Strange Vibrations...

[Gamikatsu]

where to start... ok as of recently... i'm gettin strange vibrations, i will try and describe it as much as possible.

accelerating under 1/2 to WOT and no vibrations, motor pulls great.
cruising, slight vibration
if i cruise at 45, and try to barely accelerate up to 50, IE keep it from downshifting, it feels like the rear wheels are hopping off the ground the vibrations are so bad. i can get it to cruise steady speeds, and not vibrate, but most of the time if i cruise at LOW rpms in any gear, it just vibrates like mad.

at the same time... if i'm in reverse and gun it... again 1/2 to WOT, then the FRONT vibrates really bad and you can gear a sort of grinding noise, and massive vibration thorugh the steering wheel... not sure if the two of these are related...

under advise of someone else, i tried power braking it semi gently... say 1500 rpms with foot on brake, NOT sp inning tire and NOT moving... zero vibrations in Drive... havn't tried it in reverse.

I have noticed the engine idles a little rougher as of lately, in drive, with or without AC... not bad but its thier...

any and all ideas/hints/suggestions would be greatly appreciated.

Thanks!!!
Edward

 

[badazzbulldog]

have you checked the motor mounts and trans mount?

 

[Gamikatsu]

appear solid... one of the things i checked, sorry. I'm starting to think that U joints and balancing issue... but it just happened literally overnight out of nowhere.

 

[badazzbulldog]

ujoints can do that one day fine the next day boom driveshaft on the ground

 

[TruckinAin4Sissies]

SES light? Actually do the 89 GMC Jimmy K5's have OBD?

 

[Gamikatsu]

NO SES light, and NO codes. already checked that. was thinking mabey a knock sensor or something like that. it does have OBD 1, the paper clip readable version. i'm putting new upgraded U joints in it, i'm thinking they are quite old, because the grease on them is caked and dried and requires a firm scraping with a screwdriver to remove any of it. i havn't pulled them to see what the needle bearings look like, but i have free labor from a 31 year ASE master mechanic, and already spoke with him. He's suspecting the joints as well.

 

[Gamikatsu]

Found the issue... Ended up being a Spark plug wire melted to the exhaust manifold... my "vibration" was a miss that was getting transferred to the rear end. Still suspecting a bad ujoint but we shall see... new joint is in the truck, just have to install it on the shaft.