The 2011–2016 6.6L LML Duramax uses a Bosch CP4.2 common rail system that can see fuel pressures north of 30,000 PSI. Eight high-pressure steel lines connect the rails to each injector and must seal perfectly under extreme pressure, vibration, and heat.
When these lines corrode, crack, or become contaminated, the result is more than just a fuel leak. Line issues can:
Drop rail pressure and cause drivability problems
Seed injectors with metal and debris
Contribute to or follow a CP4 pump failure
Turn into a five-figure repair if contamination spreads through the entire fuel system
This guide explains how the LML high-pressure injector lines work, why they fail, when to replace them, and how they fit into a long-term fuel system strategy.
The LML uses eight individual high pressure lines, each dedicated to a cylinder. These lines are:
Precision-formed steel tubes
Flared and tapered to seal metal-to-metal at both the rail and injector
Designed to hold and seal at over 30,000 PSI
Passenger side bank: Cylinders 1, 3, 5, 7
Driver side bank: Cylinders 2, 4, 6, 8
Firing order: 1–2–7–8–4–5–6–3
For manufacturing and service, lines are typically grouped in matched pairs:
Cylinders 1 & 8 – longest outer-cylinder lines
Cylinders 2 & 7 – driver outer and passenger inner
Cylinders 3 & 6 – inner cylinders on opposite banks
Cylinders 4 & 5 – adjacent center cylinders
Correct geometry is critical. Small changes in length, bend, or angle can prevent proper seating, introduce stress, and lead to premature failure.
Line failures often start small and worsen over time. Typical symptoms include:
Hard starting or extended cranking – rail pressure bleeds off through a leak
Power loss or hesitation under load – fuel pressure drops when demand is high
Rough idle, surging, or inconsistent running – air intrusion causes erratic injection
Diesel odor or visible seepage – staining or dampness at injector or rail fittings
Check engine light with fuel system codes – P0087, P0088, P0191, or injector codes
Decreased fuel economy – ECM compensates for pressure loss by commanding more fuel
Metal in fuel filters – shiny flakes or metallic fuzz in the filter media
Any sign of metal in the filter or fuel is a red flag that warrants immediate inspection of the CP4, rails, lines, and injectors.
External rust is easy to spot. Internal erosion is not.
Testing by diesel fuel system specialists shows that high pressure lines begin to exhibit measurable internal erosion in the 62,000–75,000 mile range. The worst damage occurs:
At narrowed or flared sections just before the injector
In areas of high velocity and turbulence
Contributing factors include:
Ultra-low sulfur diesel (ULSD) with reduced lubricity
Repeated high-pressure pulses and transient spikes
Cavitation and hydraulic shock inside the line
Thousands of thermal cycles
Certain biodiesel blends that increase corrosiveness
As the inside of the line wears, microscopic metal particles are released directly downstream into the injector – beyond the last fuel filter. The outside of the line can look perfect while the inside is shedding material and slowly chewing up injectors.
There are four main reasons the “replace only the bad one” approach usually backfires.
LML owners are painfully familiar with CP4.2 failures. When a CP4 fails, it doesn’t just kill the pump – it sends metal through the entire high-pressure side:
Pump
Rails
Injector lines
Injectors
Excess fuel and debris then travel back through the return side and into the tank.
Under 30,000 PSI, metal particles act like sandblasting media. They:
Erode line interiors
Embed into microscopic imperfections in the line walls
Lodge in injector nozzles and precision control surfaces
Flushing cannot pull embedded particles out of the line walls. Those particles release over time and contaminate new injectors and pumps.
That is why a proper CP4 failure repair usually includes:
New pump (often CP3 conversion)
Tank cleaning and upgraded filtration
Reusing old lines in a CP4 failure scenario is almost guaranteed to contaminate new components.
All eight lines see:
The same pressure pulses
The same heat cycles
The same fuel chemistry
If one line has rusted through or cracked, the others are near the same point in their life. Replacing only one line today often means pulling the truck back apart when the next one fails.
Since access to the lines is labor-intensive, it makes economic sense to replace the entire set in a single service event.
LML high pressure fittings seal with a metal-to-metal taper, usually torqued around 30 lb-ft. Each torque cycle slightly changes the sealing surface. After multiple installs:
The flare and seat no longer deform predictably
The risk of seepage and weeping increases
Torque may be reached without actually achieving a proper seal
Mixing brand-new and heavily cycled lines creates inconsistent sealing behavior from cylinder to cylinder, complicating diagnostics and creating intermittent fuel leaks and odors.
Many injector remanufacturers require new lines as part of their installation guidelines. If injectors fail and an inspection shows contaminated or reused high pressure lines, warranty claims are often denied.
Shops that have had to eat a second injector job because the original lines were reused generally make complete line replacement their standard practice going forward.
