My roommate has a 1991 W126 350SDL which had the original 3.5 liter OM603 engine replaced with a 3.0 liter OM603 out of a 1987 W124 300D. Most of the glow plugs are bad, so I want to replace them.

I know I can just install the same type that came in the 1987 3.0--right? Or will the relay in the 350SDL body mess them up?

Also, can I install the later plugs that the 3.5 liter engine would have had in it, or are they too long or something? Or would there even be a reason to put them in over the 3.0 type (upgrade)?

I'm just trying to figure out if the new 3.0 plugs my roommate has will work just fine, or if it would be better to get the ones that fit the 3.5 engine because they'll work better with the SDL relay or will give better performance.

Changing out these things is not easy.

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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles