V6 vibration at very specific RPM

[SzoylentGreen]

Hey guys,

I just bought at 1995 Nissan D21 4x4 with VG30, and manual gearbox, and there is a strong engine vibration at 2750 RPM. Its very specific to this range, and it vibrates with or without the clutch in. At higher or lower RPMs, the engine seems fine. Power seems OK, and it seems to get around 25 MPG.

The harmonic balancer looks like its on straight, at least it doesn't look like its loose or vibrating.

There does not seem to by any discernible sound associated with the vibration, although inside the cab, in neutral at this RPM, there seems to be a rattling sound, but when listening to the engine directly, there's just a shaking or vibration of the engine.

Maybe the timing belt is stretched? This truck has 160,000 miles on it, and I have no idea if the belt has been changed, and the guy I bought it from does not remember if he changed it or not.

I am thinking about pulling the engine, as the clutch is definitely weak, and I was planning on putting a belt on at the same time. But, if there's something fundamentally wrong with the engine internals, there's not much point to all this effort.

[Desert Rat]

I'd add a full ignition tune up to your list. Could be you're missing on a cylinder at higher rpm.....

Do that now...plugs, wires, cap, rotor. Spend the $100 or so on those parts and see if it helps. Cheaper than a teardown. If you don't know the timing belt history, that needs to be a priority for sure. Service interval on the older VG30s I believe is only about 60k miles.

[SzoylentGreen]

It does have new plugs, cap and wires.

I guess I'll try pulling all the plugs to see if any injectors might be stuck open or closed.

[nalz]

When i have had engine roughness issues, and had difficulty determining whether it was a balance problem or some type of misfire, i would have someone else rev/load engine while i put my ear to the tailpipe. If i heard a sputtering type noise, i knew i had a missing exhaust pulse caused by a misfire. If i had a normal, steady, smooth, sound in the exhaust, i knew i had a balance issue (damper, clutch disc/pressure plate, maybe even a broken-off piston skirt). Also, while most bad dampers are easy to spot visually, some slipped dampers will not wobble at idle, but a timing light will show that the ignition timing is way off. One more thing: i have read where an accessory can cause a vibration, and the check for that was to remove the fanbelt(s) and re-evaluate

[SzoylentGreen]

I pulled all the plugs, all looks clean, and all look exactly the same. I also pulled the timing cover off, and the belt looks fairly new, so I think I'll leave it be for now. The timing belt also looks like its tensioned correctly according to the factory service manual, and looks reasonably clean under the timing cover.

I'm narrowing this down a bit more, its starting to sound like it might be a bearing going it somewhere towards the front of the engine. Going to pull all the fan belts off and see if it still makes the rattle sound. Its weird, only does it at almost exactly 2750 RPM. Also, there is a definite noticeable vibration at exactly this RPM from inside the cab.

[Buckster62]

Not really related but how the HECK do you get 25mpg ? I have the very same truck . I have changed everything listed above , The plugs I removed looked great ! I was told 15 mpg is great and average . Sorry to interrupt your post but sure would like some input ,

P.S. Truck seems to run great.

[Buckster62]

Not really related but how the HECK do you get 25mpg ? I have the very same truck . I have changed everything listed above , The plugs I removed looked great ! I was told 15 mpg is great and average . Sorry to interrupt your post but sure would like some input ,

P.S. Truck seems to run great.

[Desert Rat]

Motor mounts could be another possible culprit for this