powerrammaster

Just picked up a nice 1990 4x4 auto reg cab 114,000 mile truck for winter use. The idle had been raised up above normal (seat of the pants/ear comparo) and when I was driving home at every stop it would just about rattle off the mirrors. I figured no biggie, I will just lower it a tad and all will be well.

Well, it's not going away at any idle. At any speed between just about to stall- to above normal idle rpm it will and has shook the glass out of the mirrors and every plastic part is a symphony of noise. I have had upwards of ten first gens and this is NOT normal. Its not the gentle rocking at low idle, everything in/on the truck visibly buzzes. it does it in park or in drive. At cruising speeds/ rpms its ok. i cant raise the idle too high being its an auto matic, nor should i have to.

list of ideas:

motor mounts? ( though they don't look abnormal)

trans/tcase mounts?- (again appear no more beat than any other truck I've had)

timing?

air entrained fueling? (no starting, running or full load issues)

I can see evidence that the idle was turned up, but otherwise she's stock. It is absolutely miserable!

Help please. I need ideas.

gyman98

You can't really tell if transmission or motor mounts are bad just by looking at them. Use a pry bar and see how much movement you get.

Is your exhaust hitting somewhere?

taterfarm

Does it change at all when in drive or reverse?

powerrammaster

Tater- its the same, drive, reverse or park

Gyman, its just a guess that it isnt mounts as I have had enough trucks 5spd and auto in worse condition, and it seems that i can see the engine vibrate and that its translating it throughout. The exhaust looked ok. i will try the prybar.

Just saw an old post about a cracked flexplate... any ideas how to rule that out?

gyman98

It could be a flexplate, usually they make a knocking or ticking noise too but might be hard to hear with the cummins. Might be able to see through the inspection plate or starter hole

If the exhaust is hitting somewhere it will have a shiny rub mark on it

Also look at your front pulleys. Especially the dampner see if something isn't spinning true.

MARF75

After getting a tranny rebuild and a tighter converter, my truck seems to shake a bit more in D with foot on the brake.

Dunno?

82F100SWB

There are at least 2 different transmission crossmembers for these trucks, varying with frame heights. My 92 had the wrong one in it when I got it, and the back of the head touched the firewall. Buzzed and rattled like you would not believe.

Greed 93

Ya a aftermarket converter will raise some Cain.. my will push it through a stop sign in winter.

93-331-29psi

Subscribing, I've got the same issue but it is only prominent from 15-30mph. I was thinking possibly the transmission mount and ordered replacement bushings.

j_martin

Just got mine back from the transmission shop. Went in for an untraceable vibration and a slight shift flare under heavy load. (Couldn't be noticed with less than 4 ton in tow.) They found 2 valve body screws in the pan.

They told me it isn't the first Cummins they've seen shake the valve body screws out.

Mine had a whine that we figured was a slight amount of air getting into the front pump. It could be easily felt in the shift lever. If the suction leak were a bit bigger, it would shake, skip, and burn.

cougar

Getting the transmission mount set correctly makes a huge difference on the transfer of vibration. It can happen because the mounts are wore out, or because the new mount is slightly misaligned.

j_martin

I was told I was hearing and feeling a non-consequential vibration from the rear end, which they had just gone through. I pulled a double 3000 miles with it, then put it back in the shop for the slight shift flare.

Lets just say I had to give my guardian angel a vacation.

powerrammaster

I just did some investigation. It appears the vibration is generated by either the engine or auto trans. It balances out at maybe 1500 rpm but below that to almost a stall it's visible looking at any part of the engine, the frequency changes with rpm up to 1500.

I have a spare truck that is real smooth with same trans so I can swap that in but would rather not overlook something simple.

Current ideas:
Belt tensioner
Harmonic balancer
Timing
Injection issue
Internal engine issue (hope not and seems unlikely)
Torque converter
Trans itself

I think I've ruled out:
Motor mounts
Trans tcase mounts
Exhaust sys

I did some prying and flexing and I did not cause change.

Even the air intake hose to the turbo transmits the vibe to the fender liner at the air cleaner house.

I was really hoping someone had this issue and could point directly to it.

j_martin

By vibration, do you mean a shake as in a miss. Just about has to be compression or fuel. The fuel side could be injectors or delivery valves.

powerrammaster

I cracked all six injector feeds and each changed the engine sound equally, but did not make it go away. I checked the temp of each cylinder at ex. manifold and they are equal.