Low RPM vibration that fades in and out at a constant pace

[rustyshackleford]

I seem to have a vibration that fades in and out at a constant pace. It's in a wave type format where if I hold the RPM steady at say 2000 RPM in Park, Neutral, or in gear, the vibration will act like this:

• Start of vibration cycle -- Smooth
  
• Increases into a vibration
• Reaches the peak of the vibration
  
• Fades out until smooth again
  
• Repeats the cycle
  

This seems to take about 2 seconds for the full cycle to complete and it happens in any gear at any speed or parked in park or neutral at any speed. It seems to be most pronounced when in gear at speeds under 20mph and again around 40-50mph and also when off the throttle, but it is most easily replicated when parked since I can easily control for road vibration and maintain a constant RPM rate. It occurs between approximately 1100-2600 rpms and then seems to be so subtle it is not easily discernable. I tried to take a video of the vibration in the mirrors, but it didn't show the vibration (it's relatively light at its peak). Although, now that I'm typing this I'm thinking about Jurassic Park, and I can probably put a glass of water in the cupholder and show the vibration in that.

Timeline of events:

• Alternator went out when driving vehicle home after purchase
• A few days after replacing the alternator the belt began to squeak, and AC issues started to appear
• Replaced the accessory drive belt and assumed the remanufactured alternator had a bad bearing
• Checked the pulleys while replacing the belt and found the AC compressor pulley bearing is shot and needs to be replaced (possibly also clutch, not sure how to check that)
• I did not check the crankshaft pulley (but I will) as I was only spinning them to check for bearing noise and didn't realize I should have wobbled the crankshaft pulley to see if it had play
• Checked the air gap on the AC compressor clutch and it appears to be just above .052" which I think is at the far end of acceptable tolerance
• At the same time as replacing the belt I replaced the gas cap (broke when I added gas the first time haha), fuel filter, air filter, PCV valve, spark plugs, and cleaned MAF and throttle body (didn't know about cleaning IAC Valve/EGR Valve until afterward so I will go clean those this week, maybe tonight)
• Plug wires looked newer, and plugs seemed fine other than the bit of oil on a couple of them, tested resistance on coil packs with a multimeter, and it came back 5.87, 5.97, 5.91, 6.11 but again, plugs look fine and no misfire engine codes.
• Noticed cam covers are leaking as well as some oil on plugs so the valve cover gaskets will need to be replaced (I don't think that would have anything to do with a vibration unless causing a misfire, just noting this for reference) Also, regarding these leaks it looks like the previous owner was using 5w-20 and I see the cap/manual calls for 10w-30 so I'm going to switch to 10w-30 high mileage + Lucas oil stabilizer after the next paycheck to see if that helps with the oil leak.
• Also replaced the front sway bar end links and cleaned mating surface of rotor to wheel of rust ridges and that seemed to kill any road vibration I may have had.
• Vibration in engine still remained as well as whine from accessory drive, watched pulleys again while idling and finally noticed from another angle that the harmonic balancer/crankshaft pulley has a slight wobble and smooths out when increasing RPM by moving throttle cable manually (didn't have someone to help so that was my improvisation to look at it)

This leads me to believe it's the harmonic balancer (unless a slight wobble at lower RPM is normal which I don't think it is) and since the AC compressor pulley is next in line that is probably taking the vibration from the balancer and killed the bearings/about to kill the clutch. The only other thing I could think of would be a warped/cracked flexplate but I don't know enough about that to troubleshoot it and I really don't want to diagnose that because if I have to pull the engine or transmission 1 month into ownership that will end my Aurora experience quickly lol.

I also took a video of the harmonic balancer, but it doesn't show the wobble (I cracked the screen on my phone somehow when replacing the fuel filter and had to go back to an old junker phone with a terrible quality camera).

For reference, prior to performing any of this maintenance I cleared the stored codes from the computer and no new codes have shown back up. No check engine lights displayed after replacing the alternator.

To sum it up, my question would be: Is the harmonic balancer the most likely culprit? If not, or if more info is needed what are my next steps?

My thoughts are to run the AC bypass belt (can't quite afford to replace the AC compressor clutch/bearing and we're heading into winter time, so that's gonna be a springtime ordeal along with a vac/recharge probably) although I've seen both the 740K6 and 745K6 recommended here for the Classic, so I don't know which is correct although it's only a 1/2" difference I don't want to mess up the belt tensioner. So, run the AC bypass belt and replace the harmonic balancer and see if that clears it up. If I do that, I can't seem to find the correct torque specs for the bolt on the harmonic balancer and have seen many numbers quoted for the Northstars online. I'd like to know what the FSM states the correct torque is for it for a Classic (1998 specifically if it makes a difference).

