Where are you located?

In answer to your question regarding swap, it is possible, but you will have less torque from an earlier 603 3.0L engine (the 3.0L is OM603.960 (W124) or OM603.961 (W126), the 3.5L is OM603.970 (W126), and OM603.971 (W140)).

Your 3.5L engine and transmission are set up for more low-end power, and a lower shift point. You can rev the engine by manually shifting to use the higher RPM, I don't believe that running 3.0L RPMs in the 3.5L transmission will cause it any harm, but the torque converter and shift points are tuned for the 3.5L.

As far as the engine physically fitting, you will need the oil pan / sump from the 3.5L, the 3.5L exhaust manifold, if you want to use the turbo from the 3.5L you will benefit from the faster spool-up. If the 3.0 comes with a Garrett turbo I would use the (pressure-actuated) wastegate from the 3.0L.

Other items you would want from the 3.5L: if you want better performance would be the IP, re-tuned to the higher RPM, the timing chain cover for its stronger design, vacuum pump, engine mount arms.

I have a good running engine from my '92 300SD, started and ran great, leaks oil though and will need a seal somewhere (shouldn't be too tough, the engine is out), I haven't had the time to mess with it and could be persuaded to part with it, and possibly a spare '87 300TD short-block which also ran great when I removed it. LMK if you're interested/local and we can figure out what it's worth. Don't toss your old engine either, there are parts on the .971 that are worth saving/selling.

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Gone to the dark side

- Jeff