UNCJD

Hey, everybody,

My '95 LS400 has 254,600 miles on it. I trailered it in from two counties away back in August, did some things to bring her back to life, and what started out as a quick flip became a "backup" car that I drive 80-90 percent of the time because it's so much more comfy than my Civic. Since I'm driving it more, I'm getting picky about things I normally wouldn't care about in a $500 Craigslist rehab.
Here's what it's doing this time around: In park, it starts to vibrate noticeably from 1800-2200 rpm. Sounds rough, feels rough. Not the smoothest thing in the world when in gear and accelerating, but it does seem better in gear. At idle, it's fine. Acceleration is about what I've been told to accept. No jerking, hesitating, sputtering, or anything like that. It just goes from smooth(ish) to rough at those rpms. The vibrations seem to diminish if I push it up to 3-3500.
No CEL, no codes and the CEL works just fine. Recently replaced both cats, all O2 sensors, fuel filter, PCV valve, cleaned the MAF sensor, repaced ECT sensor, air filter, plugs, wires, distributors and rotors. Ran a can of B12 Chemtool through the vacuum hose, a tank or two of Seafoam and one of Regane. New battery. New, heavier-gauge negative battery cable. New alternator (this thing was SCREWED when I got it). The old plugs were in crappy shape, but after 2,000 miles I checked the new plugs and they weren't damaged or oil-fouled. I had replaced the valve cover gaskets after seeing oil on the old plugs. The old PCV valve had failed, and after that and the VCGs, the oil seems to stay where it needs to be. No smoking or funny smells. It gets between 19-22 mpg combined city/highway, but that's going to be going down as we're getting into air conditioning season. I've had it as low as 18 combined. I use only premium gas.
I don't want to start throwing parts at it, so what should I test? What could this possibly be?
Of an incidental note, when I first got it and was learning, I jumped it off with cables reversed and screwed up all kinds of things that I had to get a real mechanic to fix. I've got the Digikey parts to do a computer rebuild, if you think that's what it could be.
Sorry for the long column; any thoughts would be appreciated.

m0ist

Check the engine mounts

Raid05

engine mounts

akitomk3

iTrader: (2)

What these guys said LOL, i'd check the engine mounts then the trans mount too!

jaaa

tranny mount first, it's easier

dicer

"Sounds rough" wish there was a video. Did it always do this since you got it or did it develop?

Legender

So, you haven't done the ECM cap replacements yet? That's a known issue that can cause all sorts of weird issues. I'd get that done first and then see how she runs. And you need to do it anyways.
Engine and tranny mounts for sure if they've not been done. Careful of that engine mount replacement process or you'll break your heater valve mounted near the hood on the back firewall.

UNCJD

So the tranny mount was in 2 pieces. Motor mounts looked and felt good, even after I unhooked the bottoms and jacked the motor up. Whether they ARE good or not, idk; couldn't undo the top retaining nuts. So I buttoned everything back up and started her up. The vibration is gone. Took it around the block, and all I have is a vague exhaust leak that, praise God, sounds louder at the rear of the car than at the front.
Kind of scared to do the ECU caps without a backup computer, but that's expensive. Might have to beef up my soldering skills and give it a go.
Thanks for the help, folks. Tranny mount, by the way, took less time than my typical oil change!

Legender

Easy fix... glad you found the source of your problem.

ECU caps should be done sooner rather than later so they don't cause other problems should they already be leaking. Later could be more expensive than sooner. But if this is your only ride, then I understand your dilemma.