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Vibration at 1500 RPMs

5K views 34 replies 6 participants last post by  smelonas 
#1 ·
As the title states, I have an RPM dependent vibration. So tires, driveshaft and things of the like are out of the question and I also checked them anyway. I even took driveshaft off and greased the front u-joint just to be sure..

Heres how a typical drive goes. Start car cold, no vibration (that I notice anyway), drive for a few minutes still nothing.

Get up to operating temperature and around 20+ minutes of driving and here comes the vibration. I checked through the climate control, the RPM it happens at every single time is right at '15' and by the time it rolls over to '16', the vibration is gone.

I can feel it through the gas pedal, it is also audible. I wouldnt say its very loud, but you can hear a groan kind of like a power steering pump, but not as loud I would say.

I did go ahead and check my fluids.. Its not those.

It seems to me since it is RPM related, it pretty much has to be the belt system, the torque converter (still makes the vibration when locked! so I have my doubts about this..), or a bearing on something in the belt system. My power steering pump has like 5k miles since a LEEs rebuild, my tensioner is new(thinking it might be the bearing on the pulley, I have my old good pulley still), the AC compressor I did about a year and 4 months ago I think and the alternator is the original and I am pointing towards that more and more. The water pump was replaced when I got the car in july of 2015. The bearing on the old pump casued a sound somewhat similar to this and it was completely rusted out. I did check the bearing on this pump 2 or 3 months ago when I had to put new seals on the waterpump, it spun easily and had no play.

Is there anyway I can test the alternator bearing? I guess I could take it over to good old autozone and have them test it, but that will only tell me the condition of the diodes. Wish I had another good alternator to throw on this thing to check it.

Let me know if I left out any pertinent information.
 
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#3 ·
Cant believe I forgot to mention that. It does it in neutral, yes.

It does appear to get worse as load goes up when moving as well. So it seems at least.
 
#4 ·
Damm You - And Thanks

Pretty ok detail - made me read the whole thing. Lots of google hits with "1500 rpm vibration". With that tight of a range it's way more harmonic than any bearing growl. Most results seem to center on rotted motor/tranny mounts, stretched belts..... even a gastank shield.

Some interesting Go-Pros while driving too. Since it happens in neutral why not get it on stands and set cruise and scope it out underneath. At very least throw a new belt tensioner at it?
 
#5 ·
I literally replaced the belt tensioner less than 5k miles ago. With an ACDelco one. I can throw the old one back on. Its still 'good' other than its weak spring that caused the belt to slip on foggy mornings.

Thing is that I have seen where either bushings or bearings will be bad, but not really show it and then this kind of thing will happen. Like you said, its just harmonics and once it hits that one specific frequency right, 1500 in this case, it causes a vibration. That last statement being an assumption that is a bearing.

Outside of this problem, the car drives great! Like it always has. I guess its possible the bushing for the motor/tranny mounts are junk. *sigh*

Ill look into checking the bushings and how to do that.
 
#6 ·
since it does it in neutral, pull belt and see if it still does. If it stops vibration the issue is one of the accessory's....if not then those are ruled out

exhaust grounding (from weak motor/tranny mount) could be one reason if the later of the above
 
#7 ·
I didnt even think to do that, DOH!!! Good one!

Ill pull the belt when I get home and see what happens and report back. Hopefully that isolates the problem and I can go from there :nerd:

Thankfully I do have my old tensioner and even the original belt to try different things out if that ends up being the case..
 
#8 ·
Well taking the belt off *seems* to cure the problem. But I want to test the car in a straight line drive with the belt off.

I checked for vibration twice with the belt off. Then again with the belt on. With belt on, it is much more noticeable.

I just want to confirm that taking the belt off does get rid of the vibration by doing a drive test.. thankfully it takes 2 seconds to pop the belt off and on. Reasoning for the drive test is because the accesories, mostly the power steering pump, do induce a little bit of load, so by eliminating that am I just making the sound quieter/less notoceable? That's what I hope to find.

Once I do that test, I'll report back again with my findings. Until then, if anyone has suggestions, let me know.

The price of those acdelco motor mounts is mind boggling!!!!!! You can buy those, or the off brands for about 90% less. Wtf??

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#9 ·
all accessories put load on motor. PS more when you turn the car.

If vibration seems to go away with belt off than focus on each accessory. spin by hand. does it turn smooth & freely or is there some wobble or grabbing. If the later that accessory needs attention.

Driving without belt is also a way to confirm it does not happen under drive conditions. It will be hard to steer the car (turn wheel) and your battery is running the car so just do a quick spin.

also look at the condition of the front damper. is the rubber between the inner & outer part all cracked & dry? That is a sign to replace. Mark face of damper with chalk or some marker. After you drive the car with belt on for awhile see if that mark is still straight or shows signs of outer ring of damper moving. Accessory load with belt on may exacerbate this if the damper is starting to wobble some. Look at it with engine running...it should be dead nuts smooth.
 
