You can crank longer than 15 seconds with your car....30 seconds is perfectly safe from everything I've read. IF the filter is filled or almost filled with fuel, you should not need repeated cranking cycles. It will catch then die; when you crank after that it will catch, sputter a bit, then be fine.

BTW the best way to get the filter FULL is to put it in place, wedge something under it to keep it in place, fill with diesel or ATF to the very top, screw in the bolt, then remove the wedge. Other approaches can be messy.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 161k miles
06 E320 CDI "Rutherford", Black on Tan, 180k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 147k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 81k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife