I think I've tracked down my cyclic vibration

[SUX2BU99]

It seems to me like it's coming from my motor! Coming home last night I tried some various methods to determine if it was driveline related or motor. I took it up to 50 and then backed off, both in gear and out of gear. When I popped it out of gear, I could feel the vibration for a second as the motor revved down and then it was smooth. Popped it back in gear and vibe came back as the RPM's came back up. This has always been in 3rd. I tried it in 2nd. Well, the vibe was there at the same RPM but different vehicle speed of course. Tried it in 1st, yep there too again at the same RPM (between 2000-2500).

So what the heck could this be? It's there too if I rev it to 2000 or so in Neutral but not as much I guess since it's not under load like when it's in gear. Harmonic balancer has slipped? Torque converter? Any other methods I could try to narrow it down?

[zieg85]

Check your clutch fan, I drove a wrecker that did that same thing.

[SUX2BU99]

Thanks. I see that it just bolts on to the nose of the water pump. My Haynes manual says I should remove the fan belt if I take the fan and clutch off. Why would I need to take the belt off? Shouldn't I be able to remove the shroud and and unbolt the fan clutch? Any tips?

[zieg85]

You should be able to carefully remove the fan without loosening the belt or taking off the shroud. 

[SUX2BU99]

Turns out I have a flex-fan so I removed the 4 bolts from the pulley snout and out it came. Nothing seemed loose on it but I'll try it out now without the fan to see if the vibe is there.

I looked at what I could see of my balancer from up top and the rubber ring looked okay for the 1/4 or so of the balancer that I could see, from the front.

[SUX2BU99]

Well turns out it's not my flex fan    The search goes on. Any other ideas out there?

[zieg85]

I guess the next logical place to look is the harmonic balancer. 

[eventhorizon66]

Have you tried rotating the torque converter relative to the flexplate?  Put a witness mark on both the flex plate and torque converter to indicate where they are currently bolted together and try the other two positions.

[SUX2BU99]

I could try that. I did have an issue earlier this year where the TC was loose and I retightened the bolts. Rotating the TC should be easy enough, eh?

[eventhorizon66]

Yep it's about as easy as it gets.  I just use a long large flathead screwdriver jammed between the flexplate and the ground to keep it from turning as I loosen/tighten bolts.

[murraygmc]

just an idea but maby the drive shaft is out of balance just my 2 cents

[SUX2BU99]

Thanks Event.

Murrarygmc, I was pretty sure that might have been it too but the fact that the vibe is RPM dependent and not speed dependent tells me it's not the driveshaft. The driveshaft spins only as fast as the vehicle is travelling. It doesn't spin according to the motor RPM since it's downstream of the transmission.

Do motors with rowdier cams than stock or near-stock vibrate more than tame profile cams? Never had a vehicle with one before this so I really don't know.

[eventhorizon66]

Do motors with rowdier cams than stock or near-stock vibrate more than tame profile cams? Never had a vehicle with one before this so I really don't know.

No, it's definitely not the size of the cam that's causing your problem.

[Donut]

I'm following this one also. 
I've also got a cyclic vibration that I thought was driveline, but today I tried it in 2nd gear and can almost duplicate it.  Not as pronounced at 45 mph as 70 mph (+/- 3200 rpm), but there.  I didn't hold it to see if it lessened as rpm went higher, road was to rough.

For me it's not really a problem as my truck rarely sees 70, just an irritation.
I do have a pr of u-joints that were going into the rear shaft (weather permitting)   While I'm under there, I'll be checking for anything obvious in the TC/flexplate area.  BTW, no clutch fan.