#10 ·
I was reading about the damper last night. That was going to be another thing to check. Thanks for the pro tip! I'll find some chalk or something and give that a go.

As far as driving with belt off, I was just going to find an empty lot, take the belt off drive straight, stop, put belt back on. I only need to stay in first to test it.

Thing is I checked the pulleys on everything. They all spin nice and true and smooth. That's what I found odd..

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#11 ·
Finding all of the pulleys straight is not odd, rather it is what is expected, and therefor not the problem. You have eliminated one possibility.


You should check the flex plate. They are known to break, and cause vibrations. The torque converter starts to act more like a mechanical connection at about 1500 RPMs. That could be a link to the problem.


It is a pain to troubleshoot odd problems. Sometimes they are an easy fix once you find the issue.
 
#12 · (Edited)
Okay I'm glad some said flexplate!

I took the inspection cover some number of weeks ago when I was doing my oil cooler adapter gasket and oring, and I thought I noticed a part of it looked 'broken'. But I couldn't get the cover off all the way with the starter on. So I just reattached it. Maybe this mystery is coming together now.

Reason I didn't mention this was because I can't confirm its actually broken.

Since it seems that might be a possible cause, I will need to investigate further.

Would a broken flexplate do the same thing in neutral though? I guess the TC is still spinning. However, it still does the vibration at 1500 when in lockup mode. So shouldn't it not do it that scenario?? I still need to reinspect the flexplate either way.

Thanks for the idea though! I will look into it!

EDIT: It seems the thing I saw that I thought was broken piece is common to flexplate for these cars. Looks like a weight or something. My presumption is the flexplate is okay, given this car hasn't been flogged and beaten.

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#13 ·
If the flexplate is cracked and deflection of the plate is happening at the crank, that deflection is magnified at the outer ends of the flexplate. If one part of the flexplate is off the centerline from any other part, it will throw off the balance by a country mile.
 
#14 ·
You would think if the flexplate had an issue I would feel it more the faster the engine spins.

Something I noticed yesterday on my way home as well, was that the vibration becomes very noticeable when the car has been running for a long time. It takes me longer to get home than to get to work. By the time I got home, the vibration seemed to have a wider range of RPMs it acted up in. Not a lot more, but maybe started a little at 1400, was the worst at 1500 and slowly faded into 1600. Versus in the morning where it is strictly pretty much at 1500 exactly.

It scares me reading some of these forums where people have had this issue, and in one case the guys tranny locked up. Great. Just what I need to read.

I'm leaning towards the balancer as I read that in several threads as well. Time to get out the chalk and test, test, test!


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#19 ·
....vibration becomes very noticeable when the car has been running for a long time. ....
Well that opens it up. A/C tube rubbing something. Same for an exhaust piece. Name anything that grows a little tighter when warmer.

Ask a few others for their opinion of what it sounds like, feels like, where it is coming from. Can't tell from the posts - did you ever get under it while running?
 
#15 ·
Not to freak you out even more, but running an engine for long with, say, a balancer that's started to slip or come apart, or a flexplate that's tossed a weight, can start to hammer the snot out of your crank bearings

Also, regarding your comment about RPM's and "feel it more the faster an engine spins"...

Have you ever had a wheel out of balance and it rides like a jackhammer at 55, but smooths out at 70?

I have....

YMMV
 
#16 ·
I know about the tire thing... my dad had these tires that kept separating (3 in one summer). They did that crap. Wobble like crazy at 50-60 then you 65 and she was smooth as butter. That dont make it right though.

I read about the bearings as well.. I truly think my flexplate is okay. That thing I saw was the weight they weld onto the FP.

My balancer 'looks' to be in decent shape. But that really means NOTHING unless its so far gone that its literally crumbling.

In the event it needs replaced... What are you guys using to replace these? I see ACDelco doesnt make them.. Which is neither here not there at the end of the day. Is it common for a damper to go bad before or around 170? I read where some people never do them, and others get 100k out of them before they are junk, but those are other cars.

I really need to drive test this thing without the belt to confirm, then I can move on from there.
 
#18 ·
I am making this confusing by accident...

I meant to say I saw the weight and I though that it was broken because it looks broken. That's just how it looks. To me at least. But the weight is still on the flexplate. It's welded on there, it isn't coming off just from driving.

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#20 ·
I am making this confusing by accident...

I meant to say I saw the weight and I though that it was broken because it looks broken. That's just how it looks. To me at least. But the weight is still on the flexplate. It's welded on there, it isn't coming off just from driving.

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Sorry. I was confused! :wink2:

Mike--94MSP9C1
 
#22 ·
Back to this.

