unidentifiable vibration

[Incognito892]

My truck sat for about a week and a half, the same time it started to get consitently cold outside (state college, pa). drove around after this slump and noticed a vibration shaking the whole cabin when i was in 4th. this vibration got progressively worse and has peaked in intensity now, about 200 miles later. the vibration is not there in 1st, all the way to the redline. shows up in 2nd after 3k rpm. is very bad in 3rd when the engine/trans. is under load. noticeable but not as bad in 4th when engine under load. is very bad in 5th when engine is under load.

changed the oil. took out the driveshaft and had it balanced, its okay. tires are okay. feels and sounds like its coming from up front, such as the transmission. family mechanic says the clutch and flywheel are okay, but it seems to be something with the transmission. perhaps a tranny mount?

1991 XLT manuel transmission. roughly 171,000 miles.

 

[calmac06]

You sure the tires didn't develop any "flat spots" on them from sitting? My brother bought a '91 GMC, yeah I know, that had been sitting, much longer than a week and a half, but he's tires still aren't right...

Just my .02

 

[Willard]

Tires usually round themselves out fairly quickly, I wouldn't think that is the case. Sounds like something in the drivetrain. Hard one to identify, maybe the engine is lugging a little. How are your plugs and wires?

 

[loneviking]

Do you happen to have a nice, warm garage you could put it in for a couple of days? Really cold weather, along with moisture, can cause parts that haven't been lubricated in a while to freeze. Another thought would be to check the big bushing up by the catalytic converter. This bushing sits right up next to the catalytic converter, which the heat makes the bushing dry out over time. Add some really cold weather, and the bushing could have cracked. That can give you a vibration as I had to have mine replaced. And you have to take the truck into a real alignment shop and have the frame clamped and the bushing removed--otherwise you will wreck the front end alignment.

 

[Anime]

loneviking is partly right, the control arm bushings might be worth looking at, but I doubt an alignment shop is needed to do the job. Energy Suspension polyurethane bushings are available and/or you can get Moog bushings from most parts stores.

If you're 100% sure the tires are fully balanced, get under the truck with a flashlight and check out your engine mounts and the tranny mount.

It would help if you found out whether the vibration increases with engine RPM, or just vehicle speed. If it only gets worse the faster you go, it's likely something in the drivetrain, and not a mechanical transmission/engine issue.

 

[Incognito892]

the frequency doesnt change with engine speed, amplitude does. so it gets stronger with faster engine speed. ill double check the tires just to be sure, but the vibration isnt there when im coasting in neutral, atleast not with the same intensity as it is when the engine is under load.

why would it be worse in 3rd and 5th and not so bad in 2nd and 4th. how do those gear ratios differ from one another? 5th isnt that different from 4th, right? so would it be more with the location of the shifter?

this is really whack, and i cant go back up to school until i get it figured out.