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VegasScott
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PostSubject: Vibration at idle (only when in gear)   Tue Jan 04, 2011 4:19 pm

I've got my car purring nicely at idle when in park or neutral, but I noticed when I'm at a traffic light or somewhere when I'm in gear and idling, I notice a bit more vibration. I'm hoping to do a transmission fluid and filter replacemnt this weekend if it isn't so friggin' cold! That may help my problem some, but just want to see if there are any other issues I should be on the look out for.

Thanks!
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PostSubject: Re: Vibration at idle (only when in gear)   Tue Jan 04, 2011 5:36 pm

Could be a bad engine mount, or loose interior panels, depending on what you hear. Or it could be a vibrating heat shield near the catalytic converter. You can try bending the shield slightly, and the noise might go away.

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VegasScott
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PostSubject: Re: Vibration at idle (only when in gear)   Tue Jan 04, 2011 6:59 pm

It's not so much a sound that I'm hearing... It's more of a vibration I'm feeling... I'll have to check out the factory service manual when I get home tonight.

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PostSubject: Re: Vibration at idle (only when in gear)   Tue Jan 04, 2011 7:16 pm

VegasScott wrote:
I've got my car purring nicely at idle when in park or neutral, but I noticed when I'm at a traffic light or somewhere when I'm in gear and idling, I notice a bit more vibration. I'm hoping to do a transmission fluid and filter replacemnt this weekend if it isn't so friggin' cold! That may help my problem some, but just want to see if there are any other issues I should be on the look out for.

Thanks!
VegasScott

Most likely weakening engine mounts.

Over the history of the 3800 engine, its inherent imbalance is probably legend among the (mostly now retired) engineers that had a hand in it. This is why it has a balance shaft and that beefy harmonic balancer (and if you get the factory balancer you get a high tech polymer insert, not just neoprene). Over the years many things were done to try to smooth it out. Look it up on Wikipedia and you'll see some of that history. The Series I was, I understand, the most improved of the series--later ones were better but not to the degree that the SI was improved over its predecessor. But it still vibrates so much, you gotta wonder who's putting the quarters in (think about the coin operated vibrating chairs you see at amusement places...) and make 'em stop.

So the soft mounts on the Riviera were the way chosen to use up the energy of the inherent vibration of the engine; they probably did not cost all that much compared to what they did (when new) and frankly they are not very robust. When you crawl under the car to have a look at them please try to restrain your laughter. The beefiest mount, actually, is the one at front of engine by the belts and accessories (alternator etc). The rest are little "+" shaped inserts in various castings. They are very compliant given the torque and horsepower of the engine.

IMHO all you have to do to wack out a mount is a couple months straight of 'jackrabbit' starts from a dead stop.

What to do about this?!?

Well - Len Rapkins wrote the inserts are now available. Cheaper than the whole mounts by a long shot ($$ instead of $$$)

When I get around to replacing mine (again) I think I will fill the voids with low-durometer 3M WindowWeld to soak up the energy without allowing the fatigue caused by the compliance and excursion of the stock mounts. Also elsewhere on this site is a write up about casting your own.

Albertj
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PostSubject: Re: Vibration at idle (only when in gear)   Tue Jan 04, 2011 7:24 pm

Okay, I understand that the engine may have an inbalance, but there is NO vibration at idle when the car is in P or N. It is only after it is engaged in D do I feel the vibration. As soon as I'm off idle, it's butter smooth again.

Any other ideas?

Thanks!
VegasScott
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PostSubject: Re: Vibration at idle (only when in gear)   Tue Jan 04, 2011 7:41 pm

Our Rivieras idle at a low RPM in gear. I notice some vibration in my Riv sitting at a light. It doesn't bother me at all. Not sure how severe yours is.

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PostSubject: Re: Vibration at idle (only when in gear)   Tue Jan 04, 2011 7:45 pm

VegasScott wrote:
Okay, I understand that the engine may have an inbalance, but there is NO vibration at idle when the car is in P or N. It is only after it is engaged in D do I feel the vibration. As soon as I'm off idle, it's butter smooth again.

Any other ideas?

Thanks!
VegasScott

No other ideas, just a further explanation. The reason the vibration quits in neutral is the engine torque isn't working against a stalling torque converter and the harmonic balancer and balance shaft are adequate to damp the remaining harmonic vibration. At idle, in Drive, as your mounts weaken they are not adequate to damp the vibration of the engine working against the stalled converter. You basically feel every pop. 650 rpm at 6 pops per rev gives you a buzz around 3800 hz and what you are feeling is the lower harmonic of that in time with the unbalanced firing order of the engine.

Get it? When the car is new all the damping works. But the mounts are so weak that when they wear a littl bit you really feel it fairly strongly.

