91FirebirdFmla

Well, Just finished rebuilding my T5, see thread https://www.thirdgen.org/forums/tran...-5th-gear.html if interested.

I also replaced the clutch, flywheel, and pressure plate and torqued all to related fasteners to spec. I'm more than impressed with how the transmission shifts, but it seems that now at slightly over 3000 rpm, the drivetrain vibrates in a way that shakes the entire car. The vibration is detectable in neutral, but not awful. In gear, at this same point is when the heavy vibration is present. I can power through this point all vibration goes away.

Assuming that the clutch components are properly balanced, is there any mistake I could have made during installation/removal that would cause this problem?
Is there any possibility of this being transmission related?

wanta427

try getting over the vibration speed and shut the engine off . Let it coast down and see if it vibrates at the trouble speed . with the engine off the flywheel and pressure plate aren't spinning so that may eliminate them . The other things to try are declutching (in each gear) and coasting through the vibration to see if any gear is better or worse . If it's in all gears at a specific rpm it's probably flywheel/pressure plate/disc or inputshaft/bearing as they always turn at engine speed . If it's speed related (always at say 40mph ) it's most likely from the other end ( driveshaft / rear axle ,etc ) as the rear axle dictates the speed regardless of gear. The other thing to try is jack the car and put it on stands and see if there's something wierd happening . Don't go crazy running it on stands but sometimes you can find problems(sticky brake,bad axle bearing etc) that can cause a vibration .

91FirebirdFmla

It's definitely related to engine speed rather than vehicle speed. I'll have to do some more investigating tomorrow. But the vibration is present at ~3100 RPM in neutral and in any gear, but it is much worse in gear.

91FirebirdFmla

Did some more testing today. It's definitely engine related. When rolling, or stationary, in neutral or in gear, at ~3100RPM it vibrates. When in gear with the clutch engaged, the vibration is worse, I'm assuming because the engine vibration is being transmitted through the entire drivetrain under this condition.

Since I put a new flywheel on, I pulled off the inpsection cover to verify that there was a counterweight cast into the flywheel, it was. I'm kind of at a loss for what to investigate next. I have a hard time believing that the new flywheel and pressure plate are out of balance, but its not like there are that many things to troubleshoot. Any suggestions?

I guess this thread should be moved to the General engine section of the tech boards, unless people think the transmission may still be at fault.

wanta427

I have had pressure plates that were very unbalanced . The other choice is to start pulling parts to see what's bad . Pull the trans and run the motor @ 3100 where the vibe is . Then pull the pressure plate and repeat . The car ran before with no vibration , correct ? You can see if it's the flywheel by bolting on your flexplate from the auto and running @ 3100 to see if the vibe is gone ( I thought you had done the auto to man swap but could be wrong ). If no vibe with the flexplate take both to a balance shop and have the flywheel match balanced to the flex plate ( or the original flywheel ) . Take the pressure plate and get it checked too . If you don't have a balance shop all you can do is change parts til you find ones that match . Another thought is you did get the flywheel on correctly ? I have seen people bolt them on without lining up where the dowel pin goes if it's missing . The new flywheel and the old were the same with the weight in the same place ? just some thoughts .

91FirebirdFmla

I didn't do an auto to manual swap, so I have no flex plate to test, unfortunately. After thinking about this problem more, I came to a possible conclusion, if any of my statements are incorrect, please let me know.

1. If the vibration were due to an out of balance part, such as a flywheel, or pressure plate, I would expect this vibration to be present to some extent at any engine operating speed, and especially not to get better as engine speed increases beyond 3100 at which the vibration is currently the worst. At 4000 RPM, it runs like a champ. So 3100 seems to be an engine harmonic frequency.

2. Due to the change in weigh of the new clutch, flywheel, and pressure plate, the overall dynamics of the engine have changed slightly.

3. Is it possible that the current harmonic balancer on the engine, due to being run with the previous combination for 142k miles, and deteriorated after 17 years of exposure is incapable of damping this new harmonic frequency as a result of the new components.

4. As a precaution, I have already pulled the balancer off. The rubber is cracked similar in appearance to an old tire. Additionally, the surface where the front seal rides has a groove in it about .5 millimeters deep.

Could this be my problem?

wanta427

I would not think it's the balancer . My guess would be the fly wheel or pressure plate . If it vibrates at idle and worst @ a specific rpm with your foot on the clutch the only thing that is different from before is the flywheel and pressure plate And nothing in the trans is turning . I won't use rebuilt clutches unless I have no options as I've had some so out of balance it shakes everything . Out of balance is very rpm sensitive which sounds like what you've got . They make a repair sleeve for the balancer (FelPro ) thats less than $10 . The balancer was balanced with the engine so I wouldn't change it unless it's really bad or has slipped .

RBob

iTrader: (1)

According to the GM FSM a replacement flywheel needs to be balanced on the engine. There are holes along the perimeter of the flywheel for weights to be inserted. It is a trial and error balancing.

Basically, insert weight, start engine and note vibration. Move weight 180* from current location. Start engine and note vibration. If better then add more weight, if worse move weight 60* and so on. Rinse and repeat until vibration is minimized.

RBob.

91FirebirdFmla

You wouldn't happen to know if these weight locations are accessible via the inspection cover, or a good source for a kit which the proper weights to get the balance correct?

RBob

iTrader: (1)

With the cover off the holes in the flywheel will be accessible. I really don't know what is used for weights or if there are kits available.

RBob.

91FirebirdFmla

Well, as expected, the new harmonic balancer didn't fix it, but I left it on for good measure anyways. At the very least, it is more effective at damping the imbalance.

Can anyone else reading this thread comment on their experiences balancing a flywheel post-installation.

I can't seem to find mention of this practice anywhere else on the boards(maybe i'm searching for the wrong thing) or find a place to buy the appropriate weights online.

mnorton

I have a vibration problem with a new clutch installation too.

The engine is a stock 1987 LB9 5-Speed. The Flywheel was Replaced with a brand new GM part, and a stock Clutch kit was used. The car now has a bad vibration that you can feel up through the seat of your pants. The vibration gets worse at 2400 & then again 2800 RPM with the transmission in neutral. I feels even worse while driving and decelerating to a stop. Seems to smooth out at the higher RPM's.

The work is being done at a GM Dealership. Mr. Goodwrench has now removed the flywheel and pressure plate and checked them both for balance on an external jig. They check out as neutral balance, everything looks normal so we are all stumped…. They finally installed a second brand new flywheel and clutch kit but it had the same results, it still vibrates. Next they installed a new Harmonic Balancer, no improvement...

Could it be the wrong Flywheel was used? Is there more to balancing a new flywheel on this engine? Could the problem be the Transmission input shaft?

RBob

iTrader: (1)

A one-piece RMS flywheel isn't really a 'neutral' balance. The crankshaft flywheel flange weight was moved to the flywheel. It is still what is called an 'internal balance', but the flywheel isn't balanced evenly all the way around.

There is a small unbalance in the flywheel that is the portion that used to be on the crank.

This is one main difference between the old SBC 2-piece RMS engines and the newer SBC 1-piece RMS engines. Another main difference is the flywheel flange bolt circle diameter.

Anyway, there is also a long thread on the general engine board about what the balance spec is for 1-pc RMS flywheels. IIRC, there was no real answer. Which takes us back to where the flywheel needs to be balanced on the engine.

As for the tranny input shaft, the pilot bushing in the crank is important. If it is worn or incorrect then vibration will occur.

RBob.

mnorton

Ok, so now I believe that I understand this: A 1987 LB9 305 has a one piece Rear Main Seal, and has a Flywheel that has a balance weight mounted to it. This weight is located near the outside edge and is approx. 125 grams or about 4.4 ounces. A pre-1986 305 has a two piece RMS and uses a different Flywheel that is "Neutral Balanced", does not have the 125 gram weight, and will not even bolt up to a 1987 engine. All true?

So if I were told that my brand new GM brand 1987 Flywheel P/N 14088646 was tested by a local shop for balance, and it checked out to be "Neutral Balance" should I believe that statement?

Here's a link to the Flywheel that I was told was installed:

http://www.summitracing.com/search/Y...?Ns=Rank%7cAsc

Kishan

Did you end up finding out what was wrong?