vibration issue

timeless2 · Sep 13, 2005

Yes, I know this is the wrong place, but I have some questions about my friend's '86 with a 3.8L v6. I see no where evident in any Tech forums where I should go. The v6 forums here only cover 94-04. If this needs moved, please tell me where it should go, as I only posted here because I consider it my "home" tech forum.

His car is a 1986 3.8 liter, with a 3 speed auto and has about 78,000 miles. It experiences some vibration in the powertrain around 2800 to 3400 rpm, then lets off as it gets closer to 4200, then it shifts. The vibration rumbles the floor and you can feel it in the seats. The vibration is not there below 2800rpm and it runs very smoothly at every other speed.

What could it be? Driveshaft? Tranny?

forpit2000gt · Sep 13, 2005

git-it-83 said:
Yes, I know this is the wrong place, but I have some questions about my friend's '86 with a 3.8L v6. I see no where evident in any Tech forums where I should go. The v6 forums here only cover 94-04. If this needs moved, please tell me where it should go, as I only posted here because I consider it my "home" tech forum.

His car is a 1986 3.8 liter, with a 3 speed auto and has about 78,000 miles. It experiences some vibration in the powertrain around 2800 to 3400 rpm, then lets off as it gets closer to 4200, then it shifts. The vibration rumbles the floor and you can feel it in the seats. The vibration is not there below 2800rpm and it runs very smoothly at every other speed.

What could it be? Driveshaft? Tranny?

Do you know if it is speed related or just rpm? is it from 2nd to 3rd only or 1st to 2nd at that rpm.? does it do it if you shift manually?

timeless2 · Sep 13, 2005

Thank you for the reply.

forpit2000gt said:
Do you know if it is speed related or just rpm?

RPM; doesn't matter what rate of speed we are moving.

forpit2000gt said:
is it from 2nd to 3rd only or 1st to 2nd at that rpm?

It's on both shifts. I also need to mention there is a shudder/vibration when stopped at a light, while in any forward gear. When we moved the shifter to neutral, then back into gear, the lighter shudder was gone. The shudder is never evident in reverse. We also did a flush/filter/fill to see if that was the issue--didn't help.

forpit2000gt said:
does it do it if you shift manually?

Yes.

Again, thanks for any advice you can offer.

BooWFO · Sep 13, 2005

We had a similiar issue with my buddies 92 5.0l we changed DS twice ujoints 4 times went through 4 t-5's in a year. Put an AL DS in and it solved the problem.
I'd also suggest make sure tires are balanced, and tight. Double check and make sure driveline is straight seen tranny's shift which gives funky drive angles. Check pinion angle. If none of this fix the problem I'm thinking something in the rearend.

forpit2000gt · Sep 13, 2005

did you drain the tq converter as well as the pan? Should have needed 8 or 9 qts of fluid if so. If you did that, take out some fluid and put in a quart of Trans X or Lucas trans conditioner. It sounds like from what you say, it is in the trans itself. the u-joints will not make it vibrate while sitting still and going from nuetral to drive.Neither will the rearend. Also, check the trans. mount. That is a C-4 ???

timeless2 · Sep 13, 2005

forpit2000gt said:
did you drain the tq converter as well as the pan?

Unfortunately, no, we didn't.

forpit2000gt said:
If you did that, take out some fluid and put in a quart of Trans X or Lucas trans conditioner.

We didn't try a conditioner, but did refill it with a Redline synthetic fluid.

forpit2000gt said:
It sounds like from what you say, it is in the trans itself. the u-joints will not make it vibrate while sitting still and going from nuetral to drive.Neither will the rearend. Also, check the trans. mount.

We first thought driveshaft, but then, why would it have done it at a stop? We then figured tranny, and decided to start with a flush-n-fill.
Mounts are good, as we recently pulled everything for a rear main replacement, and checked while we were there.

forpit2000gt said:
That is a C-4 ???

Dunno!?? I don't know a lot about 4-eyes. I do have his VIN, if someone could decode it: 1fabp2731gf139003 (those are zeroes, by the way ).

forpit2000gt · Sep 13, 2005

Without seeing the car in person, I would say the transmission is on its way out. I would never put anything but regular Cheap, ( I like Havoline) type F trans fluid in a trans that old. I would drain again, drain the tq converter, install another new filter( did a spring and a ball come out when you unbolted the filter?)Put in one container of transmission conditioner( take your choice, I like the two I mentioned, Lube Guard makes good stuff too) fill back up and drive it 10-20 miles and see what happens.
I had something similar and it went another 30k.
If you foot brake it and give it some gas, does it get better or worse sitting still?

I'm looking to see if that is a C4, I think so. I think that was the last year.

timeless2 · Sep 13, 2005

forpit2000gt said:
did a spring and a ball come out when you unbolted the filter?

My memory fails me here. I don't remember anything coming out except the filter itself.

forpit2000gt said:
If you foot brake it and give it some gas, does it get better or worse sitting still?

If you foot brake it (power brake) the shudder gets worse toward 1100 rpm, then dissipates (only to return at the upper rpms > 2800). It idles at about 650-700 rpm.

I know, without driving it, it's hard. We'll try another fluid flush and get the converter this time. Any tips for how best to get all of it out?

forpit2000gt · Sep 13, 2005

git-it-83 said:
My memory fails me here. I don't remember anything coming out except the filter itself.

If you foot brake it (power brake) the shudder gets worse toward 1100 rpm, then dissipates (only to return at the upper rpms > 2800). It idles at about 650-700 rpm.

