2007.5-2010 GMC Duramax LMM Fuel Lines

How the LMM Sierra HD moves fuel

Fuel leaves the tank, 26 gallons behind a standard box and 34 behind a long box, and travels to the frame-mounted filter head with its hand primer and water-in-fuel sensor. From there it reaches the Bosch CP3 in the valley between the heads. The CP3 is turned by the cam gear and does double duty, drawing the fuel up from the tank and then pressurizing it to 26,000 psi, because there is no factory electric lift pump on any 2001-2016 Duramax. High pressure goes out to two rails, then down individual hard lines to six-hole injectors. The surplus comes back through return hoses and clips. The factory system is also rated for B5 biodiesel.

What fails and why

Trailer weight is what usually brings an owner to this page. With no lift pump in the system the CP3 sometimes has a hard time keeping up, and you feel it as hesitation, sluggish acceleration and low-rail codes. Push the Sierra harder in summer heat and it turns into P0087 low rail pressure, with reduced power, a limp on acceleration and high EGT. The suspect list is broad on purpose: clogged filter, air leak, failing CP3, bad regulator or worn injectors. The other classic is the supply side letting air in. The filter housing picks up micro-cracks, seals loosen, the primer cracks, and the truck hard-starts, stumbles or cranks a long time right after a filter change. Check every line connection for an air leak before buying bigger parts.

Stock replacement vs. upgrade

For a stock Sierra pulling a stock load, replace in kind. OEM-style hoses and complete hose kits put the supply and return side back where it started, and a return line kit with metal fittings is the better choice when the original connections are cracked or chewed up. Before any of that, change the filter on schedule and inspect the filter-head seals and primer. The upgrade side of this page exists for trucks adding fuel supply: a 1/2 inch feed line kit and -6AN or -8AN high-flow CP3 supply fittings when a frame-mounted lift pump goes on, with pressure to the CP3 kept modest at roughly 10 psi or less. High-pressure injection lines are sold individually by cylinder pair, 1 and 8, 2 and 7, 3 and 6, or as a full set.

What we carry

Fleece Performance Engineering handles the high-pressure side with injection lines sold per cylinder pair and as a complete kit, and Dynomite Diesel lists oversized high-pressure injection lines in multiple lengths. Exergy Performance covers threaded hardware: three-way T fittings, rail cap plug and nut, rail plug kits, and the M12x1.5 to -6AN and -8AN high-flow CP3 supply fittings. Merchant Automotive has the OEM-style rubber, from the injection pump inlet and return hoses and the filter inlet and outlet hoses to injector return line kits, clip sets, side-specific return lines and full fuel hose kits. PPE supplies the 1/2 inch lift pump fuel line install kit.

Fitment notes

These parts are for the GMT900 Sierra HD, in WT, SLE and SLT form. The 2007 Classic Sierra HD is the earlier GMT800 body with the LBZ engine, which is why the Duramax range here begins at 2007.5. What drives tank and sump choices is box length rather than wheel count, so note whether yours is the standard or long box. Many of these hoses are listed across 2004.5-2010 or 2006-2010 and fit the LMM without issue, but nothing on this page carries to a 2011 or newer Sierra, which runs the LML engine and a CP4 pump. When a listing covers a wide span, confirm it with the fitment checker on the product page.

Frequently asked questions

My Sierra hesitates while towing and sets a low rail pressure code. Is it the lines?

It can be. On a truck with no factory lift pump the CP3 occasionally struggles to keep up under load, and anything restricting or aerating the supply makes that worse. Start with the filter and a careful inspection of the filter head, primer and every low-pressure connection, because an air leak and a tired pump can read the same way on a scan tool.

What do the -6AN and -8AN CP3 supply fittings do?

They adapt the M12x1.5 port on the CP3 to AN plumbing so a frame-mounted lift pump can feed the pump through a larger line. They are install hardware for a supply upgrade, not a fix for a pump that is already failing. Keep the pressure the lift pump delivers to the CP3 modest, around 10 psi at most.

Can I buy one high-pressure injection line instead of the whole set?

Yes. The Fleece lines are cataloged in cylinder pairs, 1 and 8, 2 and 7, and 3 and 6, as well as a complete set. Buying the single pair makes sense for one damaged line. If you are reworking the fuel system anyway, the set keeps every line at the same age and spec.

Not sure what fits? Call the shop at the number in the header, or use the fitment checker on any product page. We install everything we sell.