Strange idling on startup 320ic/325e

[Quasigriz]

For starters, I have a total Frankenstein 320i/325e cabrio. I had a valve drop into the 6th cylnder of the 320i engine while on a road trip to south France. The donar car for the new engine wound up being a 325e, not a 325i. Did an engine swap anyways using the all the 320i wiring and a bunch of the 320i components (intake etc.), and surpisingly it has worked fantastic.

Until now. I have a strange thing that happens when starting on a cold day. The car always starts on the first turn, no problems. However, sometimes the idle will jump from 1000-2000 RPM for up to 30-40 seconds, then it is fine. I can't figure out what exactly is causing it. It even contnues if I try to put it in gear right away. Almost like it's driving itself. It doesn't ever occur again afterwards, but it the car is left for a number of hours it happens again at startup. It doesn't seem like a problem, but when starting on the street everyone thinks I'm a jerk or want to race.

Hope someone out there can help!

Griz

[dan_emaps]

Odd that this is only on cold start up, is it absolutely fine once warm? at idle and across the rev range?

[Quasigriz]

Very strange indeed. It will do this up to a minute or do, then it's fine. I'm wondering if it's trying to cope with a possible timing problem, but then it would happen every time....