Rule of thumb is ten minutes max. If you're going to be out of the truck for more than 10 minutes, then it's a good idea to turn it off. If it's cold out and you have high idle enabled, then there's no problem with idling up and warming the engine. The only real problem with idling is when it's cold out and high idle IS NOT enabled. The engine cannot, at regular idle, create enough heat to warm itself if the outside temp is very low. On the newer 6.7L, idling is a big NO-NO. The DPF will not stand up to long periods of idling and will fill up pretty quick when doing so. Hope this helps.
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2001.5 White 2500 Reg-cab • 4x4 • NV4500 • 4.10 • BHAF • WALBRO GSL392 • Edge Juice + Attitude • VALAIR HD OFE • Scangauge II • 315 Cooper S/T's • Optimas • custom black grille & blue graphics • ** Diesel Fuel Treatment is here **