I'm still having probs with my car and this vibration issue. For a while I thought the motor was just out of balance, but now I'm thinking a bit different.

Background: built forged 9:1 350ci LT1, stock 4L60E (2400 stall), stock rear. Car vibrates horribly when both in and out of gear. There's definitely a loss of power, whether this is due to the vibration or not, I don't know. I don't think that the vibration was always there, but when I first had the car I didn't know much about cars at all. I can't remember when the vibration started, but it started off very minor. I'm pretty sure I started to notice it once I swapped in a poly tranny mount for the shredded stock tranny mount. I've been totally broke for a while (about 10 mths), so I haven't had money to do anything with the car. I've been driving my DD most of the time.

Finally having enough money, I recently took it to a mechanic. He dropped the trans and started up the motor, and said that the motor hardly had any vibration at all. This totally befuddled me. He said that most of the vibration is coming from the trans. So now I have no idea what's going on. The 4L60E doesn't slip at all, and shifts nice and firm. It doesn't seem like anything's wrong with it.

I'm totally stumped. What do you guys think is wrong?

 

warped drive shaft? maybe your converter is scrap?

 

I don't think it's the driveshaft, because it vibrates even when in neutral. Could be the converter, though.

Anybody wanna donate a T-56?

 

Do you feel the vibration when you're at a dead stop?

 

Yes, when revving though. It doesn't vibrate at idle and vibrates very little until around 2000 RPM.

 

I have the same problem with my 94 z28. Mine does not start until its above 2000 rpms. It does it in park to. You cant tell driving it around town but can definatley tell on the nterstate cruising at 75mph. Im lost on what it is.

 

You can definitely tell with mine around town. It's bad. It's pretty much undrivable.

 

You haven't been getting alot of rain lately have you? Just a shot in the dark, and I'm probably wrong, but it kinda sounds like a bent connecting rod from sucking up some water in a low water crossing.

 

I never drive it in the rain, but I guess it could be a bent rod... I sure as hell hope not, though.

What would be some characteristics of a bent rod? She revs smoothly.

 

If a connecting rod were bent, the motor would have a bad vibration in the neighborhood of 2,000 RPM's.


I thought you got a vibration from revving it?

 

1. Are your wheels and tires correctly balanced? I know this is very minor but sometimes it's the small things that cause the most trouble.

2. It could very well be your driveshaft. It's very important that you check the driveshaft angle or pinion angle on the rear axle. If you have in any way modified the vehicle ride height, you may have inavertintly created a u-joint vibration problem. The solution? Simply trim the rear axle and rear transmission mount to reduce the operating angles of the u-joint.

Another source could be the bearing in a two-piece driveshaft (which I suspect you don't have). So, I won't go into it.

A driveshaft runout could also be causing your vibration issues. A runout can be checked by positioning a dial indicator near the center of the driveshaft, then rotating the shaft to determine the amount of total run-out. More than .010" of runout can cause trouble. Unbolt the rear U-joint and rotate it 180° in its yoke to see if that eliminates the run-out problem. If it does not, the shaft is bent and needs to be replaced.

And finally, your driveshaft could simply be unbalanced. To test for this, raise the rear wheels off the ground while supporting the rear axle (don’t let the axle hang otherwise it may create a driveline vibration by increasing the operating angle of the U-joints).

1. Start the engine and run the rear wheels up to speed (no more than 55 mph to be safe). When the vehicle starts to shake, note the speed.

2. Stop the engine, remove the rear wheels and reinstall the lug nuts to hold the brake drums in place. Then repeat the same test to see if the vibration is still present. If the vibration does not return, the problem is not the driveshaft but wheel and tire imbalance. If the vibration is still there, proceed to Step 3.

3. Stop the engine again, remove the brake drums and repeat the test once more. If the vibration is gone, the problem is drum imbalance. If the vibration persists, it’s the driveshaft.

Caution: Do not step on the brake pedal while the drums are off, doing so will force the pistons out of the wheel cylinders. Just shut the engine off and let the drivetrain bring itself to a stop.

Solution? Send it to a machine shop that does balancing, or use an electronic on-car balancer to balance it yourself.

3. Check your motor mounts.

4. Check your harmonic balancer.

 

i also suggested to him to change his motor mounts. the harmonic balancer was changed out for a new one when it was at the mechanic supposedly. but was considered not the problem and the old one put back on.

i hardly doubt that the vibration is from his driveshaft, it vibrates very noticeable in any gear, wether moving or stopped, in nuetral or park. The shake/vibration was first evident after we installed the poly mount in his car. We were told if it was put in backwards it might cause that, so we turned it around, helped a little, but not really. This problem has everyone of us around here stumped and same with the mechanics. if its not a transmission issue and not the motor mounts the only way to find out what the problem is would be to tear down the motor. and its something that wouldnt be worth it for him..,,

bent rod....do you really think 9-10 lbs of boost is going to bend a forged rod so easy. i mean his car is hardly driven and when it is i dont think he hardly ever has it see full boost except for the 2 times its been at the track....

just for ***** and giggles, i know the plugs and wires are new, and i know the car is still missing, check the firing order and make sure the plug wires are routed correctly and that they havent melted through on the headers again.

 

Enisguy, it sounds like it could be your torque converter.

 

There's a lot of info here and I haven't read it all but I will say I've had vibration problems before and there were two different instances. The first major vibration I had was the stock driveshaft. I swapped in a LPE 3" aluminum and that solved that. The second one was a warped axle and I had it machined by Moser. I tested that by jacking the rear wheels off the ground and running it in gear. My dad watched the wheels on the back and one was shaking vigorously. I switched wheels to see if it was the wheel itself and when the vibration occured on the same side, we decided it was the axle. At first we thought it was the clutch but ruled it out with that test. So I say you either have the torque converter problem as mentioned, a warped axle, or an out of balance D/S.

 

Hehehe

Well I will put money on the Poly trans mount. I bet I have change two or three DOZEN of those out at the shop! Customer heard it was the way to go but now the car vibrates like hell and some thing HAS to be wrong.

The transmission mount is what absorbs all those vibes. When you put in the POLY mount you have all but stopped any dampening that should happen at this point. every time the motor or rear end moves now you feel it.

Here is what I do to fix this. I order a 98- Tansmission mount. They went way up in price lately but you can pick them up from people who put in poly mounts cheap then let them buy the new one LOL! The "LS1" mount has a possitive stop to keep it from breaking. I am 60 footing in the 1.6 area right now and mine has lived since 1999. You CAN take it one stepp further and add a piece of metal across the safty tabs. this makes it stronger AND limits the travel. I did it to shorten travel.

This is a fairly cheap fix and it sound like it may be your problem. If you want to test this out find a good used stock Trans mount and swap it in for a week or so and see how it feel. Then let us Know if it helped!

 

I had the same problem rescently, and it was the trans mount. Go back to stock and you'll be fine.

 

Thanks a lot for all the info, guys. I appreciate it.

 

well, i have a poly mount in my car with no such problems, the trans feels a lot more "connected" then it did before but no vibration issues. then again, i drive a mustang....

so is that what your going to try to do jon?

 

I'm just going to start from cheapest to more expensive. I seriously doubt it's anything past the tranny since it vibrates in neutral/park the same as it does in gear. I figure I'll start off replacing the mounts, and work up from there. Maybe I'll get a second opinion from the guy next to Falls View.

 

Well what some people call "More connected" others call a PITA I was just relaying what I See alot of to help and nothing more Five of the cars we had last year were Mustangs. F-Body's tend to be worse because the rear end is connected to this are with the TQ Arm

Good Luck Finding it man!