How many owners have noticed engine vibration (around 2500 rpms)?

[MSHardeman]

Glad to hear that it's not just me. I tend to get a harmonic/vibration right around the 2500 rpm range that isn't really there before or after that. I just changed my timing belt, water pump and spark plugs so I know everything is good there but the vibration was there before and after all that. Hmmmm....

[RamAirZ]

Here is a response from Jerry:

THEY ARE ALL INTERNAL BALANCED , THOSE BALANCERS /DAMPNERS DONT GIVE THAT MUCH TROUBLE UNLESS THEY HAVE BEEN PULLED ON BY THE WRONG PULLER, YOU NEVER WANT TO PULL ON THE EDGES ,

IF THE VIB,S YOU ARE TALKING ABOUT ARE THE KIND YOU CAN FEEL IN THE STEERING WHEEL OR BODY , YOU HAVE MISLOCATED ENG MOUNTS , VERY COMMON, ON THESE OR THE MT,S HAVE COLLASPED , I HAD THE SAME THING ON MINE


Then I asked him what he meant by mislocated mounts:

THE FRT MOUNT S SOME TIME CAN GET TO SAGGING OR TWISTED , AND THEYA ER HARD TO SEE , BUT TAKE A LOOK AT THE FRT MOUNTS WHERE THE TOP PLATE IS BENT OVER THE MOUNT ITSELF , IT SHOULD HAVE A GAP THERE THAT YOU CAN STICK THE ND OF YOUR FINER IN BETWEEN THE TOP PLATE AND THE MOUNT ITSELF, I BET IT IS SAGGED AND TOUCHIONG USUALLY IT IS THE PASS SIDE BUT THE DRV SIDE CAN DO THE SAME, THE REAR TRANS MOUNT SHOULD BE THE SAME WAY TAKE A LOOK AT THE MOUNT AND THERE SHOUDL BE AT LEAST A 3/4 IN GAP BETWEEN THE UPPER AND LOWER PART , IN THE MIDDLE OF THE MOUNT , THIS ONE CAN BE FIXED , BY JACKING UP THE BACK END OF THE TRANS AS HIGH AS YOU CAN GET IT AND PUTTING A PIECE OF FLATTEN 3/4 IN HTR HOSE IN THE MIDDLE AND THEN LET IT BACK DOWN, THE FRT CAN BE DONE THE SAME WAY BASICALLY BUT IT IS HARDER TO DO , NOT VERY MUCH ROOM TO WORK WITH

(he's not the best typer lol)

[RamAirZ]

Like I said, for me it's plush lol. I've driven things that shook the crap out of you at idle, mostly due to camshafts and solid mounts, that and I had a Harley for awhile lol

[Triathlete]

I had the pleasure of meeting Jerry last year in Moab. The guy will totally boggle your mind with the info he has in his head. I think the guy has every bit of Zu info memorized...truley amazing!!!!

[RamAirZ]

Ya he's an awesome guy! His trooper is one hell of a sleeper too lol

[RamAirZ]

Putting it in drive with your foot on the brake and giving it a little rev with someone watching will REALLY show you, when my Mustang's mounts went bad, the motor looked like it wanted to crawl out of the engine bay.

[SuperBoomTurbo]

...when my Mustang's mounts went bad, the motor looked like it wanted to crawl out of the engine bay.

Off topic, but do you remember that pennzoil commerical a few years back where the chevy 'burb did a wheelstand before launching the motor like a cannonball? I was working with a 60 year old mopar man who shart himself after watching it via youtube.

