Engine Vibration Mystery - HELP!

[lcoutback]

This is a long post, but I've been chasing a vibration issue in my '89 4.0 for over a year.  It has 245k and otherwise runs great.  I don't drive it often, but in the last year I've had to do a lot of work on my commuter Chevy so the MJ has been my back up for a 27 mile drive to work.

 

This vibration is most pronounced while on the road under load, but I can also detect it in the driveway, with the transmission in park and stationary.  Some days it's much more pronounced than others, but definitely still there.  Even in the driveway it's 'almost' a 'seat of the pants' vibration from roughly 1400 RPM and up, but I feel it in the steering wheel as well.  My step-son is a mechanic and can feel the same vibration, so I'm not completely nuts. (or he is humoring me... LOL)  This has caused a BUNCH of mixed messages to my old brain before getting to the point of separating engine vs drivetrain.

 

This MJ started life as a two wheel drive that I might have driven 50 miles after I bought it.  When I converted it to 4x4 I used a donor AW4/NP231 from a wrecked Cherokee (rear end collision totaled it), Ford 8.8 rear end and D30 up front.  I re-geared front and rear to 4:11 and installed a SYE.   Spring over axle rear net roughly 6" lift, which is matched up front with 3.5" coils and 2" spacer with adjustable upper and lower control arms and adjustable pan-hard bar.  Complete rebuild of both differentials (bearings, seals, carrier, ring gear, shims, rotors, ball joints, u-joints), full rebuild of both drivelines (slip yoke, u-joints) that have been balanced at two different shops.  New tires that have been rotated and re-balanced at two different shops.  I've driven with just the front, then just the rear driveshaft multiple times.  I've run it up to speed then put both transmission and transfer case in neutral.  I've put the whole truck up on jack stands and run it in gear and out of gear at various RPM. (with some scary results during one test period that I'm sure NOBODY else has every done...LMAO)

 

Up front I've replaced the water pump, harmonic balancer (the old one all but came out the front of the truck during an early test drive of the new 4x4 in my driveway) serpentine belt, injectors (upgrade to the Volvo 245) and indexed the distributor.  Should go without saying, but I've also done a full tune-up with new points, plugs and plug wires.  I've been under the hood late at night with the engine running to see if I had stray spark from a plug or coil wire.  I've spent a stupid amount of time staring at the front of the engine while it's running at various RPM to see if I have a pulley, idler or other rotating item out of round or wobbly.  I've probably touched every bolt and nut on the darn engine ten times each to verify nothing is loose.  I've tested for a vacuum leak using starter fluid while the engine is running on every surface I can reach that has a mating surface or connection that could fail.  I've cleaned the throttle body and replaced the TPS.  Recently I also replaced the O2 sensor to pass emissions.  I now have an REM II and according both it's diagnostics and every.single.parameter listed this Renix should be happy.  Yes, I've also replaced both engine mounts and the transmission mount.  I've chased every surface I can get to with the engine off AND with the wife sitting at the helm revving at different speeds to eliminate exhaust pipe-to-frame and other physical causes of vibration that I can think of.  In a long-shot fit of 'why not' I also replaced the Torque converter since I has re-used what came with the wrecked Cherokee that was my donor.

 

It always starts and idles fine.  It revs fine.  My latest troubleshooting is now on the fuel system.  I consistently have 32 psi while the engine is running and it drops to 23 'ish when I pull the vacuum off the regulator at the rail.  Seems to hold a steady 32/23 at various RPM.  This was both before and after replacing the fuel filter.  From what I have read 39/31 psi are the optimum numbers, but my 32/23 is acceptable?  Holds pressure at the rail for at least the 15 minutes I intentionally checked after shutting off the engine.  I plugged the vacuum line and took it for a test drive with the regulator disabled (31 psi at idle) and still have the same vibration issue.  I just bought a new Bosch fuel pump and pick up screen, as well as a new Bosch fuel filter that I plan on swapping out next weekend.  

 

Yes, I've refreshed grounds and been a faithful follower of all applicable tips and tricks that our great Cruiser54 has posted.

 

I've put over 6,000 miles on this pickup since converting it to 4x4 and this darn vibration is driving me bonkers.  What am I missing?

