Contrary to popular belief, that red cap belongs there. Check the service manual (25-27, 25-34). That is an auxilary fitting and the cap is supposed to remain on it.
Darn if these manuals aren't worth the money!
Might want to put it back on if you still have it.
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'01 3500 QC SLT, 5-spd. , 3. 54 LSD, Tow Pkg, Camper Pkg, White w/ Camel. (St. Louis) - Jacobs E-brake, GaugeWorks dual gauge pod, ISSPRO EGT & Boost Gauges.
[This message has been edited by lembaar (edited 08-07-2000). ]