Flutter/Vibration idle to 2400 rpm

[Bodo]

Howdy gang

Here's the patient. 1996 Jeep XJ 4.6L stroker: 258 rods/crank, Comp 68-231-4, SBC valves, 25# injectors.

I have ~12k miles on the engine and it's been great up to now. The engine starts and idles fine. When the RPM's climb above idle the engine sounds like it's missing, but it's more of a flutter. Along with the flutter is some serious vibration. This condition only occurs from idle-2400 RPM. Once it climbs above 2400 it's as smooth as glass. It's like someone is flipping a switch; it's that dramatic of a change.

Oil Pressure is good. I did a complete tune up and it didn't solve the issue (plugs, cap, rotor, wires and coil). No additional valve noise. It's a little worse when it's cold and sometimes it will not happen at all. Very confused.

Any ideas?

[Freakin A Scrib]

could it be your vibration damper?

[Bodo]

Figured out what it was.

When I built the motor, I overlooked the fact that the crank was out of a standard transmission truck. My jeep is an AW4 automatic. I didn't remove the pilot bushing/bearing when i installed the engine. So, there was a lot of pressure on the flexplate leading to its failure.

I thought it was a torque converter/transmission issue so I had one that I had laying around rebuilt. I noticed the issue this time reinstalling the tranny.

Everything seems OK now. I don't know if the thrust bearing was damaged. Time will tell. Oil pressure is good and no noises.

So the take away lesson is: Check the damned end of the crank!

[SilverXJ]

I don't think the thrust bearing was damaged as the stress was on the flex plate and the torque converter. Easy enough to check crank end play with the engine assembled though.