my 1991 190E 2.6 is having stalling/surging idle problems agian. I am hearing like air bubbles near the fuel pump/fuel tank area.

What could be causing this?

Whats new/replaced inculdes:

ovp
mas
fuel tank filter
coolant temperature sensor
spark plugs

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)