My 2003 Prius with 119,000 miles will sometimes shudder when I do a cold start. It either shudders and makes noise rocking side to side or it will rock back and forth like its going to stall does either of these a few times and then after a minute disappears as it warms. Pressing the accelerator makes it feel like it will die; so I let up until it stops doing this on its own. it only has done this a few times in the past two months but seems to be increasing and getting louder with the side to side rocking and shuddering today. No warning light as of yet so I'm not quite sure what is going on? A while back I had the warning light and Toyota did the throttle cleaning under warranty and that cleared up that problem. Any help is appreciated.
You're at a point where sparkplugs probably need to be replaced and inspected. A throttle cleaning once a year is a good idea and easy to do: Cleaning Prius Throttle Bob Wilson
It's a great idea to replace the iridium spark plugs if they had not recently been replaced. If that doesn't help, I am wondering whether you might have an intermittent connection to a fuel injector or a spark igniter which becomes bad only when the engine is cold, then goes away when the engine warms up. You might check the wiring harness connectors to make sure that each one is locked in place. If no obvious problems are apparent, you might then let the engine warm up so that it is running smoothly. Then disconnect a spark igniter connector and see whether the engine behavior is similar to what you notice when the engine is cold and running rough. You'll need to put the car in inspection mode so that the engine will run continuously. <Do a search if you don't know how to do that.>
Thank you gentlemen for your advice. Next week I was going to replace the spark plugs. I will go out and check the throttle to see its condition and if it needs cleaning again. I will report back how everything goes next week.
I replaced the Spark Plugs and all is acting normal again. Haven't replicated the problem in over a month. Thanks for the sound advice.