[rustyshackleford]

Update:
The issues of the vibration being most pronounced off throttle and at certain speeds have been cleared up by replacing a cracked PCV elbow and the throttle position sensor.

I believe the slight vibration that fades in and out is related to the harmonic balancer and the slight wobble I see on it. It is noticeable now through the entire RPM range, however, at lower RPMS (1100-2600ish) it is most noticeable as it speeds up so much as it gets higher in RPM that it seems to almost blend together. Planning on replacing the harmonic balancer and doing the AC bypass belt possibly this week. I'll be working from home Wednesday until next Tuesday so if I run into an issue it will give me a few days to come up with a solution to get it fixed.

[chokung]

What year is your 'Rora?

Please keep updated!

[rustyshackleford]

<I took a few pics (not a lot) and will edit this post to upload those after I finish>


I'm working on it today, taking a break right now. I have the new one installed, but not fully torqued. It is spinning after torquing it the 37 ft. lbs so I still have to torque it another 120*. I guess I'll have to lock the flywheel for that.

There isn't much information online that I could find about replacing the balancer on the 4.0L Northstar engine. I saw a video on YouTube for replacing the pulley on the 4.6L, and they had to drop the subframe to lower the engine. I tried to pull the balancer without lowering the engine, but I was just a few millimeters away on clearance. This was my fear in this project as this is unchartered territory for me, but the only way to gain confidence is to do your research and get in there and figure it out.

Before I did that, I wanted to make sure I could get the crankshaft bolt off. I tried an electric impact wrench, but that didn't do anything either. Then I went to the old method of jamming a breaker bar in there and bump starting the engine with the electrical connections removed from the ignition coil assembly. After a few tries and repositionings, it finally worked. I used a 3/4" socket 1/2" drive on my huge 1/2" breaker bar and got it jammed in place really good against the subframe and a jack stand and that took it off in one try. The other 2 attempts threw the socket and breaker bar across the garage so be careful!

I removed the bolts to the subframe (one in the front, 2 in the rear all 3 were 18mm). They didn't want to budge, so I tried my electric impact wrench. It took the front one off, but the back 2 required my 1/2" drive breaker bar. I would just use the breaker bar next time as that was easier than the impact gun. Then I lowered the subframe AND..... the engine didn't lower!

I figured it was probably due to the different engine mounts on the Aurora and Cadillacs. So I loosened the 4 bolts on the torque axis mount (TAM) and then jacked the engine up (I used the instructions from the forum post about replacing the belt tensioner) and removed the body mount bolt easily. Then I lowered the engine, and we were in business.

This was my first time ever using a puller, but after watching some YouTube videos, it was pretty easy. I had the pulley off in a few minutes and took a look at it. It actually looks ok, but I don't really know how to tell a good one from a bad one if it isn't falling apart. Maybe this will just give me some experience in pulling a balancer, and the problem will still be there. I guess I'll find out, and at the very least, I'll have a new balancer on there.

I put the new one on after cleaning all surfaces and lubing everything with fresh engine oil. Nearly all information online about these pulleys is related to removing them, not a lot of info about installing them. At this point, I realized I'm not sure how to tell when I have it installed all the way, so I decided to base it on being in line with the belt idler pulley. When I basically got in line with the idler pulley, I was pretty much at the edge of my strength to keep turning the pulley installer bolt anyways. I took it off and noticed there was still a minor overhang of the pulley over the edge of the crankshaft sleeve it is installed onto. I re-adjusted the installer and pushed it on a few millimeters further and all the way to the edge of my strength. I hope that's far enough <pics will be uploaded> but hopefully, someone can tell me if it is OK or not.

Then I torqued the bolt to 37 ft. lbs and went to torque it the additional 120 degrees but it just wants to spin. FYI, there are 3 evenly spaced "arms" on the balancer, and it is a circle, 360 degrees, so if you don't have a torque angle gauge you can go from one of the "arms" to the next, and that is 120 degrees.

Then I came back up to my condo (I live on the third floor but have a ground floor garage) and grabbed a couple of beers and typed this up. Gonna head back down in a few and put everything back together. I'm probably just going to leave it torqued at 37 ft. lbs until I start it up and make sure everything's OK. I really don't feel like going through the procedure to lock the flywheel today and then torque it the additional 120* lol. I'll do that within the next week I guess because who knows what will surface next.