Cummins ISX (EGR)
The engine that took Cummins through the first two tightenings, in dual-overhead-cam form. The pre-2007 version has no particulate filter, which is most of why those trucks still sell.
Output
385-600 hp
1350-2050 lb-ft peak torque
Fuel per mile
$0.658
at 6.2 mpg and $4.049 a gallon
Fuel a year
$79,005
over 120,000 miles
In-frame overhaul
$18k-$27k
rough, and shop rates move it
In the exhaust
EGR only to 2006, DPF from 2007
Exhaust gas recirculation arrives: some exhaust is fed back into the intake to cut oxides of nitrogen. It raises cylinder pressure and heat, and it introduced the EGR cooler as a thing that fails.
Known issues
EGR coolers, VGT turbo actuators sticking with soot, and injector wear. The 2007 to 2009 DPF builds attract the most complaints of any Cummins generation.
Verdict
A 2004 to 2006 ISX is a genuinely attractive used buy for an owner-operator who can wrench. A 2007 to 2009 is the one generation to be most careful with.