The car will be drivable for a long time before the mounts are so wack they are causing other problems. But that's not what you asked...

Try this at your own risk if you're feeling lucky and curious (just remember that curiosity killed the cat and satisfaction brought it back): Park the car and chock the wheels. Open the hood, start it and watch the engine. rev it a little bit, see what it does. With your parking brake set, step on the service brake (firmly) and put the car in gear while you watch the engine. The engine will rock, call the distance of the rocking the excursion. While still holding the brake flutter the accelerator a little. A couple hundred RPM will make that engine look like it's coming out to get you (or anything else within 10 feet) because your mounts are weakened, which I bet they are. you will get well more than an inch or 2 excursion. New mounts don't let the engine have much excursion under those conditions, thinking about when I last replaced mine I'd say an inch or so. Worn mounts allow much more excursion with a little throttle flutter, you'll have *no problem* seeing the engine dance about. You can get the new mounts to let the engine to move a lot too but you have to take the revs way up. When they are worn a little flutter couple hundred RPM will do it.

And dont do this a whole lot, it will fairly quickly overheat the torque converter. 15-30 seconds should prove the point.

Albertj


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PostSubject: Re: Vibration at idle (only when in gear)   Wed Jan 05, 2011 11:42 pm

Is the vibration steady, or more of an occasional popping? If occasional I would suspect injectors. I replaced my injectors with a cleaned (flow tested) set and was SHOCKED at the difference and how much it smoothed out.
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PostSubject: Re: Vibration at idle (only when in gear)   Thu Jan 06, 2011 12:07 am

Hey Derek!

I don't think it's the injectors, because the car has good power, pulls nicely, and idles great! The only time I feel that vibration is when it's idling and in gear, like at a stoplight, or stop sign. As soon as you get even 100RPM above idle, it's smooth as butter again.

When I replaced the intake manifold gaskets, I cleaned the injectors even though they looked pretty good to start with and replaced their O-rings.

Are there any vaccum lines I should check?

Thanks Guys!

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PostSubject: Re: Vibration at idle (only when in gear)   Thu Jan 06, 2011 1:36 am

As you can see Scott, a given problem can produce as many ideas as we have people on the board. happy

However, any engine will have more vibration when in gear than when in Park... It's idling at its slowest RPM, which is the most challenging condition for smooth firing, and being coupled to the transmission transfers the vibrations to the passenger compartment more clearly.

While the motor mounts ARE expensive, and bad mounts will also transmit more vibration to the body, I don't think Series II Buicks have any more external vibration than any other engine. Thanks to the improvements that Albert listed, as well as the first and most important change, the even-firing split-pin crankshaft, you can lay your hand on the engine in Park and feel how smooth it is... Unlike the pre-1977 "odd-firing" engines, these don't need the "rubber band" motor mounts that those early engines had. I remember an early-70's 231 in a GM compact wagon that would bounce all around the engine compartment when idling - in Park!

What surprises me, this being an idle problem, is that no one has suggested checking/cleaning the Idle Air Control and the Mass Airflow Sensor... I could occasionally feel my idle when at a stop light, and a gentle spray of electronics cleaner on the MAF sensor cured it - the big headache was those damned Security-Torx screws!

And yes - you should check all the vacuum fittings and hoses... They're sneaky, and they are reaching the age of "crystallization". sick
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PostSubject: Re: Vibration at idle (only when in gear)   Thu Jan 06, 2011 1:12 pm

Eldo wrote:
As you can see Scott, a given problem can produce as many ideas as we have people on the board. happy

However, any engine will have more vibration when in gear than when in Park... It's idling at its slowest RPM, which is the most challenging condition for smooth firing, and being coupled to the transmission transfers the vibrations to the passenger compartment more clearly.

While the motor mounts ARE expensive, and bad mounts will also transmit more vibration to the body, I don't think Series II Buicks have any more external vibration than any other engine. Thanks to the improvements that Albert listed, as well as the first and most important change, the even-firing split-pin crankshaft, you can lay your hand on the engine in Park and feel how smooth it is... Unlike the pre-1977 "odd-firing" engines, these don't need the "rubber band" motor mounts that those early engines had. I remember an early-70's 231 in a GM compact wagon that would bounce all around the engine compartment when idling - in Park!

What surprises me, this being an idle problem, is that no one has suggested checking/cleaning the Idle Air Control and the Mass Airflow Sensor... I could occasionally feel my idle when at a stop light, and a gentle spray of electronics cleaner on the MAF sensor cured it - the big headache was those damned Security-Torx screws!

And yes - you should check all the vacuum fittings and hoses... They're sneaky, and they are reaching the age of "crystallization". sick

great points.

