This is a 10 Hz vibration that comes exactly from the 600 RPM idle speed, which is 10 revs per second. Looking closely at the intake manifold and PCM, I was getting about 1/4" amplitude to the vibration. Less on top of the engine where the throttle body was - the intake manifold and its mounts are a bit flexible and whiplash a bit more than the engine itself.
I changed the camshaft position sensor. Didn't help. Changed camshaft position solenoid / actuator. No go.
The vibration seemed to be resonant, so I took off the intake air tube, and watched the throttle body butterfly valve for getting into a harmonic resonance with the rotational vibration. Changed its mass by adding my finger to it gently. No change.
Read some more and many members reported success by changing the motor mounts. I mocked that idea at first, but thought a bunch more and finally agreed. Bought them from parts4chevys.com (list $96, their price about $60) since aftermarket ones didn't solve the problem for some posters on the OS, so I guessed their stiffness didn't match what GM needs to get the resonance under control. When coupled with a larger mass like the vehicle, it must just work better.
The job is not easy, and frustrated the heck out of me. I had to go buy a 1/2" U-joint because I hadn't used mine in years, and it went missing, and I needed one. The nuts are 18mm, and almost impossible to get a wrench OR a socket on. The upper needs a deep socket, and on the driver's side, the PCMbracketry is right there. Ick. The inner CV tripot joint smashes into the frame when you get all four nuts off and jack up the engine with a block of wood under the oil pan. Many folks remove the engine mount bracket from the strut tower, but that's a lot easier when you don't have large coilovers like my new ones. On the passenger side, I took 3/8" off the lower stud on the motor mount and then I could worm it into place. The old one was damaged and collapsed enough that it came out easily. On the driver's side, I couldn't even get the old one out, so I dropped the engine back down, removed the CV shaft (not a 2 minute project, you all know) then raised the engine back up and got the old one out with no sweat. New one went in just as nicely.
No vibration, and the engine now sits 3/4" higher in the frame! Passenger side was worse due to proximity to the exhaust manifold, I think. There's a heat shield on it. I'm sure my habit of smashing down hard on rocks on the frame when I had only 31.5" tires had a lot to do with collapsing the mounts. Anyway, I consider this a miracle cure to the vibrating idle problem.
Jack under oil pan
Old versus new. About 3/4" difference in height when compressed. Less on the bench.
Damaged old one.
Nice new one.
Hanging up on tripot joint
CV pulled out of diff. Engine now jacks up nicely.
