Rough engine shut down

<div class='quotetop'>QUOTE(Errel @ Sep 1 2007, 08:54 AM) [snapback]505375[/snapback]</div>

My new(ish) 2007 does it as well sometimes. It's often not the first shutdown, which I've been told is frequently rough.

The way I'd describe it is that it feels like less than four cylinders are still getting gas/spark for a few revolutions. It really feels like that thing some cars used to do where they'd run for a second or two after you shut them off.

 

This is common to all Prii (at times). Your Internal Combustion Engine (ICE) runs an "Atkinson Cycle" which means that the intake valves are open for the first portion of each compression stroke (one of the unique aspects of a Prius). This puts some unburned fuel vapors back into the intake manifold (there is actually an activated carbon chamber that is there to store these vapors so that they do not get into the atmosphere).

The ICE is shut down by turning off the fuel supply, but can continue to run (roughly) for a moment on these unburned vapors. Spark is maintained during this interval to ensure that the vapors are eliminated. No harm is being done to the vehicle when this occurs, but the roughness can be noticeable.

The "run-on" that used to happen in older vehicles was a different phenomenon. They would still have idle fuel from the carburetor when spark was removed and could operate for a (hopefully) short time as a diesel engine. This could cause a carbon build up in your engine which further exacerbated the problem.

JeffD

 

From time to time I notice a "rough" engine shut down while at a stop light in my '05. If it happened on a more frequent basis I would be more concerned.

 

Jeff, thanks for the good explaination. I reported this problem to the dealer shortly after owning the car and they would not give me any details like you did. They said that all Prii do that. I appreciate knowing the real reason so I might dismiss it with comfort.