T/j over

[RamAirZ]

Well alot of it has to do with packaging, parts interchangeability, a certain goal they are trying to reach etc., and like with 4 cylinders, sometime it's just something you have to do. The 4.3's didn't start off with a balance shaft but they ended up going to one to try and smooth out vibrations. All crankshafts have counterweights but sometimes the design of the engine and the firing sequence can produce unwanted vibrations. Take the 4.3 for example, it's basically a 350 chevy with 2 cylinders chopped off. But due to the firing sequence, it's a shaker! In a V8 it works fine but in the V6 it makes it lopey. Here's a good read:

http://en.wikipedia.org/wiki/Balance_shaft

[89Vette]

Here's a good read:

http://en.wikipedia.org/wiki/Balance_shaft

Thanks. I note this paragraph at the end of the link....

"Another balance shaft design is found in many V6 engines. While an inherently balanced V6 engine would have either 60 or 120 degrees angle between the two banks of cylinders, many current V6 engines are derived from older V8 engines, which have a 90 degree angle between the two banks of cylinders. While this provides for an evenly spaced firing order in an 8 cylinder engine, in a six cylinder engine this results in a loping rhythm, where during each rotation of the crankshaft three cylinders fire at 90 degree intervals, followed by a gap of 90 degrees with no power pulse. This can be eliminated by using a more complex, and expensive, crankshaft which alters the relationship between the cylinders in the two banks to give an effective 60 degree difference, but recently many manufacturers have found it more economical to adapt the balance shaft concept, using a single shaft with counterweights spaced so as to provide a vibration which cancels out the shake inherent in the 90 degree V6."

The last couple of sentences seem to explain why some engines have a balance shaft and why the VX does not. I just noticed this evening that the 3.5L crankshaft sells for over $1000!

Since I doubt this is an availability thing (because the 3.5L was in other vehicles), I would appear a more expensive crank -- referred to in Wikipedia -- has been utilized in the VX. IOW, it doesn't have a balance shaft because it doesn't need one.

[RamAirZ]

Well NEED is a term used loosely by auto manufacturers lol. I don't think the engine needs one per se but like I stated earlier, I've driven alot rougher engines (due to my own choices). I'm not sure if the 3.5 actually uses the style of crankshaft they mention, it could just be expensive because it's not a "common" engine and specific to only one engine, which makes the market for it low, thus costing more.

[deermagnet]

I have an Isuzu tech training course for these engines and this is what it says-
Problem- Engine has a vibration at above idle.
Possible cause- Outer ring on the harmonic damper has moved.
Diagnosis- Look for abnormal harmonic damper oscillations. During a visual inspection, check to see if the inner damper spring is also damaged.

I checked the parts catalog and repair manual and can't find any info on the damper/balancer.

I don't believe it's normal for this engine to vibrate at all. In over ten years on the Vehicross forums this is the first time I've heard about any engine vibration. Mine is so smooth with 218,000 miles and at 80 mph my whole vehicle doesn't have even the slightest vibration.

Engine vibration on this 3.5L should be something that can be corrected, IF the problem can be found.

Mark Griffin

[SlowPro48]

I don't believe it's normal for this engine to vibrate at all. In over ten years on the Vehicross forums this is the first time I've heard about any engine vibration. Mine is so smooth with 218,000 miles and at 80 mph my whole vehicle doesn't have even the slightest vibration.

Engine vibration on this 3.5L should be something that can be corrected, IF the problem can be found.

Mark Griffin

Haha! Mark your VX has been rockin' along for 218K and you didn't even know it! The vibration I was talking about that is inherent in our 3.5 is not something you'll feel when you're going down the road. Put it in Park and play with the loud pedal a bit and you will feel it between 2400 and 2800 though. Also at around 1300-1400 but it's really hard to hold it steady at those low rpms. It's not bad at all and you may not even notice it if you're used to riding a Jackhammer Davidson like RamAirZ is - but go from something that has a straight six to the VX and you will feel it!

