Yamae

This is a nice write up. However I always wonder why people just replace sensors without confirming the resistance or the function which can be checked easily by a multimeter.

Sounds like an ECU issue to me.

I suspect that the ECU has leaking capacitors.

Changed the ecu? Then you can not start the engine without writing the security data or changing the key. The immobilizer prevents to start the engine.

I had a similar problem but the vibration was also noticeable when not running. I mean, I noticed the vibration at 2500rpm when parked and the gear position was P or N. It was initially only at a spot rpm but as months passed it turned out to be 2500rpm ± 100rpm then to ± 200 rpm. I also felt that the engine was misfiring because the engine sounded a bit rough when revving between 2250rpm to 2750rpm. I have checked several items but the conclusion was simply the worn transmission mount. After I replaced it, the vibration was disappeared as well as the car became quieter than before. The symptom I thought the misfiring was caused by the noise from the engine mount since the metal and the metal at the mount was barely touching because of the compressed and cracked rubber.

Aged cars tend to have problems related to electronics components. This is just about general affairs but let me introduce my experience. Old Celsiors in Japan tend to fail ECUs, Climate control units, clusters and etc. The situations are just the same in US especially at higher temperature states. I am working as a volunteer at different car repairing shops to help them to fix units which have electronics components inside. Most of workers and owners at shops are not familiar to electronics related affairs and they just try to replace the whole unit. New units are expensive and not available easily. Used units picked up from a junk yard are often used instead and these cause other problems time to time. I have seen many shop owners were not happy with those situations. One of the biggest problem is worn electrolytic capacitors. They tend to fail as aged. Some are leaking the liquid but most of them can not be recognized at a glance. Especially the Type "PF" used for early LS400 has this tendency due to the fact that the infamous liquid called the ammonium chloride was in use.The liquid inside is leaking or evaporating little by little and the capacitance is getting less with the higher impedance. Sometimes leaked liquid damages the board. Toyota has started to use this type of electrolytic capacitors a lot in late 80s and you can not escape from the problem caused by those capacitors. The LS400 is also in use of this type of electrolytic capacitors and you will be encountering the problem soon or later. When you need to use some old used unit, check inside to see the electrolytic capacitors are OK. I recommend you to replace all the electrolytic capacitors used inside.

This is about my 98 Celsior (JDM) and I used to have exactly the same problem mentioned above. Some people here were claiming that it was a nature of 98-00 Celsior but Others were denying it saying ours are not like that. I happened to clean the throttle body aiming to improve the low idle rpm. It was sometimes below 300rpm or less at D. After the cleaning, the idle went up to 550rpm. Also the hesitation problem has gone away. I mean the poor acceleration after turning the corner then the sudden acceleration was not there any more. What a big difference, I noticed. I am new here and this is my 1st post. I am a Japanese electronics engineer who used to work at San Diego CA before. I am still not good at writing English but I have some good experience to fix cars especially electronics related areas such as ECUs, motor controllers and sensors. I will be posting more in the near future.