Cummins ISB & B6.7 Problems: 6 Common Issues & Solutions
The Cummins ISB/B6.7 is the workhorse of the medium-duty diesel world, powering everything from Ram pickups to school buses, delivery trucks, and RVs. While it's a proven and reliable platform, high-mileage units develop predictable issues. Here are the 6 most common problems and how to fix them.
ISB / B6.7 Variants Overview
| Model | Years | HP Range | Applications |
|---|---|---|---|
| ISB 5.9L | 1998-2007 | 175-325 HP | Ram 2500/3500, medium-duty trucks |
| ISB 6.7L | 2007-2012 | 200-360 HP | Ram 2500/3500, buses, delivery trucks |
| B6.7 | 2013-Present | 200-360 HP | Ram 2500/3500, buses, RVs, medium-duty |
Quick Overview: Common Problems
| Problem | Severity | Repair Cost |
|---|---|---|
| EGR Cooler & Valve Failures | High | $400 - $2,500 |
| Turbocharger / VGT Issues | Medium-High | $600 - $3,500 |
| Fuel Injector Problems | High | $1,800 - $3,600 |
| DPF / SCR Aftertreatment | Medium-High | $300 - $4,000 |
| Head Gasket / Head Issues | Critical | $2,000 - $7,000 |
| Oil Leaks & CCV Issues | Medium | $50 - $1,500 |
1. EGR Cooler & EGR Valve Failures
HighAffected: 2007-2024 — most common in 2007-2012 models
Symptoms
What Causes It
The EGR cooler on the ISB/B6.7 can develop internal cracks, allowing coolant to leak into the exhaust or intake. The EGR valve itself accumulates carbon deposits over time, causing it to stick open or closed. Short-trip driving, excessive idling, and infrequent highway use accelerate carbon buildup. The 2007-2012 models with early emissions systems are most affected.
Repair Options & Cost
EGR cooler replacement costs $1,200-$2,500. EGR valve replacement costs $400-$1,000. EGR valve cleaning can sometimes resolve sticking issues ($200-$400). Regular highway driving helps burn off carbon deposits. Some operators install EGR cooler bypass kits for off-road applications.
2. Turbocharger & VGT Issues
Medium-HighAffected: All ISB/B6.7 models with VGT — most common after 200,000+ miles
Symptoms
What Causes It
The ISB/B6.7 uses a Holset variable geometry turbocharger (VGT). The variable vanes can stick due to carbon and soot buildup, especially in trucks that idle frequently or operate at low loads. The electronic turbo actuator can fail, and bearing wear develops at high mileage. The VGT system is more complex than a traditional wastegate turbo and requires cleaner operating conditions.
Repair Options & Cost
Turbo actuator replacement costs $600-$1,200. Complete turbo replacement costs $1,500-$3,500. Turbo rebuild costs $800-$1,500. VGT cleaning can sometimes restore function ($300-$600). Regular highway driving at full load helps keep VGT vanes clean. Always check for intake leaks that can accelerate turbo wear.
3. Fuel Injector Problems
HighAffected: All ISB/B6.7 models — typically after 200,000+ miles
Symptoms
What Causes It
The ISB/B6.7 uses Bosch common rail injectors that operate at extremely high pressures (up to 29,000 PSI). Over time, injector tips erode, internal seals wear, and the injector nozzle can become clogged or damaged. Contaminated fuel, water in the fuel system, and poor fuel filtration are the primary accelerators. The high injection pressures make these injectors more sensitive to fuel quality than older mechanical systems.
Repair Options & Cost
Individual injector replacement costs $300-$600 per injector. A full set of 6 costs $1,800-$3,600 with labor. Remanufactured injectors cost $200-$400 each. Always replace the fuel filter at recommended intervals and consider adding a secondary fuel filter/water separator. If multiple injectors are failing on a high-mileage engine, a remanufactured ISB ($4,000-$7,000) includes all new injectors.
4. DPF & SCR Aftertreatment Issues
Medium-HighAffected: 2007+ (DPF), 2010+ (SCR/DEF) — all current B6.7 models
Symptoms
What Causes It
The DPF (Diesel Particulate Filter) traps soot and requires periodic regeneration. The SCR (Selective Catalytic Reduction) system uses DEF to reduce NOx. Common issues include DPF clogging from excessive idle time, DEF injector crystallization, NOx sensor failures, and SCR catalyst degradation. Short-trip driving prevents proper passive regeneration. Using low-quality DEF or letting it freeze repeatedly causes system damage.
Repair Options & Cost
DPF cleaning costs $300-$600 (recommended every 100,000-150,000 miles). DPF replacement costs $2,000-$4,000. DEF injector replacement costs $400-$800. NOx sensor replacement costs $300-$600 each. SCR catalyst replacement costs $2,000-$4,000. Use API-certified DEF and CK-4 rated low-ash engine oil to extend aftertreatment life.
5. Head Gasket & Cylinder Head Issues
CriticalAffected: All ISB/B6.7 models — typically at high mileage or after overheating
Symptoms
What Causes It
Head gasket failures on the ISB/B6.7 are less common than on larger engines but do occur, especially after overheating events or at high mileage. The higher combustion pressures of modern emissions-compliant engines put more stress on the head gasket. Improper coolant maintenance (low SCA levels, wrong coolant type) can accelerate corrosion and gasket deterioration.
Repair Options & Cost
Head gasket replacement costs $2,000-$4,000 with labor. If the cylinder head is cracked, head replacement adds $1,500-$3,000. For engines with 300,000+ miles and multiple issues, a remanufactured ISB/B6.7 ($4,000-$7,000) is often more cost-effective than head work plus other needed repairs.
6. Oil Leaks & Crankcase Ventilation
MediumAffected: All ISB/B6.7 models — typically after 150,000+ miles
Symptoms
What Causes It
The ISB/B6.7 is known for developing oil leaks at several locations: the front crankshaft seal, rear main seal, valve cover gasket, and oil pan gasket. The crankcase ventilation system (CCV) can become restricted with soot and oil residue, increasing crankcase pressure and pushing oil past seals. Higher mileage engines with worn rings produce more blow-by, further stressing the CCV system.
Repair Options & Cost
Valve cover gasket replacement costs $300-$600. Front crankshaft seal replacement costs $400-$800. Rear main seal replacement costs $800-$1,500 (requires transmission removal). CCV filter replacement costs $50-$150 and should be done every 100,000 miles. Oil pan gasket replacement costs $500-$1,000. Address CCV restrictions first, as this often reduces seal leaks.
When Should You Replace Your ISB/B6.7?
The ISB/B6.7 is a durable engine, but replacement sometimes makes more sense than continued repairs: