I have a 1997 LS-400 that was previously owned by my Grandmother, so while it's pretty old it has very few miles on it (~45000) and saw regular maintenance at the local dealership throughout its entire lifetime.
Recently, I started having an issue with the engine where, while the car was in neutral, the engine would rev and then idle cyclically. It will rev up to ~2300 rpm, then drop back down to ~1200 rpm, then repeat in about a 2 second cycle. At the same time, when the car is in drive, it feels like the acceleration assist is several times its usual intensity (it is harder to brake; with the brake fully depressed at a stop the engine is at a higher rpm than typical; when the car is driving but the gas pedal is not depressed, it travels much faster than usual). I took it in to the dealer and was told that his best guess was some sort of carbon buildup that needed to cleared out. His tone basically led me to believe that he has no idea what was wrong, but felt compelled to offer a suggestion.
This issue does not occur every time I turn the car on. More often than not, I start the car and it operates as it should without any of the symptoms described above. But when it does manifest the issue, it persists through turning the car off and on again, cycling gears, etc. The only reliable way I've found to get away from the problem once it starts is to leave the car turned off for 15+ minutes before turning the ignition on again.
Does anyone have any idea what the problem might be, and / or any way to resolve it? I would greatly appreciate it.
Thanks.
