bearing noise?

So I had started a thread several weeks ago about what I thought was a misfire I was trying to diagnose...

I've just about done everything I can think of to fix this and NOTHING has indicated anything is wrong other than the direct symptoms of a misfire-type feel/noise when blip the throttle, or while accelerating under load. I can feel a loss of power compared to normal and hear the noise in the exhaust. It's kinda a "thud/tap" type noise. It only happens when the engine is under load. I can rev the thing and hold it at 3K RPM and hear/feel nothing wrong, but it shows up under load.

I have changed every sensor on the engine except the knock sensor, sync, and temperature sensors. I have tested the sync in the distributor and it appears to be working properly. The misfire is there at all engine temperatures... indicating that it has nothing to do with open/closed-loop operation differences that these sensors are used as input.
I have also changed cap/rotor/wires.
All the plugs look just fine, replaced them after running some Sea Foam through it (that company must love us jeep guys). I took the jeep for a long climb up a hill in 4th (heavy load) and then shut it down... pulled over... checked the plugs and nothing out of normal.
I have used a mechanics stethoscope and isolated the noise to the #5 or #6 cylinder.
A compression test reveals even and good compression across all cylinders.

This symptom started after i swapped everything from my XJ into my MJ, except the motor in the MJ stayed put (only 100K miles). I didn't touch the engine compartment other than to install a cold air intake and I did an oil pan and rear main seal replacement.

I'm now beginning to wonder if I botched the rear main bearing cap and this is what I'm feeling.

If a main bearing goes south, do you feel it as much as hear it? There's definitly a loss of power on this jeep, oil pressure is perfect too.

Thanks

A bearing isn't going to cause loss of power. Have you tried swapping the injectors? Move the 5&6 up to 1&2.

Yep, tried that. The issue remains in the 5/6 area. I also swapped ECU with no result. I also tried swapping injector wires between 5 and 6 and it seemed to make no difference with the noise...the motor ran just fine but had a little more vibration.

Am I wrong to expect to see some evidence on the spark plugs if that cylinder is not receiving adequate spark, or fuel injection? The injector fires just fine (i listened) and resistance is within spec... they all were. Fuel pressure is good ( i squeezed the return line and it jumped up to >60 so the pump is building pressure) and the regulator has been swapped with no result.

Maybe I have a plugged fuel rail above those injectors?

It's definitely something you feel in the driver's seat... thud... thud... thud, but at the pace of the motor.

At idle it makes no difference with the clutch engagement so i figure that's OK... and so should be the tranny input.

Thanks

(this really soured the satisfaction of getting the MJ D44'd, 4.10'd, Long Arme'd, and 4x4 swap... the engine was the ONLY thing left stock on the MJ and it ran just fine before all this work!)

I'm gonna pull the pan tomorrow and take a peek under the caps.

Somtimes it takes too long for plugs to show intermittant conditions. I've also heard of using an infrared temp sensor gun and shooting the injectors to look for temperature differences. Seems like one not flowing fuel would be hotter.

I wouldn't put this into the intermittent category. The jeep does this all the time, but just while under load. This leads me to think that the rear main bearing is shot as the crank flexes under load to place more stress on the bearing and would be supported by the other main bearings while that rear piston is along for the ride while coasting... not flexing.
I spent about 4 hours online last night reading about these bearings and I'd like to think that at least 50% of the information out there is correct... and most all I found people described these exact symptoms. The one thing I'd note that is different is that this is relatively quiet noise. Other speak of ear-shattering banging noises when bearings go south. I'm hoping to catch this one in its early stages so I dont have to do a full rebuild.
After I pull the pan I'll nab one of those thermos and see what I can find out on the injectors/plugs.

Thanks for your help

I feel stupid...

either i forgot to torque down the rear bearing cap when i did the rear main a few weeks ago, or it was not torqued straight... and shifted.

The bolts were finger tight and my guess is that the crank was bending every time the rear two cylinders fired... creating a vibration.

I took out all the crank bearings and they look identical and very minor wear. I put them back in, replaced the timing chain set, water pump, and resealed everything up.

the jeep is so quiet now it's amazing. I've never heard one so quiet and smooth. I guess I'll see how long it lasts. All the crank surfaces, bearings, and block surfaces were smooth and no signs of heat either. Wish me luck. I'm just glad i found the problem after spending about 35 hours cumulative tracking it down.