I think you're talking about either:
1. The frequency valve
2. The warmup regulator
The frequency part looks similar to an injector (shapewise). From what I remember on a BMW with the Bosch K-Jetronic system (I believe your system is KE Jetronic since you have an O2 sensor). This sensor has a fuel line that goes to the warmup regulator. They don't usually go bad (by the way, does it buzz? If the part buzzes loud when you turn the switch on then it is the frequency valve). The warmup regulator does go bad quite often and it is $$$.
Again, take a picture if you can.
1993 190E 2.6
Advanced Diesel Systems Test and Research Engineer