94 GMC 4.6L vibration

I'm starting to feel a vibration in the steering wheel. It has 250K miles on it. The vibration is slow at idle in the driveway and speeds up as I pull out and drive it. I drove it @ 80MPH for 2 hours today and of course the vibration was gone at speed but it came back at idle.

I'm thinking crankshaft harmonic balancer but don't know how to test it.

Any advice appreciated.

4.6L? What engine is that? Do you mean 4.3L V6?

Connect a scan tool, examine the misfire counts for each cylinder.

Put a timing light on the engine, see if the damper ring has shifted.

You're correct. It is 4.3L.

It's OBDI so I'm not sure I'll get misfire counts.

I'll do the timing light in the morning.

Good point about the misfire counts. I screwed up on that.

I'd still connect a scan tool, verify all the sensors, and look at the short- and long-term fuel trims. Pull the plugs for inspection, verify all the usual tune-up items--cap, rotor, plug wires, etc. A shake like you describe still sounds more like misfire than out-of-balance to me.

Can a failed harmonic balancer/damper create vibration that can be felt?
My understanding is that it damps torsional vibrations across the crankshaft, removing them such that they don't appear at the flywheel end where they could be transmitted to the drive train and thus the rest of the vehicle. Is that correct?
Is there a balancing function also? If so, on all or only the V6s?

Pinger said:

Can a failed harmonic balancer/damper create vibration that can be felt?

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Absolutely. Some torsional dampers carry an offset weight intended to "balance" the engine. Depending on the construction of that weight--and the particular failure of the damper--the engine goes out of balance and shakes like a dog shiiting razor blades.

Or, the outer ring of the damper gets loose and wobbles around, making noise and causing out-of-balance issues because the ring is no longer centered around the hub. In which case the engine shakes like a dog shiiting razor blades.

Or the damper can have a fairly "mild" failure, the outer ring shifts but doesn't get "loose"; and nobody knows about it until the engine can't be timed with a timing light. Poor power, poor fuel economy, and the thing is only caught when someone finds true TDC and compares it to the timing marks.

Pinger said:

My understanding is that it damps torsional vibrations across the crankshaft, removing them such that they don't appear at the flywheel end where they could be transmitted to the drive train and thus the rest of the vehicle. Is that correct?

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Sort of. Removing torsional vibration is primarily of benefit to the crankshaft and bearings. Reduces cracking of castings, and premature failure of bearings.

Any number of engines have no "damper" at all--including all forged-crank 283 at least up to '64. The cast-crank 283s got real-live dampers, the forged crank units got solid hubs. Seems to me that Cadillac had solid hubs rather than "dampers" also...but I'm not totally sure about it.

Pinger said:

Is there a balancing function also? If so, on all or only the V6s?

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Depends on the engine. Far as I know, with regard to the GMT400 engines, the 7.4L/454 uses offset weight in the damper. 305 and 350s do not. And I don't remember about the V-6.

Thanks Schurkey. On another forum there was someone asking if a failed harmonic balancer/damper could be the source of vibration - now I know it can be - especially as his was on a V6 (Mercedes diesel).
V6s need additional balancing and you've reminded me that Alfa Romeo's V6 has a balanced front pulley - and flywheel IIRC.

Schurkey said:

Seems to me that Cadillac had solid hubs rather than "dampers" also...but I'm not totally sure about it.

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Yup. My 472 and 500 have solid hubs that the pulley bolts to. No rubber ring anywhere.

Erik the Awful said:

Yup. My 472 and 500 have solid hubs that the pulley bolts to. No rubber ring anywhere.

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Which tells us a lot about Cadillac cost-cutting; and why a Caddy cost only $200 more to make than a Caprice.

Failure of the balancer in an LMM Duramax will result in a two-piece crank mod.