[Ωhm]

42 minutes ago, lcoutback said:

This was both before and after replacing the fuel filter.  From what I have read 39/31 psi are the optimum numbers, but my 32/23 is acceptable?

 

No.

 

Using REM what are your ST&LT fuel trim numbers? My guess is your running LEAN or you got a faulty fuel pressure gauge.

[lcoutback]

I'll be intentional and write those down this afternoon and post.  Thank you!

 

[87MJTIM]

Fuel pressure should be 31 with vacuum and 39 w/o vacuum.  If your pressure drops when vacuum is removed, check the fuel pressure regulator or just replace it.  Also, look at the pump.  If the pump isn't putting out enough pressure, it could be too lean to run properly, especially at higher RPM.  Like Ohm said, check the fuel trims.  The will give you a lot of info.

[lcoutback]

8 hours ago, Ωhm said:

 

No.

 

Using REM what are your ST&LT fuel trim numbers? My guess is your running LEAN or you got a faulty fuel pressure gauge.

Here's the scoop.  When I first start up, the ECU is in OPEN loop.  ST= 128, LT=125.  Once the ECU switches to CLOSED Loop ST drops and hovers around 90, slowly dropping and fluctuating between 55 - 70 and jumps to 128 when I Rev the engine.  LT steady at 125.

 

Being an old 'turn the screw on the carb until it runs right' guy, I still have a lot to learn about fuel injection and electronics.  Any feedback is appreciated, as I'm very much in the learning stages in this kind of troubleshooting.

[Ωhm]

Fuel trim numbers, both long or short, do two (2) things. Add fuel or subtract fuel from the INJ PW time. 128 is the centering point for this decision. Numbers less than 128 take time OFF the INJ PW (engine running RICH, calls for less fuel), while numbers greater than 128 add time ON the INJ PW (engine running LEAN, calls for more fuel).

 

Your fuel trim numbers tell me your engine is running RICH and fuel trim is calling for less fuel. NOTE:  Your numbers are well within the range of authority for the ECU and would be considered normal.

 

Back to guessing again since you don’t seem to be running LEAN, check your fuel pressure gauge for accuracy. If your gauge checks out good, than I’m barking up the wrong tree.

[cruiser54]

Closely inspect the hose from the throttle body to MAP sensor. A slight leak will cause an over rich condition. 

[lcoutback]

On 8/9/2021 at 8:05 PM, cruiser54 said:

Closely inspect the hose from the throttle body to MAP sensor. A slight leak will cause an over rich condition. 

Thanks Cruiser.  I've checked this out as well by removing and giving a full visual inspection.  I also used starter fluid all around while running with no change.  

 

NOTE:  I recently replaced the fuel pump (Bosch) and filter (again) with no change.

 

Another member (can't remember who, so if you read this Thank You!) suggested that with the engine running to try and distinguish whether in park and at various ramp up RPM if I could 'feel' a harmonic vibration coming from the engine or from the frame.  No luck.

 

Newest thought...  Could this be an issue coming from the transmission?  I read once long ago there should be a certain amount of clearance between the torque converter and ring gear to ensure the pump is fully engaged, but I've never seen spacers or any reason to do more that bolt the two together.  While there is a slight vibration in park at higher RPM, it's much more pronounced under load.  I fully expect some noise and vibration from rotating assemblies with a lift and 33" tires, but this is a distinctive vibration at higher RPM than any rotating assembly.  This hasn't changed with a new Torque Converter installed.

 

Has anyone had vibration issues with an AW4?  They appear to be pretty bulletproof, so I don't find a great deal of troubleshooting information.  This one shift fine and holds gears fine...  

[cruiser54]

If the torque converter weren't seated, you wouldn't be driving it. 

 

[Pete M]

can you hear it at all or only feel it?  what shape are your motor mounts and trans mount?  is your exhaust by the trans mount touching anything?

[lcoutback]

On 10/18/2021 at 7:09 AM, Pete M said:

can you hear it at all or only feel it?  what shape are your motor mounts and trans mount?  is your exhaust by the trans mount touching anything?

Brand new engine and transmission mounts.  No drivetrain (or exhaust) components appear to be touching the frame or body unless they are designed to.  I've had that problem with an XJ past.  I can't 'hear' anything specific.   

 

This is fairly predictable with RPMs at +/- 2k, mostly under load.  I'm generally pretty logical in my troubleshooting, but I'm still stumped. 

