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

[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