Now, if you just need something to do -- when the car is idling in neutral (park) look at the tach. If it is fluttering at all, or hunting about (revs move a bit higher/lower) it is time to pull and clean the throttle body (not just the wipe-it-out-with-a-rag thing). When things are all OK the idle does not hunt.

Albertj
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PostSubject: Re: Vibration at idle (only when in gear)   Thu Jan 06, 2011 4:30 pm

Thanks for the additional suggestions guys, but...

That's just the thing... There is no idle problem to speak of. The engine runs GREAT! I'm just trying to figure out why there's so much more vibration when in gear and idling stopped versus out of gear and idling sitting.

I like a car that at idle (whether in gear or not) will run so smoothly that you start to question whether or not the car is on. You answer that question by slightly depressing the accelerator and then hear or feel the engine.

That's the goal of this thread. Uncovering what it takes to achieve what I described above.

Thanks for the help and keep the suggestions coming. I'll report back after this weekend when I finish the rest of the PM work I want to complete on the car.

Thanks guys!

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PostSubject: Re: Vibration at idle (only when in gear)   Thu Jan 06, 2011 4:34 pm

if you are having no idle problem then vibration in gear is weak mounts...

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PostSubject: Re: Vibration at idle (only when in gear)   Thu Jan 06, 2011 5:31 pm

Quote :
There is no idle problem to speak of. The engine runs GREAT! I'm just trying to figure out why there's so much more vibration when in gear and idling stopped versus out of gear and idling sitting.
Not the solution to your problem, but it helps to understand that vibrations are basically resonances, or when materials get excited by very specific frequencies of mechanical energy. A material's molecular structure, mass (weight), and security of mounting/insulation all play a role in how much vibration is transferred from the engine, through these materials, and on to you the driver.

Idle RPM fluctuates depending on engine temperature, anywhere from 1000 or more on a cold morning down to 630 for a warming engine. For this reason, you may notice the vibration is more prominent at certain temperatures than others. At 630 RPM, the engine is emanating a frequency of about 10.5Hz into the car. At 750 RPM, the resonance is closer to 12 Hz. This may seem like a small difference, but at subsonic frequencies such as these, a range of 6 Hz makes up an entire octave.

A tiny shift in material weight, mounting/insulation, or engine RPM can really make a difference. This is why we have mass-adding materials in the floor & firewall - to increase mass, which lowers their resonant frequencies to values that to not become excited. This is also why we have rubber engine mounts - as insulation to absorb mechanical energy and change it to heat. There is a third option - to change idle RPM. You will notice in gear, with your foot on the brake, if you slightly push the gas the vibration may attenuate or even disappear. This is also why the vibrations go away in neutral, as RPM increases.

I had the same problem as you, probably a combination of loosely mounted materials and degraded insulation materials (engine mounts, etc.). These are common things in aging cars. I isolated one vibration to the heat shield under the car. Bending this slightly created tension, or more secure mounting, which quieted the sound. I also tried adding insulators to identified interior objects. Poster putty works behind speaker grills, and a very small o-ring helped the rear-mirror buttons. But in the end, there were just to many noises to hunt down. My ultimate solution was to increase the idle RPM in gear 50-100 higher than stock using PCM tuning software, which caused the resonances to stop, and perceived vibrations to cease. Some would call it a band-aid fix, but I found it well worth doing for the result.

_________________
'98 SC Riviera • 268k miles • 298 HP/370 TQ • 0-60: 5.79s • ET: 13.97 @ 99.28 • 4087 lb • 20.1 avg MPG • Nelson Ledges Lap: 1:30
3.4" pulley • AL104 plugs • 180Ί t-stat • FWI w/K&N • 1.9:1 rockers • OR pushrods • LS6 valve springs • SLP headers • ZZP fuel rails
KYB GR2 struts • MaxAir shocks • Addco sway bars • UMI bushings • GM STB • Enkei 18" EV5s w/ Dunlop DZ101s • F-body calipers
EBC bluestuff/Hawk HP plus • SS lines • Brembo slotted discs • DHP tuned • Aeroforce • Hidden Hitch


'05 GTO • 49k miles • 0-60: 4.8s • 16.9 avg MPG • Nelson Ledges Lap: 1:26
Because fun
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PostSubject: Re: Vibration at idle (only when in gear)   Thu Jan 06, 2011 6:34 pm

Mel's white riv has the same slight vibration at idle in gear and the car only has 26k miles. It's been doing it since she's owned it at 18k mi. The vibration -which is definitely not a rattling- can be felt mostly through the steering wheel. Like Aaron mentioned, it's only at about 650 rpm. I was simply going to do a motorvac treatment this spring and then raise the idle by a 50 or so rpm if that didn't do it. It had never occurred to me it might be caused by weakened mounts with such low mileage.
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Vibration at idle (only when in gear)

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