Hello,
My 96 LT1 with auto has a vibration starting at 3K rpm. I can feel it through the gas pedal and its pretty bad. I replaced the harmonic balancer and still the same. Today I just kept it in 1st gear and drove it up to amost redline and noticed that it only vibrated when I was on the gas. If I let off or just give some gas the vibration is gone no matter what rpm. So the vib seems like its there only under load.
This tells me its not a pulley or something that spins since that would vibrate with or without load. The car just hit 90K miles. The engine is crystal smooth up to 3K rpm. After 3K its pretty bad but then gets better (maybe 4K less vib) and then starts again. I don't think this is tranny related like the input shaft, torque converter ect... I've had them go bad on me before and this definetly feels like the engine. Could this be wires or plugs or something spark related? I ordered a new PVC valve and EGR valve to install hoping maybe its egr related. Thanks for any suggestions.
Craig

 

Simple things first: Check your motor and tranny mounts.

EDIT: Check the driver side motor mount first BTW. It's usually the first to go since that is where the motors torque causes allt he stress and it's also easier to get at.

 

Thanks for the quick reply. What is the best place to jack up the engine? The front cross-member? How much play should there be in the mount?... basically how much should the engine move?
thanks

 

get a board or something to place under the oil pan. The cross member isn't going anywhere - it's part of hte frame.

There shouldn't be a whole lot of play, maybe 2 inches at most. You should be able to see if the rubber is torn...bring a flashlight with you. Depends on how bad it is and where the rubber is ripped/seperated but you may need to remove the motor mount support bolt and jack the enigne up then try prying ont he mount with a large screwdriver or something to test it.

EDIT: Another tip: if you are going to unbolt hte motor mount don't do it with the engine setting down...ther ei soging ot be too much pressure ont eh bolt. After undoing the nut ont he motor mount botl you will probalby need to jack the engine up about an inch in order for there to be enough slack to pull the bolt out with your hand. Good luck

 

thanks again.
I don't know what I was thinking about with the brace.. duh..
My shop manual actual says not to lift it by the pan... that a sensor would be damaged inside the pan. I would think the a piece of wood would spread out the force though.
I'm going outside to try it out now. I'll let you know how it turns out.
thanks

 

OK, just jacked up the engine and the whole car went with it. No play at all. I was sure that the wood was just under the pan, not the frame. Looking at the mounts themselves, they seemed to be in perfect shape.

 

OK, even with brand new motor mounts you should have a little play - they are rubber...the entire reason they are rubber is to GIVE the engine play. For example i get a good 1.5 inches before i'm unabel to jack up my motor any more.

The only sensor in the oil pan is the oul level sensor - i have jacked my motor up multiple times using a 2 inche thick board across the panwithout incident (ironically, it was when i pulled my pan OFF for the rebuild that i broke the sutpid sensor). You just dont' want to put a jack direclty under the sensor and jack, becuase the pan WILL give and screw up the sensor - so in that aspect the book is right. But with a board the load should be well distributed on the pan.

The pan itself should be easy to identify fromt he cross member. The cross member is huge thick steel. The oil pan is where you drain the oil from your engine and should have sort of a plstic type feel when you tap on it (very low grade steel).

Another possibility is your tranyn mount - however with the trannby mount you would be feeling the vibration from underneat the shifter between the driver and passenger seats primarily (you would feel it in the gas pedal too though). So that's another to check for.

 

Well I jacked up the engine again... a good 6 or 8 inches or more and had my father look for movement and there was basically none. Then he jacked it an I looked soley at the driver mount and didn't see anything. I've changed out tranny mounts before... my thirdgen with tci tranny used to rip right through them because of the firm shifts. The tranny mount appears to be fine visually. I don't get any clunking going into gear, etc or vib under the floorboards. It also passed the crowbar test. Any other ideas?
thanks.

 

I wonder if the vibration you are feeling is the engine missing really bad - in which case we coudl be dealing with a plug wire/opti issue.

 

Yes, smooth as silk until 3K.

 

If it was a miss would a code be set with OBDII? Also I don't see the tach jump at all.

 

Well if it feels like a bad miss, then i'd probably go with opti if you got 90k miles on the clock, i'm assuming it's probably never been changed. Other than that the only thing i can think of is maybe worn cam/crank bearings causing issues. Remember when you let off the gas and the engine isn't under load there is going to be less stress on the bearing as the engine winds down which could explain why letting off the gas sovles the problem. It's hard to say without being in the car and feeling it. For example, I had some cam bearing worn down to the copper, i experienced bad engien vibrations from about 1800-2100 RPMs.

So i guess the question is does it feel like a bad miss? Maybe someone else can chime in with more thoughts. I'm pretty tired from staring at comptuer screens at work

 

I believe a miss is supposed to throw an SES light. But I'm not possitive 1) that is the case and 2) if that is the case what criteria is used to determine whether to throw an SES light or not on a miss. I've only scanned and programmed OBD-I myself.

 

I just hope its not the crank bearings...
About every 5 times I start the car after its been sitting for a day I'll hear them rattle for 1/2 a second until oil pressure it up. I can only hear them if I concentrate with the no background noise and the door open. How common is this? Every other 305/350 I've had went over 200K with much more abuse than what I've put to this thing. LT1s do use the same bottom end as gen 1 right?

 

LT-1 blocks are 2-bolt mains, same material you'd find in another small block. The crank in an LT-1 is cast with a different process than the gen I small blocks and is a little stronger. As far as bearings i don't remember if there is change in the LT-1 bearings compared to the TPI engins of the 80s. When i broke my LT-1 down at 135k miles the crank bearings weren't too bad ( my car was a hwy patrol vehicle for the first 90k miles of it's life...i'm sure they weren't too easy on it). It was the cam bearings (especially the front) that were bad. There was a thread the other day with some speculation concering the quality of Lt-1 cam bearings...but many people seem to go to 150k+ miles without incident. I made it to 135k and only had issues after a cam swap. My '94 T/A i recently sold was also running great at 116k miles.

After 3k RPM under throttle how is your oil pressure?