Engine/Cabin Vibration

[TripleC321]

In my 1988. When it’s in the RPM range of 900-1450ish (approximate range) there’s considerable vibration transfer to the cabin (the engine also visibly vibrates more when viewing it with the hood up. It idles at 650 RPM when in gear, and 750 when in park.
 

There’s some visual engine vibration at idle but not much at all in the cabin. 

However above 1600ish RPM, their is no vibration transfer to the cabin, and when viewing the engine it appears to have 0 visual vibration at all. If the engine is above 1600 RPM, for example 3000 RPM, if I let off the gas, as the RPM hits 1400 the vibration picks up, theirs a vibration “shudder”, then the vibrations slow back down as the engine drops to idle. 

 

I’ve replaced all 4 engine mounts and all 6 sub frame bushings. ICM/Coils replaced. No codes are being thrown. 
 

What could be the issue?

[Ronnie]

Have you replaced the "dogbone" that goes between the engine and the radiator support.? If the rubber bushing in it go bad a lot of vibration can be transferred to the chassis.

[2seater]

I don't want to muddy the discussion but has anyone noticed some similarity in the rpm bands that show up as misfires, vibrations, poor running and other observed problems? I don't know what that means, if anything. At 1500 rpm and three power pulses per revolution, it works out to 75Hz. I have no theory if there is a harmonic that manifests itself? I have a rattling heat shield on another car that only does so in a narrow rpm band, so it got me to thinking??

[TripleC321]

4 hours ago, Ronnie said:

Have you replaced the "dogbone" that goes between the engine and the radiator support.? If the rubber bushing in it go bad a lot of vibration can be transferred to the chassis.

Yes the dog bone mount was replaced

[2seater]

The subject of engine balance has come up and is related to this thread. I believe the factory LN3 balancer has a completely flat face with no openings for a puller. I know later models, some being press fit did have three bolt holes for a puller. Looking at aftermarket balancers, at least the ones with a face view, appear to have the puller access slots, no matter the year. Maybe I am off base but I am looking for a way to identify the correct balancer, if that's even possible.. 

[TripleC321]

I’ve heard the same on the balancer too. The ones with slots seem to be aftermarket and or incorrect balancer. 
 

is the balancer the culprit here?

[2seater]

12 minutes ago, TripleC321 said:

I’ve heard the same on the balancer too. The ones with slots seem to be aftermarket and or incorrect balancer. 
 

is the balancer the culprit here?

I'm Pella. Don't condemn the balancer just yet. I asked the question if anyone knows if there is a way to sort them out. The only answer I can think of at this point is to swap with a known OEM balancer and see if the vibration changes. 

[TripleC321]

7 minutes ago, 2seater said:

I'm Pella. Don't condemn the balancer just yet. I asked the question if anyone knows if there is a way to sort them out. The only answer I can think of at this point is to swap with a known OEM balancer and see if the vibration changes. 

Ah okay. Good to know this is you on the forum end. 

[2seater]

I looked at the two engines on stands, an 89 and a 90 and both have no holes in the front for puller bolts. The problem is, that doesn't necessarily mean a modern aftermarket one isn't okay. It might be as simple as consolidating part numbers? The Series 1 engine internals are very similar to the LN3 but they are not identical. I think the pistons and rods are slightly different and the crank is different in that it uses a one piece rear seal. I wouldn't think the balance factors would be wildly different, if at all, but ????

 

I was just looking through the parts book and the crankshaft and balancer are the same for all three years of the LN3. The L27 from 91 on does list a different crankshaft, balancer and flexplate/flywheel which is two piece?? That still doesn't mean much of anything as Padgett pointed out, painting a different color changed the part number.

[Ronnie]

'88-'90 had a slip fit on the balancer. Normally shouldn't need a puller to remove it. '91 has a tapered fit and needs a puller to get it off.

[DAVES89]

1 hour ago, Ronnie said:

'88-'90 had a slip fit on the balancer. Normally shouldn't need a puller to remove it. '91 has a tapered fit and needs a puller to get it off.

May be a slip fit but I've used pry bars to get it off.

[TripleC321]

How do I go about telling if they put the correct balancer on my engine?

[2seater]

The slip fit vs press is a bit of an open question for me. I remember when I purchased a new balancer for the S/C installation, I thought I would need to do some file work on the inside to get it to fit the crank snout of the 89 crank I used. It slipped right on? I think I tried the original balancer that came on the 95 donor engine and it also slipped on. I wonder if the interference fit is actually the crank snout? If I have the opportunity I am going to double check that original S/C balancer on my 1990 engine which I know has never been to the machine shop and had work done to fit pieces together.

[Ronnie]

I don't want to split hairs but a press fit (interference fit) and a taper fit are not one in the same. It might be possible to slide a slip fit (straight balancer bore with clearance for the straight crankshaft snout) onto a tapered fit crankshaft snout depending on the size of the shafts. You wouldn't think engineers would have designed the shafts so that would be possible but who knows?

[2seater]

39 minutes ago, Ronnie said:

I don't want to split hairs but a press fit (interference fit) and a taper fit are not one in the same. It might be possible to slide a slip fit (straight balancer bore with clearance for the straight crankshaft snout) onto a tapered fit crankshaft snout depending on the size of the shafts. You wouldn't think engineers would have designed the shafts so that would be possible but who knows?

Yes, you are right. I get the distinction. I have no actual idea of what sort of contact there is that requires a puller. I do know that it is important the balancer bears against the oil pump drive which is also the lower timing chain sprocket.

Dave just let me know the new spare Delco balancer he has also has the bolt access holes? I don’t know if there is an external way to know if the balancer is correct or not, but this would be the first time I heard about a balancer replacement caused a vibration

[Ronnie]

9 hours ago, 2seater said:

I have no actual idea of what sort of contact there is that requires a puller. I do know that it is important the balancer bears against the oil pump drive which is also the lower timing chain sprocket.

With a taper fit no contact with the oil pump drive would be needed. In theory, if you have a perfect taper fit on both mating surfaces, once down tightened properly, you could remove the bolt and the balancer would stay tight on the shaft and in perfect alignment. As you pull two mating parts together that are tapered the can get extremely tight depending on how much pressure is applied to pull them together.  Just a slight taper is all that is needed for that to happen.

 

I have worked on machinery where the tapered fit was so critical that we would use lapping compound on the mating surfaces to make them have perfect contact. Doing that makes them go together even tighter so the parts wouldn't move under a heavy load. We did that on the pulleys and driveshafts of a huge lathe that had two 50 horsepower motors driving them.

 

On the other hand, a slip fit like on the '88-90 balancer should slide on by hand. It depends on the hub going up against the oil pump drive as you describe. It depends on the clamping force of the bolt to keep it from moving. That is the reason that bolt is torqued so tight and hard to break lose. Maybe the reason a puller is sometimes needed to get a slip fit balance off is due to corrosion or the a burr in keyway. I don't really know. Both times I've had my balancer off it has just lid right off without a puller.

 

I still have the original balancer that was on my '88 Reatta in the attic over the garage. I will look at it today and see if it has the puller holes in it.

 

 

[2seater]

My intention to remove the balancer from my original engine didn’t work out. My good impact wrench is up north, of course, and my backup impact at home won’t budge the bolt which was of course blue loctited when installed. The one thing I could do was measure the snout diameter on the s/c crankshaft and check for taper. I found it to be 1.373” for the entire exposed length, no taper. The balancer that came off that crank does indeed have a reduced diameter area approx. 3/4” from the end. It slip fits onto the crank snout far enough to engage the key in keyway for easy lineup then it sticks on the reduced area inside the balancer. 

[Ronnie]

2 hours ago, 2seater said:

The one thing I could do was measure the snout diameter on the s/c crankshaft and check for taper. I found it to be 1.373” for the entire exposed length, no taper. The balancer that came off that crank does indeed have a reduced diameter area approx. 3/4” from the end. It slip fits onto the crank snout far enough to engage the key in keyway for easy lineup then it sticks on the reduced area inside the balancer. 

What year engine is the SC crankshaft out of and what year was the balancer intended to be used on?

 

Generally speaking, mixing a straight shaft with a tapered bore (or vice-versa) would not be advised even if they fit together and tighten up as you describe. Although tight on the shaft, the tapered bore could allow a slight rocking motion as it tightens up on the shaft causing the pulley to run out of alignment. And it might cause a vibration. When I enlarged the photo of your balancer, it appears the shaft was only making contact in the last 2/3 of the bore. That would make sense if you were putting a straight shaft into a tapered bore. Do you think that was the case? If the hub of the balancer bottoms out against the oil pump drive then alignment might not be an issue. I say "Generally speaking" because who knows... the GM engineers might have intended it to be that way.

 

I looked all through the attic for my old balancer. I couldn't find it. I must have thrown it away during one of my attic cleanouts.

[2seater]

The crankshaft is still in the 1995 S/C engine block and the balancer is the one it came with. In other words, it was previously installed on that crankshaft. I didn't get my snap gauges out to see if that tight area was tapered or not. The transition can be felt easily so I made the assumption that it was the reason for the puller to remove. Way back when, F14Crazy did a S/C swap on his existing engine and I remember him making the comment that he needed to remove a ridge as he called it. 

At this point, I do not know of any way to externally identify a balancer but if it is a slip fit, it must be for an LN3? If I was a tech that could run simulations for the various variables that would indicate a point where a vibration manifests itself into visible movement (from the original post), but I don't have those answers.

[Ronnie]

I would connect a timing light to a plug wire and shine it on the balancer. Sometimes doing that will make moving parts on an engine look like they are moving in slow motion. If it is wobbling you might be able to detect it by doing that.