I tried the chalk thing on the balancer. The chalk lines were all still aligned and seemed fine.

BUT!!! Reading through another some symptoms of a bad balancer I saw on the internet, I think I may have a going-bad balancer anyway. One of the symptoms I read was 'an odd, semi rythmical fluctuation in idle that sets no codes and all other parameters appear normal'. Something to that effect.. So maybe that does explain my odd idle that literally sounds exactly like that description that I have never been able to track down (runs perfect when on the throttle!) . And it only does the idle thing after it has been running for a few minutes and, apparently, when the balancer gets 'warm'. For the record, my current balancer does seem 'old and dry' not exactly 'fresh' looking.

So since I am here and I have a dollar to spend, which part should I buy? Dorman or Dayco? Both are on rockauto, and there dont appear to be any other *decent* choices.. I found just ONE review for the dorman and none for the Dayco, so I am kinda clueless here as to what is 'preferred'.

I did not get to drive test the car without the belt as it was my birthday this weekend and I just did not GAF at all. My truck is running so Ill just drive that till next weekend.
 
#23 ·
if your "damper" (it is not a balancer) is original 20+ years old....the rubber between outer & inner ring deteriorates which can cause the damper to wobble some. The Dayco would be fine to get. The damper is neutral balanced. The LT1 is external rear balanced motor any your flex plate has the weight on it.

You should drive without the belt to confirm the vibration is not motor or driveline sourced
 
#24 ·
Yeah.... I totally flopped on driving it this weekend. I just was being lazy and then the weather wasn't that great either...

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#25 ·
So on the topic of the damper... like I previously stated. The engine has that rhythmic vibration to it and I started the car this evening to look at the damper and check it out. The engine has the rythmic vibration as soon as you start it and it appears to get worse as the car runs. So I am definitely doing a damper. And I got raise today, so that's already spent.

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#26 ·
did you notice ANY slight wobble in the damper with the engine running?

nothing wrong with replacing a 20+ year old part.....but with that said your "rhythmic vibration" could be because of a weak or non firing cyl, intermittent injector, plug arc....etc.

A replacement Dayco or Doorman Damper is not much $ so go for it. If your problem persists you will need to do deeper diagnostics

Is your belt tensioner bouncing around a lot with engine idling??
 
#27 ·
Belt tensioner is steady (and NEW).

Damper spun true. I watched for a while tonight to see if I could detect anything. No movement at all.

You say injector. And I say I have long suspected a problem with one but have never proved it to be true. Isn't there a way to test it using glass jars and pulling the rail? Sadly that's something else I need to do still at some point.

I have 8 oem injectors I got from a junkyard for 23 bucks. Need to have those cleaned and tested.

I ohmed the current injectors when they were hot and I didn't have one that was 'bad' but I do remember one being a little out of whack to the others that were all .3 ohms within themaelves. IIRC the one was like +.6 off. So more resistance. This weekend I plan on doing the intake manifold, so while I'm there I will replace some things and check some things. Injectors being one.


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#28 ·
As a very generalized calculation, this crankshaft has probably rotated at least 500,000,000 times in its life. A little piece of rubber probably should be replaced in both the context of lifespan and reliability.

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#29 ·
Well on a side note.. I did get this lol


On amazon for 67 bucks. Not bad for probably some of the best fluid around. Also, it isn't that universal stuff which is suspect anymore..

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#30 ·

Well... I got the intake manifold resealed.. found the oil pressure sender was not plugged in. Got the F body EGR put on with a new solenoid. Checked all vacuum lines and cleaned them up.

Managed to break the plug for the MAP sensor, I'll just pray it stays together.

Also ran over my gascap and ruined it, maybe the third one will be the charm. I'm putting a leash on the next and LAST one.

Put a new PCV rubber elbow at the front of the manifold and a new grommet where the valve goes into the manifold. Astroglide literally made it fall right in.

I did dimple the manifold itself to give 'the right stuff' something to grab onto. Other wise, it's MUCH cleaner and easier on the eyes than it was before.

Also cleaned the hell out of the throttle body. Got that old thing clean as a whistle. Took the IAC valve out to clean it, and what did I find? About 1/8" to 3/16" of coking on it, cleaned it up real well and reassembled. Also put on a new TPS.

Then the mailman came. On a sunday. Thanks amazon!


Anyway. Plans to do harmonic balancer, ICM and other tests next week.

I still haven't checked the engine for vibration without the belt. Can't do that with the manifold off.. thinking this week after work I'll stop by church and test it in the lot.

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#31 ·
On my way to checking for vibration I found this while swapping the ICM...

Wonderfully exposed and ready for arcing or whatever else. So I ordered a new pigtail from rockauto.. the saga rages on.

Definitely leaning towards a bad alternator bearing or something to that effect.

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#32 ·


Here is a short video of the alternator...

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