The recommended interval for the fuel filter on the CDI is every 26k miles. However, what I've typically done with the W124 and W210 is simply wait till the car feels sluggish. It's usually pretty obvious and sudden. If it's been a while and I'm about to go on a long trip, I may change it just to be safe.

Is there any reason not to adopt this policy with the CDI? It's a different configuration: just one filter instead of a main and pre. So I was wondering if there's any downside to a longer interval.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 179k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 77k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife