Quack11

I was able to get under the car and did some additional test and checks. Engine, transmission and drive train mounts are good with absolutely no problem. With the wheels off the ground I got her up to 45 MPH and there was NO vibration, car went through all gears up and down as it should all the way up to 80 MPH. Above 45 MPH it got even smoother but I did notice a rattling noise that increase/decrease with speed. MY CONCLUSION: The vibration changes to rattling only if the rear wheels are on the ground and is caused by a defective differential. The rattling is CLEARLY coming from the differential and is very obvious by both audio and feeling. There are no excessive backlash at the input pinion or output area, this means the internal components are breaking down and sending the vibration to the vehicle. The drain and refill plugs are not moving one bit. I'm totally ruling out the xmsn (transmission) drive train and mounts unless there is a logical and mechanical explanation. The experts please chime in.

Landar, thanks for your post. I just went out again and drove the car for maybe six miles. I went though a series of what I considered "Transmission isolation" checks (My term). During normal take off and driving the car does the same thing right around 45 MPH then significantly reduces the vibration as speed increases above 48-50 MPH. I used the term "significantly" because the vibration can still be felt but very lightly or may be un-noticeable to a non-mechanically minded person. I stopped and rapidly accelerated up to 65 MPH and the car drove as new. I repeated this same procedure with the ECM switch on and it drove as it is supposed to. During various driving conditions I turned on/0ff the O/D and the tranny responded on as it should. What I did noticed is that as the car decelerates and reaches within the 45 MPH zone, the vibration starts but the RPM needle is dead steady with absolutely no erratic reading and the engine does not struggle one bit. However, the gear shifter is like an unbalanced washing machine. To answer your other questions, I last did a full tune up on this car maybe a year ago. Plugs, wires, caps, rotors, etc was changed but the car drove maybe 1500 miles since then. Tranny oil change is approaching four years with less than 5000 miles since it was changed. The car was parked for almost a year without being driven 100 miles. I cleaned the intake, TPS, MAP, inspected filters, hoses, etc and everything seem to be in good shape. This vibration only occurs at a certain RPM/MPH which leads me to think it is related to a dynamic component either out of balance or unsecured. When I get a chance I will inspect the transmission and drive train for obvious issues that could cause this. I'm thinking tranny and/or drive shaft mount is bad since these parts are still original and has been in the ice/heat since birth. I'm also ready to give her up for adoption to a new family in the MD State.

Hi all, my 91 LS400 with 150,820 original miles started acting a fool a few weeks ago. I noticed that at 42-45 MPH the car just start to jerk real bad but the engine does not miss, it does not cut off, there is no CEL, RPM is not affected and there are no other indications. If I let off the gas pedal and depress it, the car drives normal or I can keep my foot on the pedal and it will slooooowly go away between 48-51 MPH. I've also noticed that if I start off at a very fast speed and get up to 60+ MPH, the car does not have this problem, only during normal driving/acceleration conditions. I'm the second owner for the past 13 years, the vehicle is well maintained, in very good condition with only a non-working A/C that I refuse to fix. Any ideas or assistance is welcomed. This baby is a light driver with only 78K miles in thirteen (13) years. I'm also looking for a new owner.

I agree with Steve, you have defective shift solenoid which are inside the sump case. The are around $50.00 a piece and is easy to change, you will have to remove the sump pan/oil pan and they are sitting at the bottom. I had to change mine a few years ago for the same problem. Good luck.

Red Stripe beer sounds great right about now, glad you are back in business.

I think you should should go where your pocket/budget will take you. They are both good cars but I personally stay clear of air suspension.

Non-OEM works just as good in my past experience, Partsgeek will have all you need.

Good call, the left rear caliper (original part) was difficult to push back with the C-Clamp so I will be replacing it this weekend.

Problem solved......hopefully. I replaced the four rotors with brand new one but the rear rotors are defective. I returned them for replacements and the problem is gone. Just got off our local high way and at 90+ she is smooth sailing and stops like a parachute was attach to the back. At 141+K miles, I can't believe how smooth and quiet this car is. The suspension still absorb every defect in the road and the cabin is still sound proof. How much longer can these cars go? I'm the second owner and the car is still 97% original. Thanks all.

There are two small phillips head screws that secure each rotor to the hub. Can the vibration I described be created if none of these screws (four total for both front rotors)are installed?

Thanks Andre, I was questioning the brake being overheating due to a bluish color on the rotors which is an indication of excessive heat. I will check this on my next run. The tires are in excellent condition with no deformation of any kind so i've ruled out any bad tires idea. Will keep the forum updated on any progress.

I balanced the wheel and rotated them three times in the last week since problem started. The car was driven maybe a total of 50 miles in the last month and the road driven is in great condition. No maintenance was done on it prior to this issue. Transmission shifts as advertised, no issues so that is why I narrowed it down to drive shaft mount, plus I inspected the center support and it does not look too healthy, the donut portion has many small cracks and it is not centered but I have nothing to reference it to.

I also did a wheel balance and alignment with inspection of all suspension bushings, everything looks good.

91 LS 400 with 141K and in excellent running condition until last week. Car vibrates real bad above 65 MPH and sometimes at slower speed. The car does the same when the brake is applied, the problem is so bad you can see the gear shifter moving back and forth. This does not happen all the time and I've noticed that if I take off rapidly, the car drives awesome until its time to stop. I've changed both front and rear brake pads and rotors. Inspected the entire front and rear suspension, checked engine, transmission, differential and drive shaft mounts. I am not too satisfied with the condition of the drive shaft mount and the differential mounting bushing. Any assistance will be greatly appreciated. I searched the forum for previous post on this issue but did not find any. Thanks.

I agree, this should fix your problem. Pay more attention to the driver's door switch.