vibration at wot above 80mph

boogeyman

Registered User
my 90 sc has a odd vibration at wot above 80mph if i let off it goes away and as soon as i get back on the throttle it comes back my balancer is new and the u joints seem ok any idea`s im stumped
 
the engine mounts and trans mounts are fresh also i did a oil change on it and found the pan to be very close to the subframe so i changes all 3 mounts at once
 
What gears and driveshaft do you have. The shaft maybe vibrating. My stock shaft would start to vib at 80 with 3.73s then I had it balanced and it would vib at 90.
 
How bad were the old mounts?

Was the vibration present before you replaced the mounts?

What type new mounts did you use?
 
ignition breakdown

Is it dropping a cylinder ? If it only shakes at wot, I would rule out drivetrain vibration. Years ago my 90 would shake a little under wot, but in any gear. Plug wires fixed it.
 
the old mounts were not too bad just sagging a bit
the driveshaft is the stock steel piece the gears are 3.23
i think im gonna try a screaming demon coil and wires and some new plugs soon im not making tons of power right now but i like to have ignition overkill on a boosted car

im putting a 255 FI pump in the car some 42 lb injectors and a 75m tb on it next week along with intake gaskets and a ported lower intake but this vibration is killing me it pulls hard up to 80 then starts the vibration and i have to get out of it
 
i think im gonna try a screaming demon coil and wires and some new plugs soon im not making tons of power right now but i like to have ignition overkill on a boosted car.

Screaming demon coil isn't overkill its a mistake. I do not know of anyone who tried it and thought it helped. In fact I know of a few circumstances where it caused problems and a stock coil fixed the problem. I believe all of the over 400 rwhp runs
stock coils.
 
I have the exact same issue that I have not been able to figure it out. except mine isn't only at wot, its on decel too. Its a vicious vibration that starts in 3rd gear(auto), it doesn't go away with higher speed, and it goes away below 80 ish. I made a huge change to my car all at once, so its hard to pinpoint what I did that made this issue arise. its a very rapid vibration, almost like an out of balance grinding wheel on a bench and the whole bench shakes chatters. :confused:
 
Check the rear diff mounts and the rear axles. I have had both of those cause a vibration only when on the gas at highway speeds. If both of those check out, check out rear wheel bearings, and all your rear suspension bushings.
 
ok so no in th supposed good coil then.
im thinking im going to pull the shaft and have new joints put in it and have it balanced and checked out when i do the fuel pump.
how would i check the rear axle`s grab them and make sure there is no play ??
i just recently replaced the front diff bushings with poly units but the problem did not change
its really starting to puzzle me since it seems to nearly stop accellerating when the vibration starts
 
I had a vibration kind of like that, but it didn't stop accelerating. Turned out the rear U-joint hardware was loosening on the differential yolk. the joint was good, but under power was able to shift off center just enough.
Good luck, hope you fine it.
Kurt
 
mostly u joints and drive shafts arent an issue because diff doesnt move .ive never had probs in 250k miles
to me its mostley a plug or wire missing or clogged cat causing issues

ralph
 
i made a hard pull tonite after tightening the axle nuts breaking loose the rear driveshaft flange bolts and re tightening. going over the wires and pulling all plugs and changing them checking out rear wheel bearings and they felt good
it ran to 110 nice and hard then kinda fell off and started vibrating got off the throttle and it went away and as soon as i started getting back on it it came back.
im thinking it has to be that the car is leaning out since i have not yet put in my injectors and fuel pump yet im thinking im just getting a lean condition and its missing a tiny bit but im open to other idea`s
 
Tranny slip/ clutch chatter

If the engine and ignition checks out, is it possible the tranny is slipping? Clutch chatter? Give us a little better description. Does it act up after it shifts into OD? Is it in 3rd gear? I have 4:11's in mine and IIRC at 80 mph WOT it's still in 3rd. Let off the throttle a bit then it will hit OD. Really sounds like the engine is falling on it's face, the vibration is secondary.
Good luck,
JJ
 
the trans is pretty new and has good parts in it it only does it in 3rd gear at 110+ im going to reduce the plug gap down a bit more and see if that has any effect the DIS and coil are both new stock parts the crank sensor is 1 month old the cam sensor is new as of last week did a fuel filter about a month ago it does have a tune on it but i tried it with the chip out and i get exact same results but takes much longer to get to 110 lol
my boost in any gear runs up to about 13psi at 4000 or more i have stock pulleys on it has its a stage 1 morana engine mild ported heads forged pistons stock cam 76mm mass air , 3 in tube 9 inch cone filter w divider 2-1/2 exhaust into magnaflow resontor to 3 in split to dual 2-1/2 to dual magnaflows, heavy duty intercooler fan, car runs great other than the vibration up top i get good milage
 
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Some of the cars have the rear pinion marked and the DS marked for alignment. I have heard of it making a difference on some of the MN12s, but never ran into myself.
 
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