3.0si hunts a bit on idle from cold, how serious?

[mybmwhabit]

Looked through a few threads and I couldn't find this. If someone has a link handy, could you please point me there?

My new/old 2008 Z4 3.0Si Coupe seems to hunt/surge for about the first 15-30 sec from cold, but then settles in to smooth
idle.

Thoughts on how bad/potentially how expensive?

Thanks!

[tekoo]

Most probably it’s the gasoline quality you are using. Unfortunately the gas quality is not the same from one gas station to the other. You will have to find a good quality source and stick with it. Also always use the highest gas grade avialable.
Fuel additives that remove water and clean the injectors help tremendously. When the vehicle is parked for a long time some gas evaporates from fuel injectors and the engine leaving water and precipitates behind. When you start the vehicle the DME (engine ECU) runs its calculations to adjust the idle speed based on the percentages of different gasoline elements pre and post combustion. The elevated concentration of gas impurities after prolonged park throws off these calculations initially and results in significant fluctuations in idle speed during the first 30 seconds or so. Once ‘new’ fuel flows into the system it flushes the old decomposed stagnant fuel out resulting in more accurate calculations and speed normalizes.
The fact that the vehicle operates normally after the initial 30 seconds and throughout the day with no issues proves it’s NOT a mechanical, electrical or software issue!

1-Use premium gas (91 or preferably 93) from a high quality gas vendor.
Always fill from the same gas station. This is the only diagnostic and preventative measure to avoid bad quality gas.
Use the progress of your current symptoms as your indicator to the gas quality.
Never let the gas drop below 1/4 of your tank. This will also allow you to always stick with the same gas station.
2-Try the following gas additive products:
https://www.amazon.com/Liqui-Moly-20...=car-search-20

https://www.amazon.com/ISO-HEET-2820...1FL&ref=plSrch

3-make sure your spark plugs are suitable, not old and are installed according to specs.
4-make sure your battery is not approaching it’s end life.
5-Scan the vehicle for error codes.

[jclausen]

Given that is only in the cold start up phase for a short period of time I would not worry about it

[B328iC]

Check your air filter. Could cause the engine to run rich when cold and then it smooths itself out.

[cdsweet]

...as with everything that deals with idle surges. Check all your vacuum lines.