the vent valve is on the left side of the car underneath the car. To properly diagnosis check your glowplugs with a multimeter. Do you have loop style or the upgraded plugs?

How old is the fuel? have you replaced the inscreen filter?

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