Shaking/vibration at idle

[dan brooks]

This afternoon my truck started shaking/vibrating at idle, enough to cause the entire exhaust system to bounce around and make a ton of noise. So far, I've only been able to duplicate the problem at idle, in drive/reverse, and with the AC on. If I put it in park/neutral the noise and vibration goes away, and will do the same if I keep it in gear and turn the AC off.



I haven't had to use the AC much until the past week or so (it was 86* today... . not to rub it in), and I certainly don't remember it doing this during the last warm spell.



Anyone have a clue what might be causing this? I'm taking the truck on a 3000+ mile round trip at the end of next month and really don't want any "surprises" along the way.....

 

[BushWakr - RIP]

Ok, I'll bet one wobbly pop, that your problem is low idle and or some bum fuel.

Since it goes away in "N" or "P" that's my guess.

Your idle should be about 800-850 RPM in "N" or "P".

In gear no less than 725 RPM..... that's my opinion though...



bob.

 

[dan brooks]

Thanks, Bob. Wonder who I can find around here with a diesel tach - I have a real nice one for a gas engine but I can't seem to find the #1 plug wire on the Cummins.

 

[BushWakr - RIP]

Before I had a tach, I used the "mirror wobblies" test... .

Keep tweaking the idle up till there is no sign of shaking in the mirrors when in "D" or "R" with AC on... . that'll get you close...



Bob.

 

[dan brooks]

Thanks again, Bob. I haven't really looked to see if the mirrors are shaking but will take a look.



A little more info on this. It only seems to do it after driving it for a while and getting it good and hot, then shutting it down for a while. When I start it back up after 30 minutes to an hour or so, turn the AC on and drop it in gear, it starts shaking really bad. It will go away after letting it run for a minute or two.



Do you think it is still idle-speed related, or do you think there may be a problem with the AC or compressor?



On an unrelated matter, I'm also getting a grumbling feel at low speeds (a couple MPH). All the wheel bearings (F&R) are in good shape and have been serviced within the past 1000 miles.



Is it just me, or does Murphy always seem to bring his friends around when trying to get ready for a trip?

 

[BushWakr - RIP]

One way to confirm/deny the idle thought is to load it by turning on the AC and placing in "D". Does going to "N" with AC on clear it up? Does turning the AC off while in "D" clear it up? If so, then it's quite possibly idle speed related. Put it in gear, turn on AC, and apply just a touch of throttle, does it clear up with only the smallest of speed increase??? Give it a try...



What kind of "grumble" are you referring to?? How are your universals? Originals??? If you have the clubcab is your center steady bearing ok?? They can really be deceptive when you check them. They can actually feel perfectly good, no slop or excess movement, and still be in need of replacement.

I am a fan of just replacing them after a few years. Spicer makes a newer high end universal joint with excellent durability/warranty.



bob.

 

[dan brooks]

I'll give that a shot when it warms up again later this week. I would bet that dropping it in neutral or giving it a touch of gas, er, fuel will make it go away.



The grumble feels like a bad bearing. I have a standard cab so no center bearing, and I'm not getting any play in the u-joints when going from reverse to drive. There is a little slop in the tailshaft bushing but I don't think it's much more than normal. It may be something completely normal but I may be getting a little paranoid.....

 

[BushWakr - RIP]

Although it's not too common with todays bearings, it is possible to have what we used to call "boxcar bearing(s)".

That was due to poor bearing torque (over/under torqued) on install or adjustment.



If you have any doubts replace them.

I have personally seen 3 cases were the universals passed every "test" for slop of breakdown, only to find that once replaced and inspected they were shot. I'm only saying that it seems to be hard to detect universal failure with the normal "twisting/turning/yanking" method.

There is a small amount of play in the tailshaft connection. That is normal. Whether it is within spec requires more precise checking as you probably know.



Finish off this particular process of elimanating the engine shake and we can then chase down the other issues one by one... . the guys will all toss out ideas and things to check till you've got it covered.

Don't feel paranoid... . I've driven mine so long now that the slightest change in sound or feel is picked up pretty much immediately.



