So let's suppose you could lay your hands on a nice 300CE with a bad motor. What would be the problems areas if swapping a 2.5 turbo engine? Would the 722.4 fit? Differential? How about registering it after conversion?

Check in with your state laws about engine swaps and fuel type swaps. I know in washington the new engine has to be the same year or newer than the body.

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1981 300TD 4 speed manual
Euro bumpers, zender valance and skirts, H&R springs, billy HD's, leveled sls, real AMG Pentas 16x8 et11, vdo boost/egt gauges intergrated into ash tray, eurolights, led 3rd brake light

I'm as big a fan of the 2.5 turbo engine as anyone out there but I have to honestly say that if I had a CE project car I'd be thinking upgrade not downgrade. M119 5.0L or say M113K or even an OM603 3.5 turbodiesel ... any of those could be bolted to a 722.6 and make a great car.... awesome. A 2.5 swap is just going to make a great car blow smoke.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff

I think it'd be worth doing. It'd certainly be cool to have a more modern version of a 300CD. Your biggest determining factor is going to be the legality of it and trying to get it registered after jumping through a bunch of bureaucratic hoops. Unless you are fortunate enough to live in an area that is emissions exempt.

I'm sure it is possible, as there are members here on the forum with diesel swapped SL's, living in California even. I'd ask their advice

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2007 Dodge Ram 3500 Cummins Turbo Diesel 4x4
1994 GMC S-15 pickup 4.3 5MT
1985 300 SD
1978 300 CD
1962 220-S Fintail - awaiting restoration