Vibrrrrrrration at idle.

[JSBlack]

Hi All,

1st post for me. I found you guys noted at the end of an article in the Feb '09 Diesel Power magazine..

I have a 90 1-ton 4WD Cummins. Since I got it I have dealt with a pretty significant vibration at idle (the mirrors shake, the change in the ashtray jingles, gotta shut off the engine at the drive-thru). The friend I got the truck from, said he set the idle lower "to quit breaking u-joints" while towing.

The vibration goes away with the slightest bit of throttle. I had the idle dialed-up to about where it should be, but where it smooths out is just a bit higher than it should be set to. It seems worse in drive than in park or neutral.

I have shown it to two qualified diesel mechanics, and three diesel-savvy amateurs. The mechanics say it is fine. This seems strange to me: do they not want my money? Furthermore, I don't think anyone would buy a new truck, even in 1990, that vibrated so...

The amateurs say 1) harmonic balancer problem, 2+3) Bad motor-mounts, and that a harmonic balancer problem would not just be apparent at idle. I have looked at the motor mounts and they don't seem visibly degraded to me, or missing.

I suppose it is possible that the motor mounts are so hardened with age that they aren't functioning correctly. Should I replace them as a relatively cheap way to narrow-down my problem? If so, stock rubber or fancy polyurethane (if they even make those for the old 12v Cummins??)

Anyone have any suggestions? I love this truck and want to keep it a long time. I hate to think they are missing something critical and that I am doing damage to it driving...

Thanks in advance,

-John

[RSWORDS]

turn your idle back up. these trucks will shake them selves apart at idle when the idle is to low. It should be around 750-800.

It would be worse in drive then park because the tranny will pull the engine down a few RPM.

To check the engine mounts put the truck in drive. Hold the brake real hard and hit the throttle. See how far the engine rocks.

[Farmboy]

My truck does the same thing. My truck has a fast idle cable hooked up to the peddel. I don't know if this is factory or aftermarket but if I need it to quiet down I just bump up the idle. although the cable it mainly used to warm up the truck in cold weather.

[randyswelding]

ok 2 things i got to know where in ok are ya at and what kind of welder,and are they hirin where ya work?ok so 3 things welcome to the form,i found that my 92 has a bad vibration at 750 rpm to 800 and right now im at 950 its high but smooth and nice for the winter too.

[JSBlack]

Thanks gang!

For the welcome to the forum, and all the good advice. I will check back in on this string with a followup...

-John

[RumbleFish]

does your truck have mirrors like this on it?



if so, you can use a trick i picked up from pastor bob. start your truck, make sure its in neutral or park. turn the idle up until the mirror stops shaking and you can see a clear reflection in it. strangely enough, its right about where the factory spec should be.

[Ace]

Mine idles at 850. I think the 750 stock spec is too low. That might be hard on an automatic trans, tho.

[peobryant]

Mine idles at 850. I think the 750 stock spec is too low. That might be hard on an automatic trans, tho.

Quick question, why would a higher idle be hard on an automatic? I'm not doubting you, I'm just curious.

[Richie O]

It goes into gear harder. BANG!

[peobryant]

It goes into gear harder. BANG!

Well I was expecting a long, confusing explanation, but that makes perfect sense. I should've known that.