I replaced mine with a scrap of pex plumbing. I'm not sure whether the original was foam due to the desire for accurate temperature or to isolate the sensor from vibration. The more rigid tubing seems to work fine.

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85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.