Since new my 2001 Ranger when you rev the engine while in park cys out an sputters at 3000 RPM...it seems to run fine but I can rev the engine without it sputtering...Took it into dealer when it was under warranty and they did nothing to fix it...It runs and can accelerate while running I was just trying to find an answer to why. It acts kind of like there's a govenor on the engine...

 

It's the 'Rev limiter/Engine speed' govenor... Normal...