89Vette I am fully aware that the crankshaft is weighted but those weights are not for your comfort, they are there to offset the mass of the pistons and rods and thus keep the crankshaft from self destructing - as is the harmonic damper at the end of the crankshaft. Even though the rotating parts within the engine are balanced, the engine itself - as a unit - can and will vibrate however - and some configurations are worse than others. The L6 and V12 are naturally smooth whereas the V6 is not - a result of having three cylinders per side. The only way to cancel the rocking couple of our V6 is with a counter-rotating balance shaft like what is seen in the picture of the GM 4.3. That balance shaft isn't "needed" to prevent the engine from self destructing - it's there for human comfort - to keep the engine from rocking along its length which is felt as vibration. But that adds extra weight/complexity/rotating mass. The designers of the GM 4.3 thought it was worth the extra weight, etc to include a balance shaft and they were probably right since it's a 90 degree engine. A lot of sixty degree V-6s don't have one since the more shallow V means less vibration. They make do with some squishy motor mounts that turn the rocking motion into heat (and break down eventually because of it) and call it good. I guess the Isuzu engineers thought our 75 degree V6 was close enough to 60 to go "au natural" as well and for most people it is.

Bottom line is if you're looking for something that's silky smooth when revved in the driveway - from idle to redline - don't go for a V6 with no balance shaft - you need a BMW L6 or better yet a Jag V12. This "cabin resonance" at highway speeds you speak of is NOT caused by the normal amount of engine vibration of the VX though. The normal VX engine vibration isn't noticeable when driving.

Did the vehicle in question have a roof rack?

[89Vette]

Haha! Mark your VX has been rockin' along for 218K and you didn't even know it! The vibration I was talking about that is inherent in our 3.5 is not something you'll feel when you're going down the road. Put it in Park and play with the loud pedal a bit and you will feel it between 2400 and 2800 though. Also at around 1300-1400 but it's really hard to hold it steady at those low rpms. It's not bad at all and you may not even notice it if you're used to riding a Jackhammer Davidson like RamAirZ is - but go from something that has a straight six to the VX and you will feel it!

89Vette I am fully aware that the crankshaft is weighted but those weights are not for your comfort, they are there to offset the mass of the pistons and rods and thus keep the crankshaft from self destructing - as is the harmonic damper at the end of the crankshaft. Even though the rotating parts within the engine are balanced, the engine itself - as a unit - can and will vibrate however - and some configurations are worse than others. The L6 and V12 are naturally smooth whereas the V6 is not - a result of having three cylinders per side. The only way to cancel the rocking couple of our V6 is with a counter-rotating balance shaft like what is seen in the picture of the GM 4.3. That balance shaft isn't "needed" to prevent the engine from self destructing - it's there for human comfort - to keep the engine from rocking along its length which is felt as vibration. But that adds extra weight/complexity/rotating mass. The designers of the GM 4.3 thought it was worth the extra weight, etc to include a balance shaft and they were probably right since it's a 90 degree engine. A lot of sixty degree V-6s don't have one since the more shallow V means less vibration. They make do with some squishy motor mounts that turn the rocking motion into heat (and break down eventually because of it) and call it good. I guess the Isuzu engineers thought our 75 degree V6 was close enough to 60 to go "au natural" as well and for most people it is.

Bottom line is if you're looking for something that's silky smooth when revved in the driveway - from idle to redline - don't go for a V6 with no balance shaft - you need a BMW L6 or better yet a Jag V12. This "cabin resonance" at highway speeds you speak of is NOT caused by the normal amount of engine vibration of the VX though. The normal VX engine vibration isn't noticeable when driving.

Did the vehicle in question have a roof rack?

Thanks for the reply! This is the kind of feedback I was looking for. It's thought-filled, detailed, and makes a genuine attempt to explain the dynamics of what's happening.

[No offense, but blind acceptance of a vehicle as a halo project is short-sided. The motor's character was my question -- which was used in other non-concept projects. As such, it shouldn't be doled off as half-hearted by Isuzu.]

