was that which caused oil pickup plugging and failure of bearings around 30k miles.
The crankcase internal airflow path was so poorly designed it created dead air pockets, and in those areas, the carbon was separating out of the oil and making sludge balls. Ultimatly these broke away and slowly built up in the pan and around the pickup, finally resulting in pump cavitation and resultant failure.
Toyotas most recent recall?--- Frames rust out 1995/2000 Tacoma.
Factory is buying them back if rust is in evidence.
Not to shoot any carmaker down, Just pointing out they all blow it from time to time.
When they do, it pays my bills.
Later all,
Rob
 

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A guy I work with is getting his old Tacoma bought back as part of the frame rust issue. He's supposedly getting a lot more for it than he would have if he sold it privately, so he's a happy camper. He's got more than 200,000 miles on it, so it's definitely a good deal for him.

As for the sludging, I've only heard that vehicles with poor maintenance were affected. I've never heard it was a design issue, but rather poor maintenance by vehicle owners. If it were truly a design issue, then I would think that nearly all engines would have been affected. Clearly that's not the case, so I think it can be traced back to improper maintenance. Now of course different designs may be able to tolerate bad maintenance practices, but that's another discussion.
 

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TR4Runner said:
As for the sludging, I've only heard that vehicles with poor maintenance were affected. I've never heard it was a design issue, but rather poor maintenance by vehicle owners. If it were truly a design issue, then I would think that nearly all engines would have been affected.
Found this about the sludge issue:
http://www.toyoland.com/sludge.html
 

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So that article supports what I was saying:

Only 1% of vehicles are affected and although the ventilation of the crankcase can be part of the problem on some vehicles, it seems that owners who choose to ignore the oil change intervals are a much larger part of the problem. It's also interesting to see that Mercedes Benz, Volkswagen, and Chrysler have all had sludging problems recently.

This definitely illustrates the importance of proper maintenance, especially when it comes to oil changes. I'm a bit old school when it comes to changing the oil in my vehicles. I used to follow the old 3,000 interval religiously, but for the last few years I've bumped it up to 4,000 miles instead. Since the recommended interval keeps increasing due to advances in engine and oil technology, I feel comfortable at 4,000 miles for my oil change interval, but not more than that.
 

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1.5 average retail? I call that taking care of customers, not a coverup. In this age of the Internet, there's simply no such thing as covering up information like that. For whatever reason, they had problems with rust on those frames and now they're doing the right thing by taking care of their customers above and beyond what they needed to do. I think that's awesome...
 

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No question about the method the custumer is being taken care of.
I'm just stunned that this is occuring on so very young vehicles.
Truly a case of bean counter engineering.
I'll bet the original specs called for a higher grade of steel.
I'm sure you would be annoyed if it happened to you. Especially if you were not planning on vehicle replacement due to finances or whatever when the failure is found, regardless of how much trade value you were given. And, no matter how hard you tried to maintain and preserve your vehicle.
Real custumer service is providing a product that does not expose you to unneccesary expense, not try to buy your opinion.
So, try to put yourself in someone elses shoes, say just barely making it financially, and see if a high trade value on a vehicle you can't afford to replace at this time, really makes you feel good.
I can tell you I've sure not met too many who would consider that good news.

...........Rob
 
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