6.5L Diesel Fuel Pump Basics
Pre-1996 6.5 diesels only run the lift pump once oil pressure is built. An oil pressure switch (OPS) switches on the lift pump once oil pressure builds while the engine is cranking. The OPS is notorious for its frequent failures, which are do in part to the fact that the switches relatively fragile circuitry must carry the electrical load drawn by the lift pump. The engineers had good intentions when they devised this safety feature, but the results have been all but desirable. 1996 and later 6.5 diesels begin cycling the lift pump once the key is turned to the "run" position; a much more reliable system with one less thing to troubleshoot.
Symptoms of a weak or failed fuel pump include:
• Hard start condition, excessive cranking
• Engine stutters under load, bucking or jerking during hard acceleration
• Poor fuel economy and overall performance
• No start condition (typically not a lift pump issue - check oil pressure sensor & PMD)
When replacing the lift pump, consider the fact that many owners find the 1992 to 1993 model year lift pumps to outlast and outperform the later pump (rumor has it that it runs at a slightly higher rated pressure). The lift pumps are cross compatible between the model years - connector and fittings are identical.
6.5L Diesel Fuel Pump Replacement Procedures
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• Disconnect both negative battery cables.
• The lift pump is located horizontally inside the frame rail on the driver side of the vehicle, roughly halfway between the engine compartment and fuel tank (exact lateral position will vary based on truck configuration/wheelbase).
• Place a suitable drain pan on the ground beneath the lift pump and rest a shop towel/rag on the inner frame rail below the pump. It is highly recommended that latex gloves are worn during these procedures.
• Unplug the lift pump electrical connector and position it aside.
• Remove the (2) bolts securing the lift pump bracket in place with a 13mm socket. Disconnect the electrical harness and return fuel lines from the bracket, then slide the bracket towards the rear of the vehicle off of the lift pump.
• Assuming you're installing an OEM ACDelco lift pump (as you should be, unless you've opted for a superior aftermarket alternative), the nuts on the lift pump inlet/outlet require a 19mm wrench while the fittings on the fuel line side require a 5/8" wrench - flare nut wrenches are preferred to prevent stripping. Note that the hex nuts on the pump are fixed and therefore only used to hold the fuel pump in place while the fittings on the fuel line side are loosened/tightened.
• Loosen, but do not remove completely the compression fittings at the pump inlet/outlet. If you loosen the fittings to the point that fuel begins to leak, you've loosened it too much. The goal is to loosen the fittings such that they can be removed by hand in subsequent steps.
•The trick to replacing the lift pump without taking a bath in diesel fuel is to use generic, flexible vacuum line caps with roughly a 3/8" I.D. They are available at any auto parts store for a few dollars and can be used to plug the fuel lines while you are both removing and installing the lift pumps. The alternative requires impeccable speed and coordination to quickly remove the old and install the new pump; it also tends to result in an oily mess.
• Remove the male fitting from the lift pump outlet, maneuver the fuel line forward towards the front of the vehicle (the outlet line moves generously and can be rotated slightly towards the passenger side), and plug the outlet line with your finger. Next, install a vacuum line cap over the fuel outlet line fitting. The tip of the fuel line is flared, so you will need to work the cap over the flare.
• Once the outlet fitting is plugged, remove the lift pump from the fuel inlet line, then plug the line with a finger until you are able to cap the line in the same fashion as the previous step.
• Install the replacement lift pump in the opposite fashion by removing the vacuum cap and tightening down the compression fitting. I prefer to install the pump at the inlet first, followed by the outlet. However, it is not imperative which side is installed/removed first and the outcome will be the same regardless - I just find that maneuvering the outlet line is easier without the pump installed on it, so I install the pump on the inlet first.
The lift pump is directional and labeled at the outlet; do not install backwards.
• Securely tighten the inlet and outlet fittings, then reinstall the lift pump/return line bracket and reconnect the electrical connector.
• Performed correctly, the drain pan will have seemed irrelevant and you're rag should be merely damp with diesel fuel. If you're covered in diesel fuel and have succeeded in creating an oily mess, don't be discouraged - some things get easier with experience, and that's why we recommend placing a drain pan beneath the pump!
• Replace the fuel pump chassis harness connector if it is damaged, fits loosely, or is otherwise compromised.
• Bleed the fuel system; see: 6.5L diesel fuel system bleeding procedures
If you'd like to simply the process of bleeding air from the fuel system on any 6.5L GM diesel, please see: 6.5L GM Diesel Fuel Pump Cable. This tool will eliminate the need to crank the engine or cycle the key in order to run the fuel pump with the vehicle off. We highly recommend this convenient tool to any 6.5L diesel owner; simply plug it in and power the pump using the vehicle battery.
• Verify that fuel pressure is within spec; see: 6.5L diesel fuel pressure test procedures