Hey guys, [1991 LS400 73000miles]
It was really cold out last night and I noticed that once the car was completely warmed up, as I reved the engine in Park or Neutral up to around 2200-3000 RPM and let go, the tach needle would fall down and for a moment it would go below the usual idle speed (from 700rpm to maybe 550-600rpm) and 2 seconds later it was back up to 700 again. It didn't quite feel like the car was going to stall, nevertheless it did create some vibration though. What could this be? Most likely not the tranny, because it was in P or N. I don't think the car has had tune-up yet (no new spark plugs etc.). Could it need a tune-up? Never happened before. What could this be? I appreciate your response. (if you need to ask me more questions, let me know).

 

Mine seems to idle kind of low too, no vibration, but still not used to a car idling at like 500, this is when it is in (D) though, most cars idle higher in N or P cause they're not under load.

What's the normal idle in D supposed to be?

 

Mine is perfectly fine at any other time though. In D the RPM is also where it's supposed to be (700RPM). What I described happened last night for the first time and it happens once engine is warmed up.