So my 98(w/ 2000 LS1) Z28 has.. well.. a pretty hefty vibration. In the process of my engine swap, I put solid motor mounts in. I got them for a steal even though the car is mostly stock aside from LTs, catback, egr/air delete, tb bypass, k&n lid. This car is pretty haggard if you can describe a car that way. 255k on the body, 80k on the engine/trans. The rack and pinion is a rebuilt unit with new tie rod ends and a freshly aligned front end.

Just.. how much should solid mounts increase the overall vibration of the car? I ask this question with some hesitation because I dont really know what to expect from solid motor mounts as Ive never used them before. Its certainly not un-driveable but it seems.. well just slightly excessive. It gets worse when I engage the AC compressor.. its like twice as bad. That said, the moment the car is not sitting still this vibration goes away. Any forward motion and poof! The car rides like normal.

I do believe that the headers(edit: actually its the y-pipe thats doing the rubbing. it is hitting this cross section in the rail right under the driver seat area) are rubbing up against the underside of the car. You can visibly see it and Im curious is there anything I can do about that? It has a polyurethane transmission mount.

Now, if I were to guess.. I believe this combination of "mounting" parts is all lending to this problem together. They are spohn solids and they are tall, the trans mount is energy suspension polyurethane.

That leads me to my next question. I seem to be getting a different type of vibration over 55 mph or so. Seems to be a pulsating vibration. If I had to guess I would say it would be related to the driveshaft or possibly imbalanced tires? The tires on the car came from my 93 as the stock ones on this are losing air and the set that are on it are practically brand new tires.

Also I noticed when I reinstalled the driveshaft that the u-joint to pinion yoke.. there was this very small, thin metal tab that ran under/in between(cant fully recall and couldnt find a picture) the actual u-joint itself, that had a break on one end of it. Mostly just seemed to help stabilize the u-joint as it changed direction so I figured it would be ok to drive on for a short period of time until I have the chance to get it fixed.

Is there any other possibility that Im missing? Or am I headed in the right direction as far as diagnosing this is concerned? As always thanks guys. I am absolutely infatuated with this LS1! It feels sooo much faster than the LT1. Also makes me want to swap one into my LT1 :P

 

you will get bad vibration from solid motor mounts. also will get added vibration using poly trans mount. from the way you explain, you have a bad/lose u joint. that will also add vibration that will be better felt through the poly mount.

 

Yeah. Im going to swap out the u-joints here this weekend and Ill report back about whether the vibration remains. I know the u-joint straps arent loose because I checked the torque on them and then double checked it.. Then a few days later I triple checked them after running for a little while.

 

Having had solid motor mounts and then going to urethane, yes you will get more noticeable vibrations with the solid. You also have the y-pipe rubbing up against the body/crossmember, that too will give you vibrations. I'd tackle and work on the mounts and exhaust clearance before suspecting anything else. And by your description, that thin metal tab on the u-joint is just a keeper to hold the end caps from falling off when the u-joint is disconnected. It has no other mechanical function to the u-joint.

 

I dont really mind the vibration.. it doesnt bother me at all. I thought the solids might be cool because people spoke of the vibration as if it enhanced the feel of the idle.. made it more "muscley".. but honestly it kind of makes the car seem like theres something mechanically wrong with it. Like my dad took the car for a spin once I had it running and I could tell he wasnt wanting to push it hard because I think he was worried about the vibration.

Now the headers. Theres plenty of room on the passenger side but the driver side header/y-pipe section is tucked firmly up against the floor in a 6-8 inch section and I took a crowbar to see how much I could move them.. and they didnt move but a centimeter.. maybe even less. Im tempted to just leave it be and not worry about it. Im likely to do more harm than good if I take the y pipe off and try to adjust the shape

 

I had this same problem 2 weeks ago, it's still there. vibration at idle. it was looked at for free by mechanics after I spent 430 dollars there to fix my oil leak. they said it wasn't a problem after looking at it for 25 minutes without hoisting it up. They said they lowered the idle and the motor mounts are all there, just old. the salesman at O'Reilly's autoparts store said yes, unless the rpms go up at idle, there doesn't seem to be a problem; a lot of times the exhaust on older cars vibrates & causes this vibration. this vibration-only-at-idle problem happened after I had my broken transm. mount replaced. I spent a lot of money at a shop and hope to learn how to do it myself here. I'm just going to leave it alone since it doesn't happen 100 % of the time at idle--sometimes it does, sometimes it doesn't. For these not all the time problems, it might be best to just not spend anything on it and save for a rainy day until, more pressing problems, God-forbid, surface,---problems that are more pressing and require immediate attention.