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Vibration-ressonance

black 940

black 940

Member
Joined
Sep 12, 2004
Location
Charlotte, NC
Have any of you experienced this. My '93 945 T resonates terribly at highway speeds, if you let off the gas it stops. Basically anytime above 2500 rpm. Any ideas what to check?

The car has the folowing new items.
Engine mounts
Trans mount
Engine.
 
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It seems to be more RPM based then speed based. It is in the chasis not the wheel. The trans mount was just replaced (meaning within 500 miles)that was my thought. Driveshaft center bearing was another thought, however why would it do this in neutral? How do I check the center bearing?

This is not a shake like an out of balance tire, but an engine resonance (noise), vibration is very slight.
 
all i can say is good luck finding a solution to this cause my car does the exact same thing around the same rpm 2500 and above it kinda sounds like a blown out muffler to me thats what comes to mind on my car. i have had these things replaced::

motor mounts both sides

transmission mount

entire exhaust system from the cat back (stock replacement system with 2 mufflers)

complete tuneup including plug wires/cap/rotor.

my car runs great i will add and gets good gas mileage 28mpg hwy 22 in the city

my car is very sound so i just deal with the noise. my girlfriend however refuses to drive the car like this and it freaks her out.

like you said its rpm based if i am runing 75mph on the freeway and put it in neutral the noise goes away its rpm based like your car so i dont have a clue to what it might be
and neither do anyone else on this forum cause its been brought up before and everyone seems to think that its the drivesahft center support bearing but its not.

if i am sitting in the driveway with the car idling and run it up to 2500rpm it vibrates very badly I almost cant stand it . i can tell you this much before i had the motor mounts replaced it never did this.

so maybe thats it bad motor mounts?
 
Mine did it with the old motor mounts, but like you said it has gotten worse with the new mounts. My old '94 940 was quiet as a mouse on the highway, this one drives me nuts. I will check out again this weekend.
 
It might be a muffler as they are designed not to have high points of resonance when functioning. If they do they get very loud at that frequency, RPM.
 
when something of this sort, higer-decible/resonance at a particular engine speed, t turned out to be a ailed weld of an exhaust conduit. at 3000 RPM, which seems to be the beginning of power band in my app. the exhaust density or flow would increase and the failed weld joint would expand.
 
same things lower rpm

I have a 91 780 w/ b230ft with the same issue althought it begins at about 1700 rpm and fades away after about 2000rpm. It happens in park or neutral at a standstill. On the highway if i put in neutral at highway speeds it seems to get better. It makes the car feel like a piece of crap. When the turbo is on the pressure side it seems much worse.

I have a mitsubishi 13c turbo when i had a garret turbo on my 760 it seemed to be much quitier under load. I was wondering if my problem is with my turbo. Or if the mitsu unit is just loud, however that still shouldn't vibrate.

since last August (2007)
new engine mounts URO brand(from fcp)
new timing belt and pulley
new trans mount
new vibration damper
new alternator and ac bushings
new exhaust including cat and all rubber hangers (it does not rub the car anywhere)
new fan and fan clutch
new pine tree air freshener
new Volvo center mount with skf bearing

two weeks ago I replaced the trans mount again, because it seemed spongy and because the vibration is annoying. I checked center mount and it seems tight.

I have done alot of work to this car and am annoyed at this vibration. This weekend I am pulling off power steering belt and then alt belt and running it in between each removal to see if the vibration goes away. I replaced plugs this week.
When turbo is set to 10 psi it is much worse right now it is back to 6-7 and is more tolerable.
 
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If it is exhaust resonace and does it standing still stick two or three feet of pipe on the end of the tail pipe and see if it changes.
 
gm, I know how you feel. I rebuilt the engine in mine and still this annoying vibration. I will put it on the ramps tom. or the next day. It makes me feel better that others are experiencing the same thing.
 
moxy123 said:
If it is exhaust resonace and does it standing still stick two or three feet of pipe on the end of the tail pipe and see if it changes.
Click to expand...

I will try that first, I try not to rule out anything even if the part is fairly new,

If I put my hand on the engine it feels as though it is in the engine. If i drive it it feels like a tranny mount/center bearing.
 
one thing i forgot to mention was that i also had the crankshaft dampener relplaced on my car also the old one was wore out as the rubber was dry rotted and it was almost at the point where the innner pulled would have started slipping.
 
vibration

black 940 said:
Have any of you experienced this. My '93 945 T resonates terribly at highway speeds, if you let off the gas it stops. Basically anytime above 2500 rpm. Any ideas what to check?

The car has the folowing new items.
Engine mounts
Trans mount
Engine.
Click to expand...

make sure you have the bracket on the exhaust pipe to the trans bell housing conected. mine was missing when i bought my car. i bought one and it cut the noise in 1/2 but it still is there it is almost like the engine does not have a crankshaft balancer on it . i have a 78 242 with an aoutmatic and it have the vibration - noise around 55mph mine desent have a tach so i cant give you rpms but it also does it before it shifts if i run it up to 70 or 75 it seems like it goes away?
 
I put all new bushings on my accesories and that helped the problem a good deal, however even with the new engine (2nd one), it is still there. My trans. bracket is there. Have any of you heard of the dampening box on the firewall going bad and causing this?
 
black 940 said:
I put all new bushings on my accesories and that helped the problem a good deal, however even with the new engine (2nd one), it is still there. My trans. bracket is there. Have any of you heard of the dampening box on the firewall going bad and causing this?
Click to expand...


that 'damping box' is just a hunk of heavy metal, no moving parts.
 
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