[lcoutback]

It's been a minute, but after relocating to central Arizona and a new job I'm now back into troubleshooting this vibration issue.  Question:  Has anyone experienced a Harmonic balancer issue without any visible signs of rubber failure or visible signs of vibration using the ol' eye-ometer?  I would love some feedback before dumping another $75 and a couple hours to replace mine...

 

[Pete M]

9 hours ago, lcoutback said:

It's been a minute, but after relocating to central Arizona and a new job I'm now back into troubleshooting this vibration issue.  Question:  Has anyone experienced a Harmonic balancer issue without any visible signs of rubber failure or visible signs of vibration using the ol' eye-ometer?  I would love some feedback before dumping another $75 and a couple hours to replace mine...

 

 

not yet.  mine have all fallen apart.  you should probably start a new thread with that question so more eyes will see it.   or maybe alter the title?  can members do that?

[eaglescout526]

2 minutes ago, Pete M said:

can members do that?

Yes they can.

[Motorhead X]

On 8/9/2021 at 11:05 PM, cruiser54 said:

Closely inspect the hose from the throttle body to MAP sensor. A slight leak will cause an over rich condition. 

Sorry in advance for highjacking this thread. I’m chasing down a possible vacuum leak and I noticed the hose connector to the MAP is getting hardened and might not be seating as well as it should be. Many of my other lines are getting old and need to be replaced. I’ve completely blocked off the lines at the intake going to the HVAC system because the thin plastic lines are broken in places. Should I look through the yards for these parts or try making my own somehow?

 

I also installed a new MAP and the first two replacements from O’reilly’s were DOA out of the box. The third one from NAPA lasted a week. I put the old GM manufactured MAP back in this morning and at least it starts now. 

[lcoutback]

12 hours ago, Pete M said:

 

not yet.  mine have all fallen apart.  you should probably start a new thread with that question so more eyes will see it.   or maybe alter the title?  can members do that?

 

Thanks Pete...  I may just repost my original diatribe for a fresh set of eyes.  This thing is driving me bonkers!  I'm told there is a Jeep Guru about an hour north of where I live.  It pains me, but I might need to shell out few bucks for help if I can't find a new nugget from a new post... 

[lcoutback]

10 hours ago, Motorhead X said:

Sorry in advance for highjacking this thread. I’m chasing down a possible vacuum leak and I noticed the hose connector to the MAP is getting hardened and might not be seating as well as it should be. Many of my other lines are getting old and need to be replaced. I’ve completely blocked off the lines at the intake going to the HVAC system because the thin plastic lines are broken in places. Should I look through the yards for these parts or try making my own somehow?

 

I also installed a new MAP and the first two replacements from O’reilly’s were DOA out of the box. The third one from NAPA lasted a week. I put the old GM manufactured MAP back in this morning and at least it starts now. 

I had some success  buying new vacuum lines from NAPA.  Maybe it was dumb luck, but I would try a couple parts stores or Rock Auto first.  Chances are slim that +30 year old lines found at one of the yards would be any less crusty...

[eaglescout526]

23 hours ago, lcoutback said:

central Arizona

I probably am not too far from you. 

[Motorhead X]

On 11/16/2024 at 12:10 PM, lcoutback said:

It's been a minute, but after relocating to central Arizona and a new job I'm now back into troubleshooting this vibration issue.  Question:  Has anyone experienced a Harmonic balancer issue without any visible signs of rubber failure or visible signs of vibration using the ol' eye-ometer?  I would love some feedback before dumping another $75 and a couple hours to replace mine...

 

I never noticed any vibration from mine, even when the rubber was moderately deteriorating. I replaced it anyhow. 

 

This does sound like an engine issue though because you notice it in park. I had a different type of vibration that took me two years to figure out, so I REALLY understand your predicament. It ending up being two bad wheels and that totally screwed my brain. Had the wheels balanced numerous times. I even bought a new set of 4 steelies that turned out to be bogus. Finally bought some nice alloy rims and solved it, but only after replacing lots of my suspension, driveshaft, u-joint, tires, etc.. etc. I even tried different levels of AT fluid in my AW4!

 

Maybe consider trying a different set of plug wires or injectors. I have bought many new parts lately that just don’t work properly. It seems like you have covered all the bases.