I noticed mine doing the same thing late last year after we put the new motor in the Monte was put in and this was the 1 part i did not change out the IAC motor and man mine was dirty so i just went to the store and got a new one but

try cleaning the IAC (Idle are control motor ) it is super easy to do it just disconnect the neg side of the battery and a t20 torque bit and a maf sensor cleaner and a rag . or for like 29$ at autozone here is a link
Idle Air Control Valve | 2001 Chevrolet Monte Carlo 6 Cylinders K 3.8L SFI OHV | AutoZone.com

I keep my old one as a back up it was just dirty and i did clean it and test it and that was all it was .

THANKS Chiba for the PIC !!

 

Check your IAC motor (Idle Air Control). Mine was sooted up. Cleaning it and the throttle body where it seats with TB cleaner helped, but replacemet was the ultimate cure. Costs $40 for ACDelco.

 

Same problem I just gone through, Clean up the IAC valve & lube it up with wd-40 or something light.
I did that, but it only lasted about a month until now. Experiencing the same issue.
Nice signature btw ;D

 

So...I had the same problem with my 2003 3.8 Monte SS. Finally figured out the problem. One of the hydraulic lifters went bad. This causes the cylinder to leak oil instead of retain oil so only when the pressure builds back up after running for awhile would the should and shaking go away. At the shop its about a $870 fix but you might as well go ahead and replace all of them while you are in there.

 

$870 to change a lifter? What shop are you going to?

 

Might be a Fuel Pressure Regulator

Mine did the same thing until I changed it.

 

Okay here we go one of the number one causes for rough Idle when you first start your vehicle is the A.I.C. Air Idle Control valve the spring sometimes sticks as it gets older easy fix about $25-$30 right beside the Mass air flow sensor on intake. If the car still idles rough and smells of fuel check the PCV Valve . on the 3.8 and 3.4 etc. it is electronic so it is easily over looked. It should be replaced every other oil change and if the spring or O-ring is damaged they sell a kit very cheap kit compared to throwing plugs wires etc. at the car. It is located at the left rear corner under the plastic engine cover so you will have to take the cover off. Unplug the wire leads push down and turn counter clockwise. I was surprised how something so small caused so much problem. My 2002 SS runs like new again.


Hope it helps/