I have written more than once about this problem. The story is in the archives but one point should stay attached to this post.
It is very easy to evaluate this problem by removing the small air filter at the rear of the injection pump. This is for the additional air used during the warm-up cycle. The valve for the air is moved by the same action as the enrichment. When the car is cold placing one's finger over the airfilter hole will stop the motor. When warm the same activity may allow ones finger to feel vacuum but closing off the hole will not result in engine speed difference.
I always include this simple test as the first step in mixture evaluation on all those mechanically injected motors. If the plunger is seized or if the t-stat is dead (the more common problem) there will be no change in the airflow, cold to warm.
Bosch Master, ASE Master, L1
33 years MB technician