Car model and year:
07 BMW e92 328i with 160,000 KM (Second-hand)
Problem description:
Had it inspected prior to purchase and everything came back great. Well now for the past couple weeks on startup, the car will shake.
The car will start and hover around 1000rpm then slowly decrease and that's when the shaking starts, almost feels like a side to side movement. If I hold the revs above 1000 it doesn't occur but if I let them drop then it comes back. Once I start driving the car, the shaking stops. The RPM's don't bounce around or fluctuate like I've seen other people experience when idling
This only seems to occur when it's a cold start or if the car has sat for a few hours, if I stop the car after a period of driving and start again there's no shaking. There's nothing noticeable under acceleration or during normal stop and go traffic.
Suggestions:
Piece 1. Change plugs, and coils, clean out the MAF sensor. grab a can of CRC MAF sensor cleaner and blast clean the shit out of the MAF sensor. Then start it up and test drive it.
Is the car throwing any codes? SES light or CEL on?
Piece 2. I am going to bet changing your coils and plugs will do nothing for you. The symptoms you have described fit the description of leaky injectors.
Cold rough start in the morning or after sitting for a while. Yes
Car smooth out after revving a little or running for a minute or two. yes
Get an OBD2 code reader before you buy anything else. When you run the diagnostic you will have a misfire on one of the cylinders. Swap that coil with the coil from another cylinder and then clear the codes. The next time you experience the issue, check the error codes again and most likely it will be the original cylinder again.
If you have BMW change out the injector change them all at the same time. This is just preventative but it will save you the money later if another fails.
Piece 3.
Despite what a lot of people are saying in here the coils in these cars are pretty good. It's normally a waste of money to replace them all 'just because'. They mostly fail in high load situations which is not what you are describing. Their failure mode does vary, however, so the only way to really move forward without guessing is to get a BMW specific scanner such as the Foxwell NT510 for BMW or the newer NT520 for BMW. If you're tight on cash and are only looking to read BMW specific codes, get a Creator C310 for BMW. The Creator is a little cheaper, does the same code reading as the NT510/520 but it doesn't have other features like battery registration and airbag reset.
If you are handy with a computer and programming there are also DIY solutions that mimic BMW software called ISTA.
The most reliable way to fix your car is to start with a code scanner.