I know, without driving it, it's hard. We'll try another fluid flush and get the converter this time. Any tips for how best to get all of it out?

drop the pan and drain like normal. disconnect the battery if you know how to turn the engime over by hand or bump the engine until you can access the drain plug in the converter. You have to remove the dust cover to see this. It will probably be hard to break loose. Sometimes it takes a few seconds for the fluid to start coming out. let it drain until it starts to trickle. reinstall plug, ususally put thread sealant on the plug. It takes 8 or 9 quarts if I remember correctly. I wouldn't spend the money on the high priced fluids, Type F, pick your brand. put in 4- 5 quarts and crank car, let run for a minute and with foot on the brake, go to reverse and drive then back to park and turn off. put in a quart at a time until it comes up on the stick. before filling all the way, crank engine and let it come to temp, run through the gears , forward and reverse, then check with the car in park and top off on the stick. drive a few miles and check again.
Again , I would put some type of trans conditioner in there. Worst case , you need a new trans.

mikesmith66 · Sep 13, 2005

Just recently went through this vibration fiasco with my 87 GT. Turned out to be the water pump. Vibration was there in/out of gear whenever the rpm's got above 1500. May or may not save you some time/money, but pop the front belt off and start the car and see what happens. My pump bearing was so bad I could move the pulley flange 1/4" back and forth. May be worth a try. I could feel it vibrating in the seat of the car and the shifter. Swapped it out, run's perfect now.
Mike

timeless2 · Sep 13, 2005

mikesmith66 said:
Just recently went through this vibration fiasco with my 87 GT. Turned out to be the water pump. Vibration was there in/out of gear whenever the rpm's got above 1500. May or may not save you some time/money, but pop the front belt off and start the car and see what happens. My pump bearing was so bad I could move the pulley flange 1/4" back and forth. May be worth a try. I could feel it vibrating in the seat of the car and the shifter. Swapped it out, run's perfect now.
Mike

Yeah, that is a good, free test to check. I'll tell him to do that one. A water pump can really cause that much vibration? I am not certain it's going to be what's wrong in my case, as it's gear/engine speed sensitive. Did your vibration only happen at certain engine speeds?

mikesmith66 · Sep 13, 2005

git-it-83 said:
Yeah, that is a good, free test to check. I'll tell him to do that one. A water pump can really cause that much vibration? I am not certain it's going to be what's wrong in my case, as it's gear/engine speed sensitive. Did your vibration only happen at certain engine speeds?

I couldn't feel it at idle, only when the rpm's hit around 1500-2000. It did get worse at higher engine speed. What gave it away for me was that it would do it while the tranny was in neutral. I'm not going to discount anything anybody has said in this thread, but I've junked 4 auto's in my life, and my buddy has trashed 2 T-5's and 2 tremecs, and we never had one vibrate when they went bad. The auto's always slipped, whined etc. before they dumped out on me. The sticks, well, they just stopped period. What does the fluid look like? Does it smell/look burnt? Does it shift ok? Check the converter/flexplate bolts also. I've had those come lose and drive me nuts.

timeless2 · Sep 13, 2005

mikesmith66 said:
What does the fluid look like? Does it smell/look burnt? Does it shift ok? Check the converter/flexplate bolts also. I've had those come lose and drive me nuts.

Well the fluid we changed out was not smelly, just the normal reddish brown--not awful, but not great, either. It didn't smell burnt at all and was at the proper level before we drained it for the flush/fill.

It does shift OK--in fact the shifts are quick and smooth--if that is not oxymoronic?

We didn't check the converter's bolts directly--that might be another thing to check out. Thanks!

forpit2000gt · Sep 13, 2005

thats deff . something to look at. I have even seen harmonic balancers cause similar problems. You did say that in park and nuetral , there wasn't a vibration????

timeless2 · Sep 13, 2005

forpit2000gt said:
thats deff . something to look at. I have even seen harmonic balancers cause similar problems. You did say that in park and nuetral , there wasn't a vibration????

No vibration in park or neutral at any reasonable rpm (less than 4200).

Think I should check the crank pulley just to be safe?

forpit2000gt · Sep 13, 2005

check that out. I still believe it is from the flexplate back.

timeless2 · Sep 15, 2005

OK.
We did another flush/fill and got the converter this time. Conditioner was added and we took a test drive for 35 miles. Got back to town and the shudder/vibration was STILL there.

Last night, we got underneath and checked out crank pulley. We did a couple of slow rotation bursts so we could see all sides of the pulley and balancer with a flashlight.

As luck would have it the balancer weight has a crack in it and a small dime sized piece was missing. Wow! It's amazing out how that caused such issues. Guess it's time for him to hit up the Parts library and get another one to see if that's it.

Sorry for long windedness, but I thought those of you that were interested might care to know the outcome.

forpit2000gt · Sep 15, 2005

if there is a piece missing, it is a problem and should be replaced. That probably is the problem if nothing else changed in the trans.

timeless2 · Sep 15, 2005

forpit2000gt said:
if there is a piece missing, it is a problem and should be replaced. That probably is the problem if nothing else changed in the trans.

Yup, he's going to check salvage yards first--but won't likely find much, as the 3.8liter in the Foxes was rare indeed. I think the car's from Canada, myself....

forpit2000gt · Sep 15, 2005

does that have a serpentine belt? I wonder which years are the same. I may have the part you need.

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