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Ls 400 Suddenly Has No Acceleration Ability


tjcooper
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Have a 1992 LS400 with 150K miles on it. Have replaced most major engine parts in the last two years.

Returning from a 100 mile trip with engine running fine and without warning, the car could barely make it up a freeway ramp. Almost no acceleration at all. Limped home and checked error codes. Had a 52 and a 71. The 71 comes from a broken connection to the EGR value and so I know why that is there. The 52 is left knock sensor. Ohmmed out the sensor and lines to the ECU and they were not shorted to ground. Tested the knock sensor on oscilloscope with a vibrator giving "shock". The electrical signal looked like I had voltage at the 60Hz of the vibrator. I replaced both the right and left knock sensors and the error codes went away. Did a J&B weld fix on EGR valve tubing so I could get vacuum again. The error codes all went away. But the engine still has no acceleration. Any hints? I think I may need to do a swap on ECU to see if that is problem. What else has such a strong effect on acceleration? The fuel smell is strong in the exhaust so I know I have fuel. Spark looks OK. I have to think it is the ECU thinking something in the system has failed and is putting the spark advance in the "worst case failure mode" to just let the car limp home.

tjcooper

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Sounds like ECU has reverted to limp mode a safety feature to allow you to drive to safety when a problem occurs.

If its in Limp mode there should be error codes indicating what triggered the event. You could disconnecting the battery for a bit to see if reseting the system that way helps.

Maybe you've got MAF sensor problems but that is just a guess. See if the car runs the same (equally bad that is) with the MAF unplugged. Since EFI Systems are called closed loop systems it makes it tricky to solve these problems because anything within the control loop can mess things up....

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If you can smell unburnt fuel from the exhaust and given the lack of power check the driver's side ignition coil as this is a common fault,another symptom of this failing is red hot cats after a run.

This fault may not flag a fault code but a misfire on 4 cylinders may confuse the knock sensors giving you the error you had.

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