Since purchase, our '98 GS400 has had a violent surge (dip in revs, then violent surge over original, then back to normal state), so violent that at a stop-light, if you weren't pressing hard on the brake, it would surge forward hard enough to make you think you'll hit the car in front of you... we trained ourselves to shift in to N at lights. Also, it made highway cruising unpleasant, as it would also surge on cruise... enough that we didn't want to drive it on long trips. Yet this would only happened under two conditions... the OCV plug-connectors had to be engaged, and the car had to be at full operating temp. If either condition was not met (we bought it unplugged, then tried to fix it), that would keep the surge away. Replacing the OCVs with new/factory didn't help, and we were planning on a timing belt change (which it needed anyway) to see if that helped.

While replacing the alternator this week, we noticed the overflow bottle was empty, and the radiator was low. So kicking ourselves for never checking, and remembering that Toyotas hate low-coolant, in regards to idle, filled it up, and... 98% of the surge is gone, and we'll blurp the system after we confirm or buy the tool to remove the filler-hole bolt.

It all started to make sense, when we remembered that the previous owner stated he replaced the starter right before selling the car, and he said it never ran right after that, and the mechanic unplugged the OCV plugs to stop the surging. Considering the first step to replacing the starter is draining the coolant, the system was probably not blurped correctly, which led to this situation, and probably his final frustration to sell the car.

So... if you have violent surging, and you start going down the path that it's the OCV circuit, because it goes away when you unplug the OCV's... check your coolant level, and that it's been blurped correctly...

 

I personally do not see any reason to drain the coolant to replace the starter.
Sorry, nothing personal, but to me, it sounds like the story of a dilettante.
The floating engine speed definitely has a reason, but it's not related to the coolant level.
I am waiting for colleagues' opinion.

 

On the 1UZ motor, in our GS400, the starter is under the intake.
In the FSM, one of the first procedures calls for coolant to be drained.
I personally never bothered, (when removing the intake) coolant lose is minimal, as far as I'm concerned.