Choke? the 3.5 EFI doesn't have a choke. If you are disconnecting an electrical connection, you are either:
- Disconnecting a faulty part, making the system behave.
- Disconnecting a critical part, making some other behavior mask the real problem.
The system is incredibly sensitive to vacuum issues. If you're disconnecting a vacuum line, then I would look under the master cylinder, where you will find a vacuum sensor that has a thick hose running back to the manifold.
There are extensive posts in the archives of various EFI and Jetronic issues (your system). It is identical to the other pre-1975 3.5 liter motor cars, ie 300sel 3.5 and the 280se 3.5. It is almost identical to the 4.5 systems, except for how the throttle position sensor is wired in (which means the brain boxes aren't identical either).
Worse case, you need a new brain box. Rebuilt, they are 400-500$. Used, if you can find it, is about 100$.
I have a .zip file of diagnostic info if you want it, send me an email (you can't send attachments any other way).