Weird harmonic "rumble" issue has me stumped...

I've been trying to narrow down a weird harmonic "rumbling" issue that I've been fighting for a while now. We originally thought it might be the transmission (built AW4) but recently started pointing fingers at the engine since it will produce the same symptom just sitting in park.

When the motor is reved to 2-3k and held steady, the engine produces a harmonic rumble that is felt in whole chassis. In other words it's smooth for 2-3 seconds, rumbles for a second then smooths out again. Overall power seems to be down as well.

Of course I don't have a single code which is making this a nightmare to diagnose.

Background:

99 4.0L

- The head has been replaced with the newer late model casting (beefier casting). I reused all of the rockers, push rods and lifters as they all were in great shape. Everything was reinstalled in the same location. I did not check lifter preload however, but after reading more it sounds like I should? I do have a slight lifter tick coming from either #5 or #6 but it goes away once warmed up. I have maybe 6k miles on the head.

- New timing chain and harmonic balancer has been installed 10K ago.

- Flex plate bolts are tight, and has been checked for cracks.

- Comp test shows 150 across the board. I do still need to do a leak down test however.

- I have poly engine and trans mounts. They are tight and have never been an issue before.

- Ignition and fuel system have been thoroughly checked.

Any insight would be helpful!

TIA,
Bryson

Boostwerks.com said:

- The head has been replaced with the newer late model casting (beefier casting). I reused all of the rockers, push rods and lifters as they all were in great shape. Everything was reinstalled in the same location. I did not check lifter preload however, but after reading more it sounds like I should?

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Your preload would have to be off a lot to cause what you are experiencing.

- New timing chain and harmonic balancer has been installed 10K ago.

Click to expand...

Did you have the balance on the balancer checked before install? Some of those aftermarket balancers are off quite a bit.

- I have poly engine and trans mounts. They are tight and have never been an issue before.

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Could their age be a contributing factor for extra vibrations?

does your AFR gauge show anything

How about the motor mounts? A visual inspection may not reveal a bad one, so you might want to take them out to inspect, or replace.
I once had a knock that I thought was the engine, only to find that it was a bad motor mount.

Talyn said:

Your preload would have to be off a lot to cause what you are experiencing.
Did you have the balance on the balancer checked before install? Some of those aftermarket balancers are off quite a bit.
Could their age be a contributing factor for extra vibrations?

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I didn't check the balancer, but it's been on for a while now and hasn't been an issue.

The engine mounts aren't that old, and visually look fine. I have little to no movement of the motor, even under load.

does your AFR gauge show anything

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AFR is steady and plugs look good. Injector spray pattern is good, and cap/rotor/wires have been changed as well with no change.

I'm going to try and dig into it more this weekend.