jcfp05 wrote:Hi ImStricken; I read your Mar 20 2012 description of why our 4-cyl cars dip the RPM's and shake, but I have a doubt; why it doesn't happen when in Park or Neutral?
in park & neutral, the load of the vehicle, is removed from the engine via the transmission.
ever notice how once you put your vehicle into N or P - the RPM instantly increases by at least a couple hundred rpm? well, you just released the strain, that was on the engine while in gear. its like taking weight off your shoulders.
when your in drive, or any gear for that matter - at 0mpg release your brake and the vehicle starts to move. now apply the brake and your brakes are fighting the engine, and the vehicles moving weight(momentum). At idle coasting guess who is going to win? 4 brake calipers exerting thousands of pounds of pressure are going to win over a small engine thats not being given throttle. (give it throttle, and now the engine will win)
so while sitting at a red light, and applying the brakes - there is still a small amount of strain on your engine. if you just release your brake pedal, you would find the vehicle instantly starting to move, and the RPM would return upward by as much as 200 or more RPM.
and you mentioned that you drove other 4cyl engines and things were a bit smoother; well i dont doubt you. every car is different. from motor mounts, to the HP in those engines, to the weight of the vehicle, etc. but you must understand that our car has a CVT. this transmission coupled with an early locking torque converter is armed and ready to get the car moving much earlier and faster than other cars without the CVT. so what your actually getting is more feedback aka vibrations. its nothing to be concerned about, and not something to stress over. every car is different by power, weight, design, mechanics, etc.
small fuel efficient engine + CVT transmission + early locking Torque Converter = more vibrations when standing still while in gear. especially if the AC pump is active, or the alternator has strain/drain.