Hi,

I've posted this message on another section of this site, but thought maybe more people visit this part, so I write again. This is what happened the other night with my 91 LS400 with 73000 miles on it. When the car has warmed up completely and it's in Park or Neutral, as I rev the engine up to around 2100-3000 RPM and let go, the tach needle drops down to about 500-600 and then back up to normal idle speed which is 700 RPM. This small decline doesn't quite feel like the car wants to stall, nevertheless it does cause some vibration and disturbing the super smooth lexus engine feel. The decline lasts for about a second or two and then goes back to 700 RPM and everything is normal again. What could this be? Most likely not the tranny (remember, tranny is in P or N). Weak spark plugs(tune-up?) ? what?
appreciate your response.

 

So far, the only thing that sounds abnormal is the vibration. Not the idle speed control. With that, I'd say have the motor and trans mount inspected. Very common repair for older LS.

 

Can I ask why you are reving the engine ?
To me , it's a waste of gas and unlike the OLD
days of carburation engines , it doesn't do anything
for you.

That said , it may be a sudden drop in pressure
in the fuel lines ? Sticky Throttle Body , where it closes
too slowly or as said before, engine mounts.

 

in addition to what mlevk said, it may be too that your fuel pump is getting tired and the rapid reving is creating a sudden drop in fuel pressure. This brings to mind the fuel pressure regulator valve, also. Maybe that is starting to become faulty.

Does this happen if you just start the car and don't rev it?

steviej