Bob.

 

[AMikonis]

I have a standard cab so no center bearing

Your sig says you have '92 250 2wd. I have a '92 D250 2wd standard cab and it has the center bearing. The '92 W250 4x4 standard cab I had did not.



Both of mine are Getrags. Do the 2wd automatics not have the center bearing?

 

[dan brooks]

Originally posted by AMikonis

Both of mine are Getrags. Do the 2wd automatics not have the center bearing?

I don't have a clue. I looked at a couple 5-speed trucks but didn't check them out that close after deciding I'd rather have an auto.



BushWakr - I'm really thinking this may be more temperature related, either with the idle speed or the AC. It was fairly cool yesterday (in the 60's), but I still ran with the AC on and it idled fine, in gear, without any shake. The mirrors had a barely detectable vibration. Is there something with the injection pump that may cause a change in idle speed after experiencing heat soak?



This is my first diesel so I have little or no idea what "feels" or "sounds" are normal. I'm just hoping to avoid a repeat of last year's fiasco - three out of three vehicles not capable of a long trip, and having to rent a car for vacation - or worse, breaking down in the middle of nowhere.

 

[BushWakr - RIP]

When the engine is cold there are a couple things that cause harsh idle.

If the temp is below 59*F the manifold hearters will kick in to warm incoming air before entering the cylinders. These little babies can draw some serious amps..... When they trigger on and off you'll see your alternator guage doing nose dives and recovering repeatedly. The cycle time will vary with temp.

Also, the KSB solenoid triggers on under cool/cold temps. This has the effect of increasing internal pump pressure, which in turn, increases the pump timing. The increased timing helps to reduce the smoke and shakes at idle, as well as slightly raising the idle RPM's. Once the engine reaches a certain temp, the KSB releases the pump's internal pressure and it falls back to it's 'normal preset level'.

These are two circuits that can affect your initial cold start/idle/running.

A bad or poor battery can magnify the draw down of the 'heaters' because there is little current coming from the battery to supplement the alternator current being sent to the heater block. That means that pretty much the entire load is dumped on the alternator. With that kind of current demand the alternator will really pull your RPM's down... . heck, even a good battery will need some serious support from your alternator.

If you idle is low to begin with, and then you compoud things with high demand by the manifold heaters, you're going to see the engine idle do a face plant every time the heater relay kicks on/off.



Now after all of that, the temps you mention don't sound like they are cold enough to cause a serious or prolonged demand on the heater grids, or KSB.

This is a bit of a mystery. Do you have any odd sounds when it idles rough??? Have you tried upping the idle a bit to see if it cures your problem. I have poor idle on winter fuel, in fact I loose about 50-75 RPM and the idle quality is rough too.



Bob.

 

[dan brooks]

I can't really hear any strange noises while it's idling... at least nothing I can identify as strange. That is the one thing I haven't quite got used to - it's pretty hard to track down noises when standing next to a running Cummins.



I paid closer attention today when idling in gear, with and without the AC on, and if I give it a little extra fuel it smooths out what vibration there is in the mirror.



I didn't think about the winter fuel dropping RPM - with it being so warm last week and running the AC on winter fuel, that may be all it needed to drop the RPM below where it needs to be. It's supposed to be in the 60's to low 70's for highs for the next several days so I don't know when I'll have a chance try to duplicate the problem. I'd really like for it to be something as simple as winter fuel.....



Thanks again for the great advice.

 

[dan brooks]

Update...

I decided to bump the idle up a little a couple days ago. That seemed to help but I wasn't sure how close I was to being within spec, so I took the truck to a shop today and had it checked. Turns out it was about 60 RPM too low - even after I turned it up - so I'm pretty sure that was the problem. The guy also said everything sounded fine, and was running just as a 54k mile engine should - guess all those rattles, shakes, and other things are normal.



As far as the grumbling goes, I haven't noticed it lately so maybe it was just the road surface or maybe a rock hung in the brakes..... at least, I'm hoping that's all it was.



Thanks again.