I agree the motor seems to have some inherent vibration. Surprisingly, the only other motors I've noticed this on (in my 37yrs of driving) is a 2-cylinder motorcycle. OTOH, this is the first V6 I've driven with the intent of ownership. (My father has a V6 Buick which is also smooth -- but that's in the cabin. Maybe it has a balance shaft too?)

To determine whats "normal", I placed my hand directly ON the 3.5L motor. I repeated this for another 3.5L motor from 2001. With the isolation of the mounts removed from my perception, I could feel the similarity in cylinder pulses, variances at rpm, and stubbling during decel. With everything I felt, the character was the same. The only difference was that the strength of vibrations were approximately double in the newer 76k mile engine. (The other had 120k on the clock.)

Though it's possible that the lower mileage vehicle also transmits more vibration thru the mounts (to the vehicle), it seems more like the character of the motor itself.

Obviously, part/weight tolerances from the factory could make a difference, or these a specific action that could smooth the vehicle. Since air, fuel, and compression (plus timing) all play a part in the strength of an indidual cylinder pulse, this particular motor might see improvement thru: 1) spark plug inspection/replacement, 2) injector service/replacement, 3) coil replacement?, or 4) cylinder compression check. But I haven't gone so far as to do those myself. But, the owner was KIND enough to send it to a mechanic for the "once over". I have no idea if any of these individual points were examined.

That said, the mechanic gave his thumbs up. Though he also drove it, felt the vibration, and acknowledged it, his final judgement was to categorize it as a typical 9-yr-old vehicle. My problem with that assessment is the vibration CAN be felt and acknowledged, but an explanation could only be found thru my efforts here. (Thanks btw for that!)

I can feel the vibration on the hwy around those rpms. Because it can also be heard, one logical conclusion is the vibration is being transmitted to the frame via hardened/touching motor mounts. Just as an exhaust pipe can create exponential noise when touching a car's frame/body, so could an engine.

So, it would seem like the mounts might be the culprit. OTOH, because it's vibration does feel somewhat stronger than another 6VE1 specimen, maybe there's a tune-up issue that could affect improvement.

The issue is not resolved IMO, though I've gotten alot closer to determining the issue. Because there are SO many other GOOD things about the unit in question, I'm not so eager to assume it's just the nature of the beast or that a VX just isn't the vehicle for me. That approach just doesn't provide enough investigation/thought IMO. If you can't tell, I'm more eager to make this work than to give up!

Happy Memorial Day to Everyone!!!

gp

[RamAirZ]

Well also a vibration to one person might not be one at all to another. So it's hard to say. But it's very easy for any engine to vibrate when mounts go bad, which they can/will do over time. Like you said, should be able to be corrected. I'm thinking mounts or balancer, I know the earlier 3.2 had balancer issues.

[89Vette]

Well also a vibration to one person might not be one at all to another. So it's hard to say. But it's very easy for any engine to vibrate when mounts go bad, which they can/will do over time. Like you said, should be able to be corrected. I'm thinking mounts or balancer, I know the earlier 3.2 had balancer issues.

I agree this is a DIFFICULT topic to talk about/solve -- as one man's vibration might be another's soothing back rub! LOL

The vibration I'm talking about is definitely more than you could sell a new car with (and enough so that I can comfortably classify it as an "issue"). It's also enough that it would drive me toward a solution (or sale if never corrected). It's one that creates cabin resonance at hwy speed where passengers are annoyed (mainly in the back seat). But, it's definitely coming from the motor (since it is present when parked).

After reading Jerry's response, I tend to think this is a motor mount issue. Plus, the current owner had the belt removed, checked for balancer play, and if the vibration still existed w/o the belt. It does and there's no play.

That pretty much means it's the mounts (or the motor). When/if I can confirm the mounts, I'd like to purchase the VX in question.

The downside is the same shop said they did not see a problem with the mounts either. But that's probably harder for the average mom/pop wrench-turning shop to determine. It's one of those areas where a (dealership that's seen more specimens) might better diagnose. OTOH, lots of dealers have seen very few of these!