Location: Campbelltown NSW
Posts: 1,782
Club: M.A.L.A.K.A.S.
Car: 2020 AMG GT C
ok so.
tell me if im wrong.
Putting this here because the question keeps coming up monthly and im tired of typing it out in PM. A 300CE-24 (W124 with the 24v M104 straight six) approaching 200k has a service profile that is different to the 12v M103 cars of the same era. Different parts age differently. The most common diagnostic mistake on a 200k M104 is treating it like an M103. Here is what actually matters from what i have seen come through Campbelltown.
The wiring loom in the engine bay is the big one. The insulation Mercedes used on the M104 engine harness for production years 1991-1995 degrades over time. By 200k the harness is brittle, insulation crumbles, short-to-ground events happen. Either replace the loom with new one (about $1800 OE) or remediate it with modern wire of equivalent gauge (about $400). Jurgen wrote up the remediation walkthrough years ago, search his thread.
Head gasket. The M104 head gasket lasts somewhere between 220-280k in Sydney conditions. By 200k you are past mid-life. Buy a block tester at Repco, $80, do the radiator cap test, check the oil filler cap for the mayonnaise. If either, fix it before it lets go. If neither, leave it but plan for it.
Timing chain is usually fine. Single row chain with hydraulic tensioner, tensioner is reliable, chain stretches over time. At 200k check the timing-mark deviation in the WIS. Replace if more than about 4 degrees.
Crankshaft harmonic balancer. The rubber-bonded outer ring delaminates with age. Inspect visually, the outer ring sits slightly off-axis if youre looking carefully. Do not run a delaminated one, it kills the crank bearings. About $380 for the part, 90 minutes labour.
Climate control vacuum pump inside the heater box. Symptom is blend doors stop responding to temperature changes. About $145 for the OE pump, user-replaceable if you can stand the contortions.
Diff and subframe mounts. Rubber perishes. About $90 for the diff mount pair, $320 for the subframe set, half a day on jack stands.
What is NOT urgent at 200k on an M104: the cylinder head itself (no overhaul unless the head gasket comes off and you find wear), the valve seals (M104 seals last 280-350k easy), camshafts (400k+ in normal conditions), and crank bearings (they last forever if you have been changing the oil).
Budget for a 200k M104 health restoration: about $3,800-5,500 depending on whether you do the wiring harness as full replacement or remediation, and how many of the optional items above your individual car actually needs.
Leaving the thread open for additions.
