My 97 ls 400 is doing something at higher speeds. Sounds like the rear end. I changed out the rear diff fluit with Mobil 1 70-90 LS and I'm wondering if that was a mistake?

 

Can you be more specific? Did the problems start immediately after the fluid change? Did you fill the differential until the new fluid started to come out of the filler hole?

http://www.lexls.com/tutorials/suspe...iffchange.html

 

check the bushings..
if the car is shimmying at high speed most likely rear diff bushing is bad.

 

This sounds about right. I don't drive the car but occasionally. It has done this to me on the interstate at high speed. I noticed a vibration coming from the rear. Then a sudden jerk. Like the power was shut off hitting the rev limiter. My daughter said it was doing this all the way to school this morning. I thought the slight vibration was the new tires I had put on not being balanced correctly. Would this bushing cause the jerking?

Thank you in advance

 

Hate to revive this thread but...........Still dealing with this. I don't think the trans has anything to do with the jerking at this point. Saturday morning I took the car to 7-11 for coffee. Our first cold front of the year here in FL, a brisk 65 and lower humidity! The car really didn't like it! At first touch of the throttle the car would hesitate, and then if I continued to accelerate slowly the car would start the jerking. Though if I stab the accelerator with authority it would take off and run through the gears. 40mph is top speed where I live to give an idea of driving. Once I got coffee and left the 7-11 it was fine. The jerking really seems related to the amount of throttle given no matter what speed. TPS sensor? Cel comes and goes, each time read it is right side O2 sensor, maybe that's the cause but I doubt it. I also found the below paragraph relating to knock sensors. Description is pretty spot on except mine only jerks under light acceleration or maintaining speeds. And at high speeds when it happens I get a loud bang that sound like it's coming from the rear. The speed limiter description below is spot on. Like jerking from a manual trans by someone not good with a stick.

How will I know if I have a faulty knock sensor?

We're not being funny here, but you would experience engine knocking and vibration. The knocking can become increasingly loud where you can hear it over your engine noise. If that doesn't happen, you will experience rough acceleration with a jerking sensation like you are hitting a speed limiter. You may also experience misfires, backfires, or smell a strong engine exhaust or burning smell from the pre-ignition happening in the cylinders.