Okay, I'm not new to this site, as I've used it for a few years now helping diagnose any problem or questions I have.

In any event, I've got a 98 Z28 intake, headers, and maybe some internal work? Anywho since I've owned this car I have had so many problems and as soon as I fix and button up one, im chasing another.

I first got the car with the only stated issue being a bad power steering pump. Funny thing is that has never bothered me enough to fix. Let's get to the chase. A dreaded vibration... under WOT it wil vibrate pretty bad and make a rattling among other pleasant noises. So first thing I thought was rear end, it felt like it was coming from behind me at first. Ended up breaking my pinion gear while test driving the car with a driveline specialist. So I got it rebuilt with 3.73 gears. Just like it had before. Then still had a vibe to it and it was close to the end of driving season as winter was upon us. I parked it for the winter knowing I had to do some work to it. Never got around to working on it through the winter. I took it out after about 5 months, and after a week of it freshly being out I moved to Las Vegas. Road tripped it all the way from Minnesota, actually wasn't too bad, I had the cut out open the whole time. When I made it I needed to do a tune up, and needed to replace my MAF fuse. Cutting to the chase here. I have replaced my motor mounts, oems, trans mounts poly, torque arm bushings, u-joints, and made sure my piņon angle was correct, and my input and output shaft was within 3 degrees parallel also made sure the drive line was balanced. So after all that I still had a vibe. It's always there, park neutral, worse between 1800-3000 and it's horrible sounding, so the driveline shop said to check my torque converter or my harmonic balancer. So I got under it and watched the balancer. Had a wobble but wasn't sure if the belt writing was playing tricks on me, removed belt, and it looked to clearly wobble at that point. So I said hell yeah finally got it figured out. NOPE, removed and replaced, the proper way... Seen some weird ways others had on here haha. Still vibes same exact feeling as before. If any one has any suggestions or questions to help diagnose further please God ask, or help advise me what to do. I'm ready to roll it off a cliff and collect a check. Funny thing is I'm still winning races. I'm aware I shouldn't be beating on it, but it doesn't seem to affect performance for now anyway. Any help would be appreciated. Car is an auto. Torque converter is next.

 

Ever had the tires checked? Might simply have a bad rim on one.

 

you say the vibration is there in park and neutral.is that while the car is moving or at a stop, rev the motor to 1800-3000 rpms? if you are moving it could be a slipped belt or egg in a tire, unbalanced tires, wheel bearings, panhard bar or lca bushings, something could be lose. put it on a lift and check all suspension bolts and bushings for too much wear.

 

If it's doing it in park car isn't moving. If you're winning races, that shouldn't indicate misfires.

Two things I'd check are plugged cats and out of balance flex plate or torque converter. In park I'd think flex plate more likely.

The best damper IMO is the ATI. Be worth swapping that if you're still stock.

But of a long shot here but your driveshaft might be twisted slightly so the u joints are out of phase. Wouldn't explain P/N vibrations though which is why the long shot disclaimer.

 

Okay so it's not the tires I can rule that out. It does it in all gears, park, and neutral, between 2-3k. I had it on a GM TECH 2 Scanner tonight, isolated all the injectors, and coils. I had it on the scanner for over an hour, I checked everything and had nothing, until the fourth time on a misfire graph and I had it bouncing around 3-4K and the second cylinder was showing a miss fire. Obviously couldn't really do much as the engine was hot. I had a fresh tune up in March, replaced plugs and wires, can't remember what ones �� Mid grade I think. I'm going to pull the plug tomorrow after work and inspect it, also thinking it could be a bent push rod? Not sure, how deep I'm going to get until this weekend, but I want to pull the valve covers if it isn't this plug. I'll keep you posted, it's definetly the engine though that I'm positive of.

-New OE motor mounts
-New tranny mounts poly
-new torque arm mounts and bushings
-new U-joints
-took it to a driveline specialist in Las Vegas, they made sure the pinon gear was set correctly, and the input and out out shafts were within 3 degrees of parallel, also made sure drive shaft was balanced
-just replaced the harmonic balancer Sunday night
-I'm ruling out the TC for now, I have no problems shifting, no noises or pings pops nothing weird. I would like to make sure the TC bolts are all there and tight, but I really doubt it's that.

