The car could be either a 300SDL (W126) or a 300D Turbo or N/A (W124) -- I don't know what Mercedes exported to OZ in those years. Either way it should have an OM603 with electronic idle speed control. If so, then there's a problem with the EGR electronics 'cause that's where the idle speed is controlled from.

The first to check is the over-voltage protection relay (OVP) and its 10 Amp fuse. Next is the IP and its various electrical connections. The EGR box itself is last, only because it's historically very reliable, there's nothing you can do with it, and it's very expensive.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970