Not sure about the 240D, but my 300D glow plug system continues to heat for about ONE minute. I believe the glow light going out at the 5 second interval is only a "suggestion" as to when to try to start the car.

Try keeping the ignition switch in the "glow" position for 30 seconds and see if that helps.

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)