Cummins Repair in Southern Utah

The 6.7L Cummins uses a grid heater rather than individual glow plugs. A grid element in the intake warms the incoming air for the whole engine, and the controller cycles it before and after the start. When the element, its relay or the controller circuit fails, the engine has cold air and cold cylinder walls to work with and the crank stretches out on a cold morning.

The other half of the hard-start picture is fuel pressure. The high-pressure common rail has to reach the pressure the ECM wants before it commands injection. Air drawn in past a fuel filter housing seal, a weak or failed lift pump, a leaking injector bleeding the rail down overnight, or a suction-side restriction all delay that.

We watch rail pressure during the crank, check grid heater operation and current draw, and run a return-flow test on the injectors. Which of those the data points at determines whether the fix is an electrical circuit or a fuel system component.

White smoke is fuel entering the cylinder without burning cleanly, or coolant turning to steam. Cold white smoke that clears with warmth usually means poor atomisation or ignition — a worn injector nozzle, a grid heater not doing its job, or low compression on one cylinder. White smoke that persists warm, with coolant disappearing, points at coolant reaching the combustion chamber.

Black smoke is the opposite problem: too much fuel for the available air, or fuel that is burning late. Restricted intake air from a dust-loaded filter, a turbo not making commanded boost, a sticking variable-geometry vane mechanism, or an injector delivering more fuel than it should all produce it.

We separate them with data rather than by eye — injector correction values at idle, boost commanded versus actual, air filter restriction, and where the picture calls for it, a compression or coolant-intrusion test.

An oil level that climbs between changes, with a strong diesel smell on the dipstick, means fuel is getting into the crankcase. On a common-rail Cummins there are two main routes. An injector that leaks past its seat or a cracked injector body puts raw fuel down the cylinder and past the rings. A failing high-pressure pump can also allow fuel into the oil circuit.

It also arrives through the exhaust side. Post-injection fuel used to raise exhaust temperature for a particulate filter regeneration is metered to burn in the exhaust, but on a truck whose regens keep aborting, some of that fuel washes down the cylinder wall and dilutes the oil.

Fuel dilution is not a cosmetic problem. Diluted oil loses viscosity and film strength, and bearings and the turbo are what pay for it. We confirm dilution, then find its source — injector return flow testing, rail pressure behaviour, and the truck's regen history all get read before anything is replaced.

A derate is the ECM deliberately capping fuel and boost after a monitored value went out of range. On the 6.7L the usual triggers are a turbocharger actuator or vane position fault, a boost figure that does not match the commanded value, low fuel rail pressure, an exhaust temperature above target, or a DEF system fault the truck has already been warning about.

Emissions-related derates escalate on a schedule set by regulation — a warning, then reduced power, then a speed limit. That escalation is why a DEF message left alone for weeks turns into a truck that cannot pull a trailer.

Diagnosis is a full code read with freeze-frame data preserved, then live data under load: commanded versus actual boost, rail pressure at throttle, NOx sensor values compared against each other. Clearing the code first destroys the evidence.

Two very different transmissions sit behind the 6.7L. The 68RFE is a six-speed automatic used behind the standard-output engine, and the Aisin AS69RC is the heavier unit used behind the high-output engine in 3500 applications. They do not share fluid specifications, service procedures or failure patterns, and a shop that treats them as interchangeable will get both wrong.

The 68RFE is the one owners ask about. It is a clutch-to-clutch design that relies on precise line pressure and clean fluid, and it lives behind an engine making a great deal of torque. Harsh 1-2 shifts, a flare between gears, a shudder under light throttle or a slip when the converter locks are the symptoms, and heat is the accelerant — a loaded trailer on the Virgin River Gorge climb is exactly the duty cycle that finds its limits.

Chrysler and Ram specify Mopar ASRC fluid for the Aisin AS69RC, and the 68RFE has its own specified fluid. Using the wrong one causes shift quality problems that look like mechanical failure. We identify which transmission is actually in the truck, check fluid condition and level at temperature, read line pressure and clutch volume data on the scan tool, and log a road test before calling anything internal.

The Holset variable-geometry turbo on the 6.7L moves a sliding nozzle to change effective turbine housing size — small at low speed for quick response, large at high flow. That mechanism also doubles as the exhaust brake, which is why it moves constantly, and it lives in soot.

When the mechanism sticks or the electronic actuator loses calibration, actual boost stops tracking commanded boost. You get a derate, poor throttle response, a weak or non-functioning exhaust brake, or a code for turbo actuator position. Ignoring it lets the mechanism seize in one position, which turns a cleaning and calibration into a turbo.

We command the actuator through its sweep with a scan tool and watch position feedback, compare commanded and actual boost on a load, and inspect the mechanism for soot binding. Where the actuator itself is at fault, calibration to the specific turbo is part of the repair.

The 6.7L filters fuel in two stages — a frame-mounted filter and water separator ahead of the pump, and a finer engine-mounted filter after it. The water-in-fuel sensor sits in the separator bowl, and the light means water has accumulated to the level the sensor watches.

Water is not a nuisance on a common-rail diesel; it is the direct enemy of the components fuel lubricates. The high-pressure pump and the injectors rely on diesel's own lubricity, and water displaces that film. Driving on a water-in-fuel light is one of the cheapest ways to buy an expensive fuel system.

The separator gets drained, the fuel is inspected for water and debris, both filter stages are checked, and where contamination is significant the tank and lines get addressed rather than just the filter. If the light returns immediately after a drain, the sensor circuit itself gets tested.

Blue smoke is oil burning, and on a 6.7L the shortest path to it is the turbocharger. Oil passing a turbo seal enters the charge air stream, collects in the intercooler and the charge pipes, and burns in the cylinder. Wear in the rings or valve guides produces the same colour, so the intercooler and pipe interiors are worth looking at first — oil pooling there names the turbo without further disassembly.

A boost leak produces no smoke of its own but shows up as black smoke, sluggish response and a truck that will not pull. The charge air system on these trucks runs a long path from turbo to intercooler to intake, with clamped rubber couplers that harden in heat and eventually release under peak boost on a grade. The truck feels fine around town and falls over towing the Virgin River Gorge, because that is the only place it sees full boost for minutes at a time.

EGR cooler symptoms sit next to both. The cooler passes exhaust gas through engine coolant, so an internal failure sends coolant into the intake — white vapour from the exhaust, a coolant level that drops without a puddle, and soot fouling downstream. A cooler that is plugged rather than leaking causes high EGR flow errors and rising exhaust temperature instead.

We pressure-test the charge air system to find leaks rather than chase them by feel, inspect the intercooler for oil, pressure-test the cooling system, and read EGR flow and differential values against commanded position. These systems are diagnosed and repaired to factory specification — we do not remove or disable emissions equipment on any vehicle.