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91 Ls400 Jerking At 45 Mph.


Quack11
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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.

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Well, Quack, if you are correct that RPM is not affected, then I would say the transmission is the culprit. However, you could be tricked with the RPM needle being dampened so that it cannot follow a quick stutter of the engine. That is what I suspect. So, the first thing that comes to mind are plugs, wires and caps/rotors.

There are a myriad of other issues that can affect a 91, like trunk hinge wiring, bad ECU caps, dirty throttle body, Throttle position sensor....etc. But always suspect the 'simple' things first. When was the last time the plugs, wires and caps/rotors were changed?

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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.

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Oh, so it is vibration rather than a real bad jerking? That's a big difference. Yes, I would now inspect your transmission mounts, engine mounts and driveshaft. Knowing it is vibration, I would give the engine a clean bill of health and focus on the other parts of the drive train. Try to determine if the vibration is worse under deceleration or very light acceleration around the 45 mph range. That info can further serve to isolate the issue.

So, how much do you want for her? My wife would kill me, but I might know of someone who is interested. ;-)

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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.

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Seeing as how it sat for that long, I'd look to the gasoline as the culprit since you are not getting a CEL. Perhaps try a complete fuel system cleaner. Gasoline lasts less than 6 months before it begins tom turn to varnish and then all kinds of bad things can happen. I'd try the cleaner (the $10 one) and a little WOT. With a little luck, it may clear up after driving awhile.

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