When it is time to replace lines, owners typically choose between:
Genuine GM/Bosch OEM lines
High-quality aftermarket kits
Both can be good options when sourced from reputable suppliers.
Both OEM and quality aftermarket lines:
Use high-strength steel tubing
Are formed to precise engineering tolerances
Are designed and tested for 30,000+ PSI fuel pressure
Where aftermarket can have an advantage is external corrosion protection. Many kits use zinc plating or similar coatings that significantly slow rust formation compared to bare steel OEM lines, particularly in salted-road or coastal environments.
Properly engineered aftermarket kits:
Match OEM routing, lengths, and bend profiles
Include correct thread pitch and flare geometry
Drop in without modification
Cheaper, off-brand lines can have:
Slightly off bends that require “persuasion”
Inconsistent flare quality
Poor surface finish inside the fittings
Those issues introduce sealing problems and long-term reliability concerns, so choosing a known, quality-focused supplier is critical.
Typical ranges:
Individual OEM lines: around $40–$45 each, $320–$360+ for a full set
OEM-style full kits: roughly $295–$330
Quality aftermarket full kits: roughly $275–$300 with enhanced coatings
Given that injector and line access is an 8–12 hour job, the small price spread is irrelevant compared to the labor. Line quality and corrosion resistance matter far more than saving a few dollars up front.
There are four major triggers, plus one mileage-based guideline.
If the CP4 has failed or metal is found in the fuel filter:
Do not reuse rails, lines, or injectors without a proper contamination plan
Expect to replace all eight lines as part of the repair
Skipping the lines nearly guarantees contamination of new components.
Any time injectors are replaced for balance rates, hard start, smoke, or performance issues, the best practice is:
Replace all eight high pressure lines at the same time
The cost of the lines is small compared to a second injector job caused by contamination or sealing issues.
Replace lines if there is:
External rust and flaking
Wetness or staining at fittings
Cracks, kinks, or rub-through damage
Even if only one line is visibly bad, the remaining lines have seen the same conditions and are not far behind.
Because internal erosion accelerates beyond roughly 75,000–90,000 miles, a conservative preventive interval is:
About 100,000 miles or 8–10 years, whichever occurs first
Trucks that see heavy towing, biodiesel, poor fuel quality, or higher-than-stock rail pressures may justify earlier replacement.
High pressure line installation on an LML is all about cleanliness, alignment, and torque.
Thoroughly clean around rails and injectors before loosening anything
Use compressed air to blow debris away from fittings
Cap or cover open ports while lines are off
Inspect injector and rail seats for corrosion or damage
Any dirt or rust that enters the system during service can damage new components just as badly as old erosion.
Typical key specs:
High pressure line fittings (rail and injector): around 30 lb-ft (41 N·m), or per line manufacturer requirement
Best practices:
Always start fittings by hand
Snug all connections before final torque
Do not use thread sealant – the metal taper provides the seal
If using a crow’s foot, correct the torque setting for the effective length
Common related fasteners:
Injector hold-down bolts: around 22 lb-ft (new bolts recommended where specified)
Injector bracket bolts: around 22 lb-ft
Always confirm current torque data for the exact engine and component set being serviced.
After line replacement:
Bleed at the fuel filter housing with the primer until bubble-free fuel flows.
Bleed at the rail bleeder (if equipped) until fuel is clear.
Prime the low-pressure side (manual primer or lift pump key-cycles).
Crank in short bursts, allowing rest periods, until the engine starts.
Inspect for leaks at idle and again at full operating temperature.
If rail pressure is not building or the engine will not start, recheck connections and repeat bleeding steps.
The CP4.2 used on the LML is less tolerant of poor lubrication and contamination than the previous CP3 design. Risk factors include:
Low lubricity ULSD
Running the tank low and ingesting air
Water or dirt contamination
Aggressive tuning and higher pressures
When a CP4 fails, it frequently:
Sends metal into rails, lines, and injectors
Contaminates the return side and tank
Quality, corrosion-resistant lines with proper installation help ensure:
A reliable seal at high pressure
Reduced risk of external corrosion shedding rust flakes into the system later
Fewer repeat failures caused by marginal seating surfaces or poor flare geometry
Many owners combine line replacement with a CP3 conversion and upgraded filtration to eliminate CP4 from the equation entirely.
High pressure lines are just one piece of an LML fuel system reliability plan.
Key components of a strategy:
Use high-quality fuel from high-turnover stations
Consider a lubricity/cleaner additive
Avoid running the tank below 1/4
Change fuel filters religiously and inspect media for metal or debris
Consider supplemental filtration and a lift pump system
Monitor injectors via balance rates and drivability
Plan for injector and line service before catastrophic failure
Treating high pressure lines as wear items, rather than “lifetime parts,” dramatically reduces the risk of catastrophic fuel system damage and the repair bills that follow.
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