80 series vibration, any ideas?

[gr8fulabe]

Hey Guys,
So two days ago the 80 ('96 80 series) was running perfectly. Then, out of the blue, it started feeling like it was lacking power a little bit & had a weird vibration starting at around 30mph. Seems like it smoothed out at 60ish again, but don't know what could have caused it.

It idles perfectly smoothly, and the vibration disappears if I take my foot of the accelerator while moving, but comes back instantly if I step on the gas again. So I'm assuming that eliminates wheel balance or axle issues. Seems like it must be actual driveline.

Does anyone have any ideas of what I should be looking at first to try & identify?
Thanks,
Abe

 

[Hulk]

U-joint?

 

[gr8fulabe]

U-joint?

I was kind of thinking the same thing. When I climb under the truck & just turn the drive shafts, they seem tight. But I don't really know what that means. Was just searching to figure out how to test a U-joint to see if it needs to be replaced. Or is there no test & you just replace, assuming they are bad?
THanks,
abe

 

[SteveH]

Try greasing the ujoints to see if it makes a temporary difference in the vibration. If you have a CDL, you could drop one driveshaft and see how it feels using the other driveshaft.

 

[gr8fulabe]

Seems like they needed it, and seemed like power was normal & no vibration at any speed. But then when I rolled the windows up & turned on the A/C, there was a huge vibration & loud noise coming out from under the dash. So maybe the blower fan is going out... I'll have to explore that more unfortunately.

Thanks for the help though guys!

 

[gr8fulabe]

I spoke too soon. Once the truck heated up, the vibration came back. Seems like it comes on at 30mph & pretty much just stays after that.

 

[Corbet]

The lack of power comment in post has me confused. Otherwise I also say check your driveshafts first. Pull one at a time and lock the CDL to see if it goes away.

Are the symptoms in post #5 a separate issue or the same noise? I'm confused there.

Between 30-60 when you hear the noise does it change or go away if you manually change gears?

 

[LARGEONE]

The lack of power makes me wonder if the engine is stumbling a bit...like either a leak in the air intake manifold, or maybe even a fouled plug, wire, or injector?

But you would think either of these would present at idle as well.

What about a bad motor mount? Seems like the problem comes with a bit of torque, but relieves when torque is removed?

 

[Romer]

See if this helps you http://forum.ih8mud.com/threads/drive-shaft-pm-and-drive-train-troubleshooting-for-faq.80904/

 

[SteveH]

The rubber tube between the air flow sensor and the intake manifold is always a suspect for weird running issues - so be sure to check that for cracks. A buddy of mine had random power loss issues, and it was due to a crack in this tube.