I'll update tomorrow after work. Thanks for the help so far, I appreciate it.

 

I think it's a dead misfire, won't throw a code. I want to make sure that it's tight, and the plug is undamaged, luckily it's #2 cylinder. I was in a pinch to get it fixed so I went with OE balancer. No cats. Hooker long tubes.

 

I'd think a bent pushrod wouldn't explain why it is there some moments and not others. A bad connection to the spark plug, or just a bad plug in general might. Or possibly a loose vacuum hose. A bad or clogged injector. It seems the engine is likely the culprit at this point though

 

It really smooths out after it gets through 3k. It's weird guys, hard to explain almost. Im positive the tires have nothing to do with it. Or any suspension components, it vibes stagnant or moving. It wasn't until tonight that I realized a misfire maybe causing. I initially ruled it out because I recently replaced my plugs and wires, and just now I remembered I had a mobile car was service detail the engine, most likely had a pressure washer in there so who knows what that may have done. Still won't be sure until I check the plug. The vibration & high temps out here caused the dash to crack also. So sometimes you can't win haha! It's a labor of love though. Refuse to sell it, and give up.

 

Luckily I work with a guy who used to work at a Chevy dealership in town, and he hooked up the scanner, but even he was baffled. the only thing that engine was showing to be a problem was a misfire, but it took a long time to get it to misfire. Also doesn't matter if the engine is cold or warm still has the same vibration.

 

It makes no odd noises either, I don't think it's lifters or valve springs, or even pushrods, but I'd still like to dig into it and see.

 

Update. Checked the #2 plug, no visual damage, clip and gap was fine. Was a little loose. Didn't seem to change after I tightened it all the way down this time. Maybe a sticky lifter? Pray for me.

 

Update numero two for the night. Got home jacked the damn thing up, wanted to check the header clearance over the k member. At first I thought I figured it out.


Looks super close less than a 1/4 " clearance. You can see where it's rubbed itself down. So I had the neighbor come over and rev it to vibration, but didn't look to touch at all. At this point I just don't know. Still going to unbolt the TC this weekend and see if it goes away or not to rule that out. Flex plate maybe? What would a bad flex plate feel like?

 

Update. It's not the weekend yet, but I got under it tonight, stuck the throttle to 2200 rpm, laid underneath, had my hand on the oil pan, couldn't feel a vibration. Moved it to the transmission pan, and it was vibrating like crazy. I'm thinking it's the torque converter at this point. Also noticed a new noise tonight. Windows down quiet road, no music I was listening and took off from a dead stop, and could feel/hear a cluck right under my feet. Very slight, and it happened quickly. I'm going to unbolt the TC Saturday afternoon and see what happens. Wish me luck.

 

If anybody's been following I've got an update. I dropped my started, and pulled off the plastic inspection cover I got to the first TC bolt, and it was an effing Allen head smh. I knew it didn't look right, so I grabbed an Allen socket, and fumbled my ratchet down to my chest, I stuck my hand up toward the flex plate to feel the TC bolt.... it moved freely. I got some red loctite, and wrenched it down, checked the rest but they seemed to be fine so I put it all together, vibe was still there. So I took it all apart and checked to make sure the bolts stayed tight. They did, so I'm guessing it's internal or the flexplate is cracked. I've ordered a new SFI certified flex plate, and a 2800 stall converter from summit. It's an FTI, also ordered a new borla muffler, so I'm looking forward to the weekend. Wish me luck. Also made sure to order new bolts for the flex plate and TC. Praying the trans is alright.

 

OP, to answer your question an out of balance flex plate would feel like an out of balance wheel except it would vibrate at engine speed not wheel speed.

Engines tend to balance themselves at higher rpm, which will help explain why it gets better with higher rpm. Just like on the highway there are certain speeds where vibration is worst.

 

Yeah it makes scenes, I'm just hoping this is it. When I was looking at it the plate seemed OK... I could onl see a slice of it though, I think my parts are coming today, shipped from sparks Nevada at like 11 pm, I'm in vegas so fingers crossed.