FWIW ;

Many cars but have a device that slows the return to idle as over run (using the engine to slow down) produces a lot of emissions .

Fuel injected cars tend to have a valve that's adjustable .

Carburated engines tend to use dashpots .

This might be work looking into - way back in the 1970's I was driving a Ford Currier pickup delivery truck and down shifting was useless until I discovered there was a switch on the clutch pedal's arm that activated this device .

When VW Bugs went to fuel injection in 1975 they used a remote vacuu
m delay valve to operate the dashpot, it was adjustable .

I hope you get this sorted and let us know what the end fix was .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better