Running out of fuel in the middle of nowhere isn’t just inconvenient—it can be dangerous, especially if you’re towing heavy loads or exploring remote trails in your 2008 Duramax. Many owners solve this by installing an auxiliary fuel tank, but the real challenge lies in how to plumb an auxiliary tank into a 2008 Duramaxsafely and effectively. This guide walks you through every critical step, tool, and safety consideration—so you can extend your range without compromising performance or reliability.
Why Install an Auxiliary Fuel Tank on a 2008 Duramax?
The 2008 Chevrolet Silverado or GMC Sierra with the 6.6L LMM Duramax engine comes with a standard 26-gallon fuel tank. While that’s decent for daily driving, it falls short for long-haul towing, overlanding, or rural work. An auxiliary tank can double your range, reduce refueling stops, and even serve as a backup during emergencies.
According to the U.S. Department of Energy, diesel trucks like the Duramax average 14–18 MPG on the highway. With a 26-gallon tank, that’s roughly 364–468 miles per tank. Add a 30-gallon auxiliary tank, and you’re looking at over 800 miles—a game-changer for serious drivers.
“Auxiliary tanks aren’t just about convenience—they’re a strategic upgrade for anyone who depends on their truck,” says Mike Jenkins, lead technician at Diesel Power Magazine.
What You’ll Need: Tools & Parts Checklist
Before you begin, gather these essentials:
Auxiliary fuel tank (30–40 gallon recommended; aluminum or polyethylene)
Fuel transfer pump (12V electric, self-priming, ~5–7 GPM flow rate)
Diesel-rated fuel lines (SAE J1527 or equivalent, 3/8″ or 1/2″ ID)
Inline fuel filter (pre-pump, 10-micron)
Relay, fuse, and wiring harness (30A fuse, 12-gauge wire)
Tank mounting hardware (brackets, bolts, rubber isolators)
T-fittings or Y-valve (for seamless integration with OEM fuel line)
Safety gear: gloves, eye protection, fire extinguisher
⚠️ Never use gasoline-rated hoses or fittings—diesel requires specific materials resistant to degradation.
Step-by-Step: How to Plumb an Auxiliary Tank into a 2008 Duramax
Follow these steps carefully. Rushing can lead to leaks, air intrusion, or fuel starvation.
Step 1: Choose the Right Tank Location
Most owners mount the auxiliary tank behind the cab or under the bed. Ensure:
Clearance from exhaust components (>12 inches)
No interference with suspension or frame rails
Easy access for maintenance
Use rubber isolators to prevent metal-on-metal contact and vibration damage.
Step 2: Install the Auxiliary Tank Securely
Bolt the tank using grade 5 or higher hardware.
Angle the tank slightly (1–2°) toward the outlet to aid drainage.
Ground the tank to the chassis to prevent static buildup.
Step 3: Route the Fuel Lines
Run diesel-rated hose from the auxiliary tank outlet to the inlet of the transfer pump.
From the pump outlet, connect to a T-fitting installed on the OEM fuel line between the factory tank and the engine’s fuel filter.
Use stainless steel clamps—never zip ties.
🔍 Pro Tip: Install an inline shut-off valve on both tanks. This lets you isolate either tank for service or in case of a leak.
Step 4: Wire the Transfer Pump
Connect the pump to a dedicated 30A fused circuit via a relay.
Trigger the relay with a dash-mounted toggle switch (illuminated for visibility).
Route wiring away from heat sources and moving parts.
Test the pump before final assembly: it should move fuel within 5 seconds of activation.
Step 5: Prime and Test the System
Fill the auxiliary tank with clean diesel.
Turn on the pump for 10–15 seconds to prime the line.
Start the engine and monitor for air bubbles, leaks, or rough idling.
Switch between tanks using the valves to confirm smooth operation.
📌 Note: The 2008 Duramax uses a high-pressure common-rail system. Air in the fuel can cause hard starts or injector damage. Always bleed the system properly if air is suspected.
✅ Extended range (ideal for towing or remote work)
✅ Fuel cost savings (buy in bulk or at lower prices)
✅ Emergency backup during breakdowns or detours
Disadvantages:
❌ Added weight (~250 lbs when full)
❌ Initial cost ($500–$1,200 for quality setup)
❌ Potential warranty issues (if not installed professionally)
💡 Most users report ROI within 6–12 months through reduced fuel stops and better route flexibility.
Legal & Safety Considerations
DOT Compliance: In the U.S., auxiliary tanks must meet FMVSS 301 standards if used on public roads.
State Laws: Some states (e.g., California) require certification or inspection. Check local regulations.
Fire Safety: Never install near catalytic converters or turbochargers. Keep a Class B fire extinguisher onboard.
FAQ Section
Q1: Can I run both tanks at the same time?
A: No. Running both simultaneously can overwhelm the fuel system or cause pressure imbalances. Use a manual valve or solenoid switch to select one tank at a time.
Q2: Will this void my Duramax warranty?
A: If your truck is still under factory warranty (unlikely for a 2008 model), modifications could affect coverage. However, since the 2008 Duramax is well beyond its warranty period, this isn’t a concern for most owners.
Q3: Do I need a second fuel gauge?
A: Not required, but highly recommended. Install a float sensor in the auxiliary tank wired to a secondary gauge or use a digital fuel monitor that tracks usage from both tanks.
Q4: Can I gravity-feed the auxiliary tank?
A: Not advisable on the 2008 Duramax. The CP3 injection pump relies on consistent fuel pressure. Gravity feed may cause air ingestion or insufficient flow, leading to engine damage.
Q5: How long does installation take?
A: A skilled DIYer can complete it in 6–8 hours. Beginners should allow a full weekend. Always test thoroughly before long trips.
Q6: What’s the best brand for auxiliary tanks?
A: Top choices include Transfer Flow, Titan Fuel Tanks, and RDS. These offer DOT-compliant designs, baffled interiors (to reduce slosh), and direct-fit kits for GM trucks.
Conclusion
Knowing how to plumb an auxiliary tank into a 2008 Duramaxempowers you to travel farther, work harder, and drive with confidence. With the right parts, careful planning, and attention to diesel-specific requirements, this upgrade is both safe and highly rewarding.
If you found this guide helpful, share it with fellow Duramax owners on Facebook, Reddit (r/Duramax), or your local truck forum. Your next adventure might just depend on that extra 30 gallons!
Got questions? Drop them in the comments—we’re here to help you build a better, more capable truck.
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