car shaking in park or sitting still???

[transamgirl]

my 99 trans am for about a year has been shaking erratically during park or sitting still in gear. it's not a big shake but feels like your going over some small bumps in the road but your sitting still. you can feel it do this when in the car.when i stand behind the car i can hear this happen. when it shakes the sound getts quiet for like a second. almost like it's pausing.it will do this every few seconds but it's not a constant steady shaking. I can see the engine shake a little more when this happens as well. it only does this after the cars been running for about 5 minutes.when i start moving it's great very smooth and no problems at all. i just had my alternator replaced with a 150 amp b/c i was told the regulator in my stock one was bad.my husband said that would fix the shaking since the car hadn't been getting a lot of power from it and he also said he'd heard that a bad alternator being left on a car can start to cause problems with the engine since it's putting a strain on it. i checked my plugs which are about 2 yrs old and they are fine. any clues???

[vetteemt]

Any ses codes? If you can hook up a hand held scan tool to see if the engine is registering any miss fires or any other errors. A misfire at idle could cause your engine to shack once in a while. Do you have any after market parts on the engine i.e. cam or heads?

[rbob93]

LS1 engines run slightly rough

[transamgirl]

only aftermarket on the car are the air box since mine wasn't ram air and now is and the exhaust that was put on before i got the car

[juiced99ws6]

what is your car idling at?

[transamgirl]

around 6 or 700 rpms. it's always been that since i've had the car for the past 5 years.

[Eagle]

Sounds about normal to me for that RPM. The cars idle a bit on the rough side. Its the price you pay to have a LS1.

-Eagle

[transamgirl]

thanks, I might just now be noticing the shaking b/c my car being over 118,000 miles. before 100,000 I never worried about anything going wrong and now i get worried even though i take good care of it and keep everything changed as I should.

[Eagle]

If it gets excessive with mileage that high, you might look into replacing motor and trans mounts to firm it back up again. GL!

[vetteemt]

I 2nd eagle's post. Check the mounts, they might just be worn out.

[transamgirl